Order 1226379: Management Unit 5

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ESSENTIAL ENVIRONMENT T H E S C I E N C E B E H I N D T H E S T O R I E S

Fourth Edition

JAY WITHGOTT • MATTHEW LAPOSATA

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Science and Sustainability: An Introduction to Environmental Science Upon completing this chapter, you will be able to:

➤ Define the term environment and describe the field of environmental science ➤ Explain the importance of natural resources and ecosystem services to our lives ➤ Discuss the consequences of population growth and resource consumption ➤ Describe the steps of the scientific method ➤ Understand the nature and importance of science, and characterize aspects of the process of science ➤ Compare and contrast various approaches in environmental ethics ➤ Diagnose and illustrate major pressures on the global environment ➤ Articulate the concepts of sustainability and sustainable development

Our Island, Earth

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global conditions are changing more quickly than ever. Right now, we are gaining scientific knowledge more rap- idly than ever. And right now, the window of opportunity for acting to solve problems is still open. With such boun- tiful challenges and opportunities, this moment in history is an exciting time to be alive—and to be studying environ- mental science.

We rely on natural resources An island by definition is finite and bounded, and its inhabit- ants must cope with limitations in the materials they need. On our island, Earth, human beings, like all living things, ultimately face environmental constraints. Specifically, there are limits to many of our natural resources, the various substances and energy sources we take from our environ- ment and that we rely on to survive. Natural resources that are replenished over short periods are known as renewable natural resources. Some renewable resources, such as sun- light, wind, and wave energy, are perpetually renewed and essentially inexhaustible. Others, such as timber, water, and soil, renew themselves over months, years, or decades. In contrast, nonrenewable natural resources, such as minerals and crude oil, are in finite supply and are formed much more slowly than we use them. Once we deplete a nonrenewable resource, it is no longer available.

We can view the renewability of natural resources as a continuum (FIGURE 1.1). Renewable resources such as timber, water, and soil can be depleted if we use them faster than they are replenished. For example, overpumping groundwater can deplete underground aquifers and turn lush landscapes into deserts. Populations of animals and plants we harvest from the wild may vanish if we overharvest them.

We rely on ecosystem services If we think of natural resources as “goods” produced by na- ture, then it is also true that Earth’s natural systems provide “services” on which we depend. Our planet’s ecological sys- tems purify air and water, cycle nutrients, regulate climate, pollinate plants, and receive and recycle our waste. Such es- sential services are commonly called ecosystem services. Ecosystem services arise from the normal functioning of nat- ural systems and are not meant for our benefit, yet we could not survive without them. Later we will examine the count- less and profound ways that ecosystem services support our lives and civilization (pp. 36, 90, 94–95).

Just as we can deplete natural resources if we take too many of them, we can degrade ecosystem services by deplet- ing resources, destroying habitat, or generating pollution. In recent years, our depletion of nature’s goods and our disrup- tion of nature’s services have intensified, driven by rising af- fluence and a human population that grows larger every day.

Population growth amplifies our impact For nearly all of human history, fewer than a million peo- ple populated Earth at any one time. Today our population has grown beyond 7 billion people—several thousand times more! FIGURE 1.2 shows just how recently and suddenly this monumental change has come about.

OUR ISLAND, EARTH Viewed from space, our home planet resembles a small blue marble suspended in a vast inky-black void. Earth may seem enormous to us as we go about our lives on its surface, but the astronaut’s view reveals that Earth and its systems are finite and limited. From this perspective, it becomes clear that as our population and our consumption of resources increase, so does our capacity to alter our planet and damage the very systems that keep us alive.

Our environment surrounds us A photograph of Earth offers a revealing perspective, but it cannot convey the complexity of our environment. Our en- vironment consists of all the living and nonliving things around us. It includes the continents, oceans, clouds, and ice caps you can see in the photo of Earth from space, as well as the animals, plants, forests, and farms that comprise the landscapes surrounding us. In a more inclusive sense, it en- compasses the structures, urban centers, and living spaces that people have created. In its broadest sense, our environ- ment also includes the complex webs of social relationships and institutions that shape our daily lives.

People commonly use the term environment in the first, most narrow sense—to mean a nonhuman or “natural” world apart from human society. This usage is unfortunate, because it masks the important fact that people exist within the environ- ment and are part of nature. As one of many species on Earth, we share dependence on a healthy planet. The limitations of language make it all too easy to speak of “people and nature,” or “humans and the environment,” as though they were sepa- rate and did not interact. However, the fundamental insight of environmental science is that we are part of the “natural” world and that our interactions with the rest of it matter a great deal.

Environmental science explores our interactions with the world Understanding our relationship with the world around us is vital because we depend utterly on our environment for air, water, food, shelter, and everything else essential for living. Moreover, we modify our environment. Many of our actions have enriched our lives, bringing us better health, longer life spans, and greater material wealth, mobility, and leisure time—but they have also often degraded the natural systems that sustain us. Impacts such as air and water pollution, soil erosion, and species extinction compromise our well-being, pose risks to human life, and jeopardize our ability to build a society that will survive and thrive in the long term.

Environmental science is the study of how the natural world works, how our environment affects us, and how we af- fect our environment. We need to understand how we interact with our environment so that we can devise solutions to our most pressing challenges. It can be daunting to reflect on the sheer magnitude of environmental dilemmas that confront us today, but these problems also bring countless opportunities for creative solutions.

Environmental scientists study the issues most cen- trally important to our world and its future. Right now,

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Renewable natural resources

Nonrenewable natural resources

• Sunlight • Wind energy • Wave energy • Geothermal energy

• Fresh water • Forest products • Agricultural crops • Soils

• Crude oil • Natural gas • Coal • Copper, aluminum, and other metals

FIGURE 1.1 ▲ Natural resources lie along a continuum from perpetually renewable (left) to nonrenewable (right). Perpetually renewable, or inexhaustible, resources, such as sunlight and wind energy, will always be there for us. Renewable resources such as timber, soils, and fresh water may be replenished on intermediate time scales, if we are careful not to deplete them. Nonrenewable resources, such as oil and coal, exist in limited amounts that could one day be gone.

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FIGURE 1.2 ▲ For almost all of human history, our population was low and relatively stable. It increased after the agricultural revolution and then skyrocketed as a result of the industrial revolution. Our growing population has given rise to congested ur- ban areas, such as this city (inset) in Java, Indonesia. Data compiled from U.S. Census Bureau, U.N. Population Division, and other sources.

Two phenomena triggered remarkable increases in popu- lation size. The first was our transition from a hunter-gath- erer lifestyle to an agricultural way of life. This change began around 10,000 years ago and is known as the agricultural revolution. As people began to grow crops, raise domestic animals, and live sedentary lives on farms and in villages, they found it easier to meet their nutritional needs. As a result, they began to live longer and to produce more children.

The second notable phenomenon, known as the industrial revolution, began in the mid-1700s. It entailed a shift from rural life, animal-powered agriculture, and handcrafted goods to an urban society provisioned by the mass production of factory- made goods and powered by fossil fuels (nonrenewable energy sources including oil, coal, and natural gas (pp. 328–335)). Industrialization brought technological advances and improve- ments in sanitation and medicine, and it enhanced agricultural production through the use of fossil-fuel-powered equipment and synthetic pesticides and fertilizers (p 136).

The factors driving population growth have brought us better lives in many ways. But as our world fills up with peo- ple, population growth has begun to threaten our well-being. We must ask how well the planet can accommodate 7 billion of us—or the 9 billion forecast by 2050. Already our sheer numbers, unparalleled in history, are putting unprecedented stress on natural systems and the availability of resources.

Resource consumption exerts social and environmental pressures Population growth is unquestionably at the root of many en- vironmental concerns, but the growth in resource consump- tion also plays a role. As the industrial revolution enhanced the material affluence of many of the world’s people, it con- siderably increased our consumption of natural resources and manufactured goods.

The “tragedy of the commons” When publicly ac- cessible resources are open to unregulated exploitation, they inevitably become overused and, as a result, are damaged or depleted. So argued the late Garrett Hardin of the University of California at Santa Barbara in his 1968 essay in the journal Science, titled “The Tragedy of the Commons.”

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Basing his argument on a scenario described in a 19th-century pamphlet, Hardin explained that in a public pasture, or “common,” open to unregulated grazing, each person who grazes animals will be motivated by self-interest to increase the number of his or her animals in the pasture. Because no single person owns the pasture, no one has incen- tive to expend effort taking care of it, and everyone takes what he or she can until the resource is depleted. This is known as the tragedy of the commons. Ultimately, overgrazing will cause the pasture’s food production to collapse.

Some argue that private ownership best addresses this problem. Others point to cases in which people sharing a common resource have voluntarily organized and cooperated to enforce its responsible use. Still others maintain that the di- lemma justifies government regulation of the use of resources held in common by the public, from grazing land and forests to clean air and water.

Our ecological footprint As global affluence has risen, human society has consumed more and more of the planet’s resources. We can quantify resource consumption using the concept of the ecological footprint, developed in the 1990s by environmental scientists Mathis Wackernagel and William Rees. An ecological footprint expresses environmental im- pact in terms of the cumulative area of biologically produc- tive land and water required to provide the resources a person or population consumes and to dispose of or recycle the waste the person or population produces (FIGURE 1.3). It measures the total area of Earth’s biologically productive surface that a given person or population “uses” once all direct and indirect impacts are totaled up.

For humanity as a whole, Wackernagel and his colleagues calculate that our species is now using 50% more of the plan- et’s resources than are available on a sustainable basis. That is, we are depleting renewable resources by using them 50% faster than they are being replenished—like drawing the prin- cipal out of a bank account rather than living off the interest. This excess use has been termed overshoot, because we have overshot, or surpassed, Earth’s capacity to sustainably support us (FIGURE 1.4). Moreover, people from wealthy nations such

FIGURE 1.3 ▲ An ecological footprint represents the total area of biologically productive land and water needed to produce the resources and dispose of the waste for a given person or population. The footprint of an average citizen of an affluent nation is much larger than the physical area in which the person lives day to day. Adapted from an illustration by Philip Testemale in Wackernagel, M., and W. Rees, 1996. Our ecological footprint: Reducing

human impact on the Earth. Gabriola Island, British Columbia: New Society

Publishers.

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FIGURE 1.4 ▲ The global ecological footprint of the human population is over 2.5 times larger than it was a half-century ago and now exceeds what Earth can bear in the long run, scientists have calculated. Data indicate that we have already overshot Earth’s biocapacity—its capacity to support us—by 50%. That is, we are using renewable natural resources 50% faster than they are being replenished. Data from WWF International, 2010. Living planet report 2010. Published in 2010 by WWF-World Wide Fund

for Nature.

The Tragedy of the Commons Imagine you make your living by fishing. You are free to boat anywhere and set out as many lines and traps as you like, and your catches have been

good. However, the fishing grounds are getting crowded, and you find yourself competing with more people for fewer fish. Catches decline year by year, leaving you and the other fishers with catches too meager to support your families. Some call for dividing the waters and selling access to individuals plot by plot. Others implore the government to regulate how many fish can be caught. Still others want to team up, set quotas themselves, and prevent newcomers from entering the market. What do you think is the best way to combat this tragedy of the commons and restore the fishery, and why?

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as the United States have much larger ecological footprints than do people from poorer nations. If all the world’s people consumed resources at the rate of U.S. citizens, we would need the equivalent of four-and-a-half planet Earths.

Environmental science can help us avoid past mistakes It remains to be seen what consequences resource consump- tion and population growth will have for today’s global so- ciety, but we have historical evidence that civilizations can crumble when pressures from population and consumption overwhelm resource availability. Easter Island is a classic case (see THE SCIENCE BEHIND THE STORY, pp. 6–7).

Many great civilizations have fallen after degrading their environments, and each has left devastated landscapes in its wake: the Greek and Roman empires; the Angkor civilization of Southeast Asia; and the Maya, Anasazi, and other civiliza- tions of the New World. In Iraq and other regions of the Mid- dle East, areas that are barren desert today were lush enough to support the origin of agriculture when ancient civilizations thrived there. In his 2005 book Collapse, scientist and author Jared Diamond analyzed existing research and formulated general reasons why civilizations succeed and persist, or fail and collapse. Success and persistence, he argued, depend largely on how societies interact with their environments and on how they respond to problems.

In today’s globalized society, the stakes are higher than ever because our environmental impacts are global. If we cannot forge sustainable solutions to our problems, then the resulting societal collapse will be global. Fortunately, envi- ronmental science holds keys to building a better world. By studying environmental science, you will learn to evaluate the changes happening around us and to think critically and crea- tively about actions to take in response.

THE NATURE OF ENVIRONMENTAL SCIENCE Environmental scientists aim to comprehend how Earth’s natural systems function, how these systems influence peo- ple, and how we are influencing these systems. Many envi- ronmental scientists are motivated by a desire to develop so- lutions to environmental problems. These solutions (such as new technologies, policy decisions, or resource management strategies) are applications of environmental science. The study of such applications and their consequences is, in turn, also part of environmental science.

Environmental science is interdisciplinary Studying our interactions with our environment is a com- plex endeavor that requires expertise from many disci- plines, including ecology, Earth science, chemistry, biology, geography, economics, political science, demography, eth- ics, and others. Environmental science is thus an interdis- ciplinary field—one that borrows techniques from multiple

disciplines and brings their research results together in a broad synthesis (FIGURE 1.5).

Traditional established disciplines are valuable because their scholars delve deeply into topics, uncovering new knowl- edge and developing expertise in particular areas. In contrast, interdisciplinary fields are valuable because their practition- ers consolidate and synthesize the specialized knowledge from many different disciplines and make sense of it in a broad context to better serve the multifaceted interests of society.

Environmental science is especially broad because it encompasses not only the natural sciences (disciplines that examine the natural world), but also the social sciences (disciplines that address human interactions and institu- tions). Most environmental science programs focus pre- dominantly on the natural sciences, whereas programs that incorporate the social sciences extensively often use the term environmental studies. Whichever approach one takes, these fields reflect many diverse perspectives and sources of knowledge.

Just as an interdisciplinary approach to studying issues can help us better understand them, an integrated approach to addressing environmental problems can produce effective solutions for society. As one example, we used to add lead to gasoline to make cars run more smoothly, even though researchers knew that lead emissions from tailpipes caused health problems, including brain damage and premature death. In 1970 air pollution was severe, and motor vehicles accounted for 78% of U.S. lead emissions. Over the following years, environmental scientists, engineers, medical research- ers, and policymakers all merged their knowledge and skills into a process that eventually brought about a ban on leaded gasoline. By 1996 all gasoline sold in the United States was unleaded, and the nation’s largest source of atmospheric lead emissions had been completely eliminated.

Environmental science

EcologyEthics

Economics

Anthropology

History

Political science

Engineering

Archaeology Geography

Geology

Atmospheric science

Chemistry

Biology

Oceanography

FIGURE 1.5 ▲ Environmental science is an interdisciplinary pursuit, involving input from many different established fields of study across the natural sciences and social sciences.

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The Lesson of Easter Island

Historians and anthropologists wondered how people without wheels or ropes, on an island without trees, could have moved statues 10 m (33 ft) high weighing 90 metric tons (99 tons) as far as 10 km (6.2 mi) from the quarries where they were chiseled to the coastal sites where they were erected. The explanation, scientists discovered, was

that the island did not always lack trees. Indeed, scientific research tells us

that the island had once been lushly forested and had supported a pros- perous society of 6,000 to 30,000 people. Tragically, this once-flourishing civilization overused its resources and cut down all its trees, destroying itself in a downward spiral of starvation and conflict. Today Easter Island stands as a parable and a warning for what can happen when a population consumes too much of the limited resources that support it.

To explore the mystery of Easter Island’s past, scientists have used various methods. Some, such as British scientist John Flenley, have excavated sediments from the bottom of the island’s lakes, drilling cores deep into the mud and examining ancient grains of pollen preserved there. Pollen grains vary from one plant species to another, so scientists can reconstruct, layer by layer, the history of vegetation in a region through time. By analyzing pollen grains under scanning electron microscopes, Flenley and other re- searchers found that when Polynesian people arrived (likely between A.D. 300 and 900), the island was covered with a species of palm tree related to the Chilean wine palm, a tall and thick- trunked tree.

Moreover, archaeologists located ancient palm nut casings in caves and crevices, and a geologist found

carbon-lined channels in the soil that matched root channels typical of the Chilean wine palm. Scientists decipher- ing the island people’s script on stone tablets discerned characters etched in the form of palm trees.

By studying pollen and the re- mains of wood from charcoal, scientists such as French archaeologist Catherine Orliac found that at least 21 other spe- cies of plants, many of them trees, had also been common but are now com- pletely gone. The island had clearly supported a diverse forest. However, starting around A.D. 750, tree popula- tions declined and ferns and grasses became more common, according to pollen analysis from one lake site. By A.D. 950, the trees were largely gone, and around A.D. 1400 overall pollen levels plummeted, indicating a dearth of vegetation.

The same sequence of events occurred two centuries later at the other two lake sites, which were higher and more remote from village areas. Researchers first hypothesized that the forest loss was due to cli- mate change, but evidence instead supported the hypothesis that the people had gradually denuded their own island.

their research rigorously objective and free from ideology, personal values, and preconceptions. Remaining open to whatever conclusions the data demand is a hallmark of the effective scientist.

THE NATURE OF SCIENCE Modern scientists describe science as a systematic process for learning about the world and testing our understanding of it. The term science is also used to refer to the accumulated body of knowledge that arises from this dynamic process of questioning, observation, testing, and discovery.

Knowledge gained from science can be applied to address societal needs—for instance, to develop technology

Easter Island’s immense statues

Environmental science is not the same as environmentalism Although many environmental scientists are interested in solving problems, it would be incorrect to confuse environ- mental science with environmentalism, or environmental ac- tivism. They are not the same.

Environmental science is the pursuit of knowledge about the workings of the environment and our interactions with it. In contrast, environmentalism is a social movement dedicated to protecting the natural world—and, by exten- sion, people—from undesirable changes brought about by human actions. Although environmental scientists search for solutions to environmental problems, they strive to keep

Easter Island is one of the most remote spots on the globe, located in the Pacific Ocean 3,750 km (2,325 mi) from South America and 2,250 km (1,395 mi) from the nearest inhabited island. When the first European explorers reached the island (today called Rapa Nui) in 1722, they found a barren landscape populated by fewer than 2,000 people, who lived in caves and eked out a marginal existence from a few meager crops. However, explorers also noted that the desolate island featured hundreds of gigantic statues of carved stone, evidence that a sophisticated civilization had once inhabited the island.

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The trees provided fuelwood, building material for houses and ca- noes, fruit to eat, fiber for clothing, and, presumably, logs with which to move the stone statues. By hiring groups of men to recreate the feat, anthropolo- gists experimentally tested hypotheses about how the islanders moved their monoliths down from the quarries. The methods that have worked involve using numerous tree trunks as rollers or sleds, along with great quantities of rope. The only likely source of rope on the island would have been the fibrous inner bark of the hauhau tree, a species that today is near extinction.

With the trees gone, rain would have eroded soil away—a

phenomenon confirmed by data from the lake bottoms, where large quanti- ties of sediment accumulated. Erosion of the islanders’ agricultural land would have lowered yields of bananas, sugar cane, and sweet potatoes, leading to starvation and population decline.

Archaeological evidence sup- ports such a scenario of environ- mental degradation and civilization decline. Analysis of 6,500 bones by archaeologist David Steadman has shown that at least 31 species of birds nested on Easter Island and served as a food source for the islanders. Today, only one native bird species is left.

Remains from charcoal fires aged by radiocarbon dating show that besides crops and birds, early islanders feasted on the bounty of the sea, including porpoises, fish, sharks, turtles, octopus, and shellfish. But analysis of islanders’ diets in the later years indicated that the people consumed little seafood. With the trees gone, the islanders could no longer build the great double canoes their proud Polynesian ancestors had used for centuries to fish and travel among islands.

As resources declined, archae- ologists found, the islanders began keeping their main domesticated food animal, chickens, in stone fortresses with entrances designed to prevent theft. The once prosperous and peaceful civilization fell into clan warfare, as revealed by unearthed weapons, skeletons, and skulls with head wounds.

Is the story of Easter Island as unique and isolated as the island itself, or does it hold lessons for our world today? Like the Easter Islanders, we are all stranded together on an island with limited resources. Earth may be vastly larger and richer in resources than Easter Island, but Earth’s human population is also much greater.

The Easter Islanders must have seen that they were depleting their resources, but it seems that they could not stop. Whether we can learn from the history of Easter Island and act more wisely to conserve the resources of our island, Earth, is entirely up to us.

or to inform policy and management decisions (FIGURE 1.6). Many scientists are motivated by the potential to develop use- ful applications, whereas others are motivated simply by a de- sire to understand how the world works.

Scientists test ideas by critically examining evidence Science is all about asking and answering questions. Scientists examine how the world works by making observations, taking measurements, and testing whether their ideas are supported by evidence. The effective scientist thinks critically and does not simply accept conventional wisdom from others. The scientist becomes excited by novel ideas but is skeptical and

judges ideas by the strength of evidence that supports them. In these ways, scientists are good role models for the rest of us, because every one of us can benefit from learning to think critically in our everyday lives.

A great deal of scientific work is observational science or descriptive science, research in which scientists gather basic information about organisms, materials, systems, or processes that are not well known or that cannot be manipu- lated in experiments. In this approach, researchers explore new frontiers of knowledge by observing and measuring phe- nomena to gain a better understanding of them. Such research is common in traditional fields such as astronomy, paleontol- ogy, and taxonomy, as well as in newer, fast-growing fields such as molecular biology and genomics.

The haunting statues of Easter Island (Rapa Nui) were erected by a sophisticated civilization that collapsed after depleting its resource base and devastating its island environment.

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of information. However, scientists of all persuasions broadly agree on fundamental elements of the process of scientific in- quiry. The scientific method (FIGURE 1.7) typically consists of the steps outlined below.

Make observations Advances in science typically begin with the observation of some phenomenon that the scientist wishes to explain. Observations set the scientific method in motion and also function throughout the process.

Ask questions Curiosity is a fundamental human charac- teristic. Just observe the explorations of babies or young chil- dren in a new environment—they want to touch, taste, watch, and listen to anything that catches their attention, and as soon as they can speak, they begin asking questions. Scientists, in this respect, are kids at heart. Why are certain plants or ani- mals less common today than they once were? Why are storms becoming more severe and flooding more frequent? What is causing excessive growth of algae in local ponds? When pesti- cides poison fish or frogs, are people also affected? All of these are questions environmental scientists ask.

Develop a hypothesis Scientists address their questions by devising explanations they can test. A hypothesis is a statement that attempts to explain a phenomenon or answer a

Once enough general information is known about a sub- ject, scientists can begin posing more specific questions that seek deeper explanations about how and why things are the way they are. At this point they may pursue hypothesis-driven science, research that proceeds in a targeted and structured manner, using experiments to test hypotheses within a frame- work traditionally known as the scientific method.

The scientific method is a traditional approach to research The scientific method is a technique for testing ideas with observations. There is nothing mysterious about the scientific method; it is merely a formalized version of the way any of us might naturally use logic to resolve a question. Because science is an active, creative process, an innovative scientist may some- times find good reason to depart from the traditional scientific method. Moreover, scientists in different fields may approach their work differently because they deal with dissimilar types

(a) Methanol-powered fuel-cell car

(b) Prescribed burning

FIGURE 1.6 ▲ Scientific knowledge can be applied in engineering and technology and in policy and management decisions. Energy- efficient automobiles (a) are technological advances made possible by materials and energy research. Prescribed burning (b), shown here in the Ouachita National Forest, Arkansas, is a management practice to restore healthy forests that is informed by scientific research into forest ecology.

Scientific method

Observations

Questions

Hypothesis

Predictions

Fail to reject hypothesis

Test

Results

Reject hypothesis

FIGURE 1.7 ▲ The scientific method is the traditional experimental approach that scientists use to learn how the world works. This dia- gram is a simplified generalization that, although useful for instruc- tive purposes, cannot convey the true dynamic and creative nature of science. Moreover, researchers from different disciplines may pursue their work in ways that vary legitimately from this model.

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able, a variable the scientist manipulates, whereas the quan- tity of algae that results is the dependent variable, one that depends on the fertilizer input. If the two ponds are identical except for a single independent variable (fertilizer input), then any differences that arise between the ponds can be attributed to that variable. Such an experiment is known as a controlled experiment because the scientist controls for the effects of all variables except the one whose effect he or she is testing. In our example, the pond left unfertilized serves as a control, an unmanipulated point of comparison for the manipulated treatment pond.

Whenever possible, it is best to replicate one’s experi- ment; that is, to stage multiple tests of the same comparison of control and treatment. Our scientist could perform a repli- cated experiment on, say, 10 pairs of ponds, adding fertilizer to one of each pair.

Analyze and interpret results Scientists record data, or information, from their studies (FIGURE 1.8). They partic- ularly value quantitative data (information expressed using numbers) because numbers provide precision and are easy to compare. The scientist running the fertilization experiment, for instance, might quantify the area of water surface covered by algae in each pond or might measure the dry weight of algae in a certain volume of water taken from each.

However, even with the precision that numbers provide, a scientist’s results may not be clear-cut. Data from treatments and controls may vary only slightly, or replicates may yield different results. The researcher must therefore analyze the data using statistical tests. With these mathematical methods, scientists can determine objectively and precisely the strength and reliability of patterns they find.

If experiments disprove a hypothesis, the scientist will re- ject the hypothesis and may formulate a new one to replace it. If experiments fail to disprove a hypothesis, this lends support to the hypothesis but does not prove it is correct. The scientist may choose to generate new predictions to test the hypothesis in different ways and further assess its likelihood of being true. Thus, the scientific method loops back on itself, often giving rise to repeated rounds of hypothesis revision and new experi- mentation (see Figure 1.7).

If repeated tests fail to reject a particular hypothesis, evidence in favor of it accumulates, and the researcher may eventually conclude that the hypothesis is well supported. Ide- ally, the scientist would want to test all possible explanations. For instance, our researcher might formulate an additional hypothesis, proposing that algae increase in fertilized ponds because chemical fertilizers diminish the numbers of fish or invertebrate animals that eat algae. It is possible, of course, that both hypotheses could be correct and that each may ex- plain some portion of the initial observation that local ponds were experiencing algal blooms.

We test hypotheses in different ways An experiment in which the researcher actively chooses and manipulates the independent variable is known as a manip- ulative experiment. A manipulative experiment provides the strongest type of evidence a scientist can obtain because it can reveal causal relationships, showing that changes in an

scientific question. For example, a scientist investigating why algae are growing excessively in local ponds might observe chemical fertilizers being applied on farm fields nearby. The scientist might then state a hypothesis as follows: “Agricul- tural fertilizers running into ponds cause the amount of algae in the ponds to increase.”

Make predictions The scientist next uses the hypothesis to generate predictions, specific statements that can be di- rectly and unequivocally tested. In our algae example, a re- searcher might predict: “If agricultural fertilizers are added to a pond, the quantity of algae in the pond will increase.”

Test the predictions Scientists test predictions one at a time by gathering evidence that could potentially refute the prediction and thus disprove the hypothesis. An experiment is an activity designed to test the validity of a prediction or a hypothesis. It involves manipulating variables, or conditions that can change.

For example, a scientist could test the prediction linking algal growth to fertilizer by selecting two identical ponds and adding fertilizer to one while leaving the other in its natural state. In this example, fertilizer input is an independent vari-

FIGURE 1.8  Dr. Jennifer Smith of the Scripps Institution of Oceanography in San Diego uses a quadrat with a digital camera to photograph sites along a transect of a coral reef at a remote atoll in the South Pacific. Data from analysis of the photos will help her test hypotheses about how human impacts affect the condition and community structure of coral reefs.

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The scientific process continues beyond the scientific method Scientific work takes place within the context of a commu- nity of peers. To have an impact, a researcher’s work must be published and made accessible to this community. Thus, the scientific method is embedded within a larger process involv- ing the scientific community as a whole (FIGURE 1.9).

Peer review When a researcher’s work is done and the results analyzed, he or she writes up the findings and sub- mits them to a journal (a scholarly publication in which sci- entists share their work). The journal’s editor asks several other scientists who specialize in the subject area to examine the manuscript, provide comments and criticism (generally anonymously), and judge whether the work merits publica- tion in the journal. This procedure, known as peer review, is an essential part of the scientific process.

Peer review is a valuable guard against faulty science con- taminating the literature on which all scientists rely. However, because scientists are human, personal biases and politics can sometimes creep into the review process. Fortunately, just as individual scientists strive to remain objective in conducting their research, the scientific community does its best to ensure fair review of all work. Winston Churchill once called democ- racy the worst form of government, except for all the others that had been tried. The same might be said about peer review: It is an imperfect system, yet it is the best we have.

Conference presentations Scientists frequently present their work at professional conferences, where they interact

independent variable cause changes in a dependent variable. In practice, however, we cannot run manipulative experi- ments for all questions, especially for processes that operate at large spatial scales or on long time scales. For example, in studying the effects of global climate change (Chapter 14), we cannot add carbon dioxide to 10 treatment planets and 10 control planets and then compare the results! Thus, in environmental science, it is common for scientists to run natural experiments that compare how dependent variables are expressed in naturally different contexts, and to search for correlation, or statistical association among variables.

For instance, let’s suppose our scientist studying algae surveys 50 ponds, 25 of which happen to be fed by fertilizer runoff from nearby farms and 25 of which are not. Let’s say he or she finds seven times more algal growth in the fertilized ponds. The scientist would conclude that algal growth is cor- related with fertilizer input; that is, that one tends to increase along with the other.

This type of evidence is weaker than the causal demonstra- tion that manipulative experiments can provide, but sometimes a natural experiment is the only feasible approach for a subject of immense scale, such as an ecosystem or a planet. Because many questions in environmental science are complex and ex- ist on large scales, they must be addressed with correlative data. As such, environmental scientists cannot always provide clear- cut, black-and-white answers to questions from policymakers and the public. Nonetheless, good correlative studies can make for strong science, and they preserve the real-world complexity that manipulative experiments often sacrifice. Whenever pos- sible, scientists try to integrate both natural and manipulative experiments to gain the advantages of each.

Scientific process (as practiced by scientific community)

Scientific method (as practiced by individual researcher or research group)

Observations

Questions

Hypothesis

Predictions

Test

Results

Scientific paper

Peer review

Paper accepted

Further research

by scientific community

Reject hypothesis

Revise paper

Fail to reject hypothesis

Publication in

scientific journal

Paper rejected

FIGURE 1.9  The scientific method (inner yellow box) followed by individual researchers or research teams exists within the context of the overall process of science at the level of the scientific community (outer green box). This process includes peer review and publication of research, acquisition of funding, and the elaboration of theory through the cumulative work of many researchers.

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Science goes through “paradigm shifts” As the scientific community accumulates data in a given area of research, interpretations may change. Thomas Kuhn’s 1962 book The Structure of Scientific Revolutions argued that sci- ence goes through periodic revolutions: dramatic upheavals in thought, in which one paradigm, or dominant view, is abandoned for another. For example, before the 16th century, scientists believed that Earth was at the center of the uni- verse. Their data on the movements of planets fit that concept quite well, yet the idea eventually was disproved by Nicolaus Copernicus, who showed that placing the sun at the center of the solar system explained the planetary data even better.

Another paradigm shift occurred in the 1960s, when ge- ologists accepted plate tectonics (pp. 228–229). By this time, evidence for the movement of continents and the action of tectonic plates had accumulated and become overwhelmingly convincing. Such paradigm shifts demonstrate the strength and vitality of science, showing it to be a process that refines and improves itself through time.

Understanding how science works is vital to assessing how scientific interpretations improve through time as information accrues. This process is especially relevant in environmental science, a young field that is changing rapidly as we learn vast amounts of new information. However, to understand and ad- dress environmental problems, and to assess our actions and their consequences, we need more than science. We also need ethics. Science does not take place in a vacuum—it is influenced by the worldviews and cultural backgrounds of the scientists who practice it. Cultural influences also guide how engineers, managers, policymakers, and citizens apply scientific knowl- edge. Thus, our examination of ethics below (and of economics and policy in Chapter 5) will help us learn how values shape human behavior and how information from the sciences is in- terpreted and put to use in our society.

ENVIRONMENTAL ETHICS Ethics is a branch of philosophy that involves the study of good and bad, of right and wrong. The term ethics can also refer to the set of moral principles or values held by a person or a society. Ethicists help clarify how people judge right from wrong by elucidating the criteria, standards, or rules that peo- ple use in making these judgments. Such criteria are grounded in values—for instance, promoting human welfare, maximiz- ing individual freedom, or minimizing pain and suffering.

People of different cultures or with different worldviews may differ in their values and thus in the specific actions they consider to be right or wrong. This is why some ethicists are relativists, who believe that ethics do and should vary with social context. However, different human societies show a remarkable extent of agreement on what moral standards are appropriate. Thus, many ethicists are universalists, who maintain that there exist objec- tive notions of right and wrong that hold across cultures and contexts. For both relativists and universalists, ethics is not just descriptive, but prescriptive; it tells us how we ought to behave.

Ethical standards are the criteria that help differentiate right from wrong. One classic ethical standard is the categorical imperative proposed by German philosopher Immanuel Kant,

with colleagues and receive informal comments on their re- search. Such feedback can help improve a researcher’s work before it is submitted for publication.

Grants and funding To fund their research, most sci- entists need to spend enormous amounts of time requesting grant money from private foundations or from government agencies such as the National Science Foundation. Grant ap- plications undergo peer review just as scientific papers do, and competition for funding is generally intense.

Scientists’ reliance on funding sources can occasionally lead to conflicts of interest. A researcher who obtains data showing his or her funding source in an unfavorable light may feel reluctant to publish the results for fear of losing funding—or worse yet, may be tempted to doctor the results. This situation can arise, for instance, when an industry funds research to test its products for safety or environmental im- pact. Most scientists resist these pressures, but when you are critically assessing a scientific study, it is always a good idea to note where the researchers obtained their funding.

Repeatability The careful scientist may test a hypothesis repeatedly in various ways, and after the research results are published, other scientists may seek to reproduce the results in their own experiments. Scientists are inherently cautious about accepting a novel hypothesis, so the more a result can be reproduced by different research teams, the more confi- dence scientists will have that it provides the correct explana- tion for an observed phenomenon.

Theories If a hypothesis survives repeated testing by nu- merous research teams and continues to predict experimental outcomes and observations accurately, it may be incorporat- ed into a theory. A theory is a widely accepted, well-tested ex- planation of one or more cause-and-effect relationships that has been extensively validated by a great amount of research. Whereas a hypothesis is a simple explanatory statement that may be disproven by a single experiment, a theory consoli- dates many related hypotheses that have been supported by a large body of data.

Note that scientific use of the word theory differs from popular usage of the word. In everyday language, when we say something is “just a theory” we are suggesting it is a specula- tive idea without much substance. Scientists, however, mean just the opposite when they use the term. To them, a theory is a conceptual framework that explains a phenomenon and has undergone extensive and rigorous testing, such that confi- dence in it is extremely strong.

For example, Darwin’s theory of evolution by natural selection (pp. 46–48) has been supported and elaborated by many thousands of studies over 150 years of intensive re- search. Research has shown repeatedly and in great detail how plants and animals change over generations, or evolve, expressing characteristics that best promote survival and re- production. Because of its strong support and explanatory power, evolutionary theory is the central unifying principle of modern biology. Other prominent scientific theories include atomic theory, cell theory, big bang theory, plate tectonics, and general relativity.

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person who feels responsibility for the welfare of insects would answer the third question very differently from a person whose domain of ethical concern ends with people. Three loosely conceived categories summarize differences among personal domains of ethical concern. These three ethical perspectives, or worldviews, are anthropocentrism, biocentrism, and ecocen- trism (FIGURE 1.10).

Anthropocentrism describes a human-centered view of our relationship with the environment. An anthropocentrist denies or ignores the notion that nonhuman entities can have rights. An anthropocentrist measures the costs and benefits of actions solely according to their impact on people. For exam- ple, if development of a mining project would provide signifi- cant economic benefits while doing little harm to aesthetics or human health, an anthropocentrist would conclude it was a worthwhile venture, even if it would destroy many plants and animals. Conversely, if protecting the area from development would provide greater economic, spiritual, or other benefits to people, an anthropocentrist would favor its protection. In the anthropocentric perspective, anything not providing benefit to people is considered to be of negligible value.

In contrast, biocentrism ascribes value to certain living things or to the biotic realm in general. In this perspective, human life and nonhuman life both have ethical standing. In the case of a mining proposal, a biocentrist might oppose mine development if it would destroy many plants and ani- mals, even if it would generate economic benefits and pose no threat to human health.

Ecocentrism judges actions in terms of their effects on whole ecological systems, which consist of living and nonliving ele- ments and the relationships among them. An ecocentrist values the well-being of entire species, communities, or ecosystems (we

which advises us to treat others as we would prefer to be treated ourselves. In Christianity this is called the “golden rule,” and most of the world’s religions teach this same lesson. Another standard is the principle of utility, elaborated by British philosophers Jere- my Bentham and John Stuart Mill. The utilitarian principle holds that something is right when it produces the greatest practical benefits for the most people. We all employ such ethical stand- ards as tools for making countless decisions in our everyday lives.

Environmental ethics pertains to people and the environment The application of ethical standards to relationships between people and nonhuman entities is known as environmental ethics. Our interactions with our environment frequently give rise to ethical questions that can be difficult to resolve. Consider some examples:

1. Is the present generation obligated to conserve resources for future generations? If so, how much should we sacrifice?

2. Can we justify exposing some communities to a dispro- portionate share of pollution? If not, what actions are war- ranted to prevent this?

3. Are humans justified in driving species to extinction? If de- stroying a forest would drive extinct an insect species few people have heard of but would create jobs for 10,000 people, would that action be ethically admissible? What if it were an owl species? What if only 100 jobs would be created?

Answers to such questions depend partly on what ethical standard(s) a person adopts. They also depend on the breadth and inclusiveness of the person’s domain of ethical concern. A

Ecocentric

Biocentric

Anthropocentric

FIGURE 1.10  We can categorize people’s ethical perspectives as anthropocentric, biocentric, or ecocentric. An anthropocentrist grants ethical standing only to human beings and judges actions solely in terms of their effects on people. A biocentrist values and considers all living things, human and otherwise. An ecocentrist extends ethi- cal consideration to living and nonliving components of the environment and takes a holistic view of the connec- tions among these components, valuing the larger functional systems of which they are a part.

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argument based on the principle of utility). “Everybody needs beauty as well as bread,” he wrote, “Places to play in and pray in, where nature may heal and give strength to body and soul alike.”

Some of the factors that motivated Muir also inspired the forester Gifford Pinchot (1865–1946; FIGURE 1.12), who founded what would become the U.S. Forest Service and served as its chief in Theodore Roosevelt’s administration. Like Muir, Pinchot op- posed the deforestation and unregulated development of North American lands. However, Pinchot took a more anthropocentric view of how and why we should value nature. He espoused the conservation ethic, which holds that people should put natural resources to use but that we have a responsibility to manage them wisely. The conservation ethic employs a utilitarian standard, stating that we should allocate resources in a way that provides the greatest good to the greatest number of people for the longest time. Whereas preservation aims to preserve nature for its own sake and for our aesthetic and spiritual benefit, conservation pro- motes the prudent, efficient, and sustainable extraction and use of natural resources for the benefit of present and future generations.

Pinchot and Muir came to represent different branches of the American environmental movement, and their contrasting ethical approaches often pitted them against one another on pol- icy issues of the day. Nonetheless, they both represented reac- tions against a prevailing “development ethic,” which holds that people are and should be masters of nature and which promotes economic development without regard to its negative conse- quences. Both Pinchot and Muir left legacies that reverberate today in the various ethical approaches to environmentalism.

will study these in Chapters 2–4) over the welfare of a given in- dividual. Implicit in this view is that preserving systems generally protects their components, whereas just protecting certain com- ponents may not safeguard the entire system. Ecocentrism is a more holistic perspective than biocentrism or anthropocentrism. It encompasses a wider variety of entities and seeks to preserve the connections that tie them together into functional systems.

Conservation and preservation arose with the 20th century With the onset of the industrial revolution, more people be- gan adopting biocentric and ecocentric worldviews. In the 19th and 20th centuries, worldviews of people in the United States evolved quickly as the nation pushed west, urbanized, and exploited the continent’s resources, boosting affluence and dramatically altering the landscape in the process.

A key voice for restraint during this period of rapid growth and change was John Muir (1838–1914), a Scottish immigrant to the United States who made California’s Yosemite Valley his wilderness home. Although Muir chose to live in solitude in his beloved Sierra Nevada for long stretches of time, he also became politically active and won fame as a tireless advocate for the preservation of wilderness (FIGURE 1.11).

Muir was motivated by the rapid deforestation he wit- nessed throughout North America and by his belief that the natural world should be treated with the same respect we give to cathedrals. He promoted the preservation ethic, which holds that we should protect our environment in a pristine, unaltered state. Muir argued that nature deserved protection for its own inherent value (an ecocentric argument), but he also maintained that nature promoted human happiness (an anthropocentric

FIGURE 1.11  A pioneering advocate of the preservation ethic, John Muir is also remembered for his efforts to protect the Sierra Nevada from development and for his role in founding the Sierra Club, a leading environmental organization. Here Muir (right) is shown with President Theodore Roosevelt in Yosemite National Park. After his 1903 wilderness camping trip with Muir, the presi- dent instructed his interior secretary to increase protected areas in the Sierra Nevada.

FIGURE 1.12  Gifford Pinchot, the first chief of what would be- come the U.S. Forest Service, was a leading proponent of the con- servation ethic. The conservation ethic holds that people should use natural resources but should strive to ensure the greatest good for the greatest number of people for the longest time.

Preservation and Conservation With which ethic do you identify more—preservation or conservation? Think of a forest or wetland or other important natural resource in your region. Give an

example of a situation in which you might adopt a preserva- tion ethic and an example of one in which you might adopt a conservation ethic. Are there conditions under which you’d follow neither, but instead adopt a “development ethic”?

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the same community and that we are obligated to treat the land in an ethical manner. In his 1949 essay “The Land Ethic,” he wrote:

All ethics so far evolved rest upon a single premise: that the individual is a member of a community of interdependent parts. . . . The land ethic simply enlarges the boundaries of the community to include soils, waters, plants, and ani- mals, or collectively: the land. . . . A land ethic changes the role of Homo sapiens from conqueror of the land-commu- nity to plain member and citizen of it. . . . It implies respect for his fellow-members, and also respect for the commu- nity as such.

Leopold intended that the land ethic would help guide deci- sion making. “A thing is right,” he wrote, “when it tends to preserve the integrity, stability, and beauty of the biotic com- munity. It is wrong when it tends otherwise.” Leopold died before seeing “The Land Ethic” and his best-known book, A Sand County Almanac, in print, but today many view him as the most eloquent and important philosopher of environmen- tal ethics.

Environmental justice seeks fair treatment for all people Our society’s domain of ethical concern has been expanding from rich to poor and from majority races and ethnic groups to minority ones. This ethical expansion involves applying a standard of fairness and equality, and it has given rise to the environmental justice movement. Environmental justice involves the fair and equitable treatment of all people with respect to environmental policy and practice, regardless of their income, race, or ethnicity.

The environmental justice movement has been fueled by the perception that poor people tend to be exposed to a greater share of pollution, hazards, and environmental degra- dation than are richer people. Environental justice advocates also note that racial and ethnic minorities tend to suffer more exposure to hazards than whites. Indeed, studies across North America repeatedly document that poor and nonwhite com- munities each bear heavier burdens of air pollution, lead poi- soning, pesticide exposure, toxic waste exposure, and work- place hazards. This is thought to occur because lower-income and minority communities often have less access to informa- tion on environmental health risks, less political power with which to protect their interests, and less money to spend on avoiding or alleviating risks. Environmental justice propo- nents also sometimes blame institutionalized racism and in- adequate government policies.

A protest in the early 1980s by African Americans in Warren County, North Carolina, against a toxic waste dump in their community is widely seen as the beginning of the movement (FIGURE 1.14). The state had chosen to es- tablish the dump in the county with the highest percentage of African Americans.

Likewise, white residents of the Appalachian region have long been the focus of environmental justice concerns. Mountaintop coal mining practices (p. 240) in this eco- nomically neglected region provide some jobs to local resi- dents, but also pollute water, bury streams, degrade forests,

Aldo Leopold’s land ethic inspires many people As a young forester and wildlife manager, Aldo Leopold (1887–1949; FIGURE 1.13) began his career in the conserva- tionist camp, having graduated from Yale Forestry School, which Pinchot had helped found just as Roosevelt and Pin- chot were advancing conservation on the national stage. As a forest manager in Arizona and New Mexico, Leopold em- braced the government policy of shooting predators, such as wolves, to increase populations of deer and other game animals.

At the same time, Leopold followed the advance of eco- logical science. He eventually ceased to view certain species as “good” or “bad” and instead came to see that healthy ecologi- cal systems depend on protecting all their interacting parts. Drawing an analogy to mechanical maintenance, he wrote, “to keep every cog and wheel is the first precaution of intelligent tinkering.”

It was more than science that pulled Leopold from an anthropocentric perspective toward a more holistic one. One day he shot a wolf, and when he reached the animal, Leopold was transfixed by “a fierce green fire dying in her eyes.” The experience remained with him for the rest of his life and helped lead him to an ecocentric ethical outlook. Years later, as a University of Wisconsin professor, Leopold argued that people should view themselves and “the land” as members of

FIGURE 1.13 ▲ Aldo Leopold, a wildlife manager and pioneering environmental philosopher, articulated a new relationship be- tween people and the environment. In his essay “The Land Ethic,” he called on people to include the environment in their ethical framework.

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Just as wealthy people often impose their pollution on poorer people, wealthy nations do the same to poorer nations. For instance, the millions of tons of hazardous waste that we in developed nations produce in our factories, power plants, and incinerators must go somewhere (p. 394). Proper disposal is ex- pensive, so companies often find it cheaper to pay cash-strapped nations to take the waste—and cheaper still to dump it illegally. In developing nations with lax environmental and health regu- lations, workers and residents are often uninformed of or un- protected against the dangers from this waste. An international treaty, the Basel Convention, prohibits the international export of waste, but trade and illegal dumping continue. Although 169 nations have ratified the treaty, the United States, the world’s largest exporter, has not. Environmental justice at all levels is a key component in pursuing the environmental, economic, and social goals of the modern drive for sustainability.

SUSTAINABILITY AND THE FUTURE OF OUR WORLD Recall the ethical question posed earlier (p. 12): Is the present generation obligated to conserve resources for future genera- tions? This question cuts to the core of sustainability, a guid- ing principle of modern environmental science and a concept you will encounter throughout this book.

Sustainability means living within our planet’s means, such that Earth and its resources can sustain us—and all life—for the foreseeable future. Sustainability means leaving our children and grandchildren a world as rich and full as the world we live in now. It means conserving Earth’s resources so that our descendants may enjoy them as we have. It means de- veloping solutions that work in the long term. Sustainability requires maintaining fully functioning ecological systems, be- cause we cannot sustain human civilization without sustain- ing the natural systems that nourish it.

and cause flooding. Low-income residents of affected Ap- palachian communities have historically had little political power to voice complaints over the impacts of these mining practices.

Today, although our economies have grown, the gaps be- tween rich and poor have widened. And despite much pro- gress toward racial equality in Western societies, significant inequities remain. Although environmental laws have prolif- erated, minorities and the poor still suffer substandard envi- ronmental conditions (FIGURE 1.15). Yet today more people are fighting environmental hazards in their communities and winning.

One success story is in California’s San Joaquin Valley. The poor, mostly Latino, farm workers in this region who help harvest much of the U.S. food supply also suffer some of the nation’s worst air pollution. Industrial agriculture produces pesticide emissions, dairy feedlot emissions, and windblown dust from eroding farmland, yet this pollution was not being regulated. Valley residents enlisted the help of several organi- zations, including the Center on Race, Poverty, and the Environment, a San Francisco–based environmental justice law firm, and succeeded in convincing California regulators to enforce Clean Air Act provisions and California legislators to pass new laws regulating agricultural emissions.

Environmental Justice Consider the place where you grew up. Where were the factories, waste dumps, and polluting facilities located, and who lived closest to them? Who lives nearest

them in the town or city that hosts your campus? Do you think the concerns of environmental justice advocates are justified? If so, what could be done to ensure that poor communities are no more polluted than wealthy ones?

FIGURE 1.14  Communities of poor people and people of color have suffered more than their share of environmental hazards, a situation that has given rise to the environmental justice move- ment. The movement gained prominence with this protest against a toxic waste dump in Warren County, North Carolina.

FIGURE 1.15  Hurricane Katrina revealed our ongoing need for environmental justice because the people affected most by the storm and its aftermath were poor and nonwhite. These girls are playing in the Lower Ninth Ward of New Orleans, where many homes were destroyed and water remained unsafe to drink long afterwards. Their mother had moved back here after Katrina destroyed her home, but poverty forced her to accept donated gutted housing once the Federal Emergency Management Agency cut off payments.

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The most comprehensive assessment of the condition of the world’s ecological systems and their capacity to continue supporting us was completed in 2005, when over 2,000 leading environmental scientists from nearly 100 nations completed the Millennium Ecosystem Assessment. The Millennium Ecosystem Assessment makes clear that our degradation of environmental systems is having negative impacts on all of us, but that with care and diligence we can still turn many of these trends around.

Sustainable solutions abound Humanity’s challenge is to develop solutions that enhance our quality of life while protecting and restoring the envi- ronment that supports us. How we tackle this challenge will largely determine the nature of our lives in the 21st century and beyond. Fortunately, many workable solutions are at hand. For instance:

▶ Renewable energy sources (Chapter 16) are being de- veloped to replace fossil fuels. In extracting fossil fu- els, we have been splurging on a one-time, short-lived

We can think of our planet’s resources as a bank account. If we deplete resources, we draw down the bank account. How- ever, we can choose instead to use the interest and leave the principal intact so that we can continue using the interest far into the future. Currently we are drawing down Earth’s natu- ral capital, its accumulated wealth of resources. Recall (p. 4) that one research group estimates that we are withdrawing our planet’s natural capital 50% faster than it is being replenished. To live off nature’s interest—its replenishable resources—is sus- tainable. To draw down resources faster than they are replaced is to eat into nature’s capital, and we cannot get away with this for long.

Population and consumption drive environmental impact Humanity is placing an ever-greater burden on Earth’s sys- tems. We add over 200,000 people to the planet each day, and the ongoing growth of human population amplifies nearly all of our environmental impacts (Chapter 6). Our consumption of resources has risen even faster than our population. The modern rise in affluence has been a positive development for humanity, and our conversion of the planet’s natural capital has made life more pleasant for us so far. However, like ris- ing population, rising per capita consumption magnifies the demands we make on our environment.

Moreover, the world’s citizens have not benefited equally from the overall rise in affluence. Today the 20 wealthiest na- tions boast over 55 times the per capita income of the 20 poor- est nations—nearly three times the gap that existed just four decades ago. The ecological footprint of the average citizen of a developed nation such as the United States is considerably larger than that of the average resident of a developing coun- try (FIGURE 1.16).

United States

(8.0 ha) Afghanistan (0.6 ha)

Haiti (0.7 ha)

India (0.9 ha)

Mozambique (0.8 ha)

Canada (7.0 ha)

Israel (4.8 ha)

World average (2.7 ha)

China (2.2 ha)

Indonesia (1.2 ha)

Mexico (3.0 ha)

Brazil (2.9 ha)

France (5.0 ha)

FIGURE 1.16 ▲ The citizens of some nations have much larger eco- logical footprints than the citizens of others. Shown are ecological footprints for average citizens of several developed and develop- ing nations, along with the world’s average per capita footprint of 2.7 hectares. One hectare (ha) = 2.47 acres. Data are for 2007, from Global Footprint Network, 2010.

Ecological Footprints What do you think accounts for the variation in sizes of per capita ecological footprints among societies? Do you think that nations with larger footprints have an

ethical obligation to reduce their environmental impact, so as to leave more resources available for nations with smaller footprints? Why or why not?

Our growing population and consumption are intensify- ing the many environmental impacts we examine in this book, including erosion and other impacts from agriculture (Chap- ter 7), deforestation (Chapter 9), toxic substances (Chapter 10), mineral extraction and mining impacts (Chapter 11), fresh water depletion (Chapter 12), fisheries declines (Chap- ter 12), air and water pollution (Chapters 12 and 13), waste generation (Chapter 17), and, of course, global climate change (Chapter 14). These impacts degrade our health and quality of life, and they alter the ecosystems and landscapes in which we all live (FIGURE 1.17). They are also driving the loss of Earth’s biodiversity (Chapter 8)—perhaps our greatest problem, be- cause extinction is irreversible; once a species becomes ex- tinct, it is lost forever.

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bonanza, and researchers estimate that we have de- pleted nearly half the world’s oil supplies (Chapter 15). Today scientists, engineers, and policymakers are work- ing to develop alternative energy options and to increase energy efficiency.

▶ In response to agricultural impacts, scientists and others have developed and promoted soil conservation, high- efficiency irrigation, and organic agriculture (Chapter 7).

▶ Legislation and technological advances have reduced the pollution emitted by industry and automobiles in wealthier countries (Chapters 5 and 13).

▶ Conservation biologists are helping to protect habitat, slow extinction, and safeguard endangered species (Chapter 8).

▶ Recycling is helping to conserve resources and relieve waste disposal problems (Chapter 17).

▶ Governments, businesses, and individuals are taking steps to reduce emissions of the greenhouse gases that drive cli- mate change (Chapter 14).

Sustainable development involves environmental protection, economic well-being, and social justice Today’s search for sustainable solutions centers on sustaina- ble development, the use of resources in a manner that sat- isfies our current needs but does not compromise the future availability of resources. Sustainable development aims to enhance people’s quality of life while preserving environ- mental quality. The modern drive for sustainable develop- ment arose from the recognition that society’s poorer people often suffer the most from environmental degradation. This realization led advocates of environmental protection, ad- vocates of economic development, and advocates of social justice to begin working together toward common goals. Increasingly, sustainable development efforts by govern- ments, businesses, industries, organizations, and individu- als everywhere—from students on campus (pp. 414–415) to international representatives at the United Nations (FIGURE 1.18)—are generating sustainable solutions that satisfy a

Human Influence Index

High

Low

FIGURE 1.17  Human activity has heavily influenced much of the United States, especially in cities, on farms, and across the eastern portion of the nation. This map summarizes influence on terrestrial ecosystems by human settlement, roads and transportation networks, nighttime light pollution, and agriculture and other land use. It demon- strates that we live in a highly modified environment and suggests we would be wise to carefully nurture natural systems and manage remaining resources. Used by permission of the Center for International

Earth Science Information Network (CIESIN),

The Earth Institute, Columbia University.

FIGURE 1.18  Former South African President Thabo Mbeki hugs a boy who performed in the welcoming ceremony of the United Nations–sponsored World Summit on Sustainable Development. Held in Johannesburg, South Africa, in 2002, the summit hosted 10,000 delegates from 200 nations who set sustainable develop- ment goals. Sustainability requires that each generation leave enough resources for future generations to live as well or better.

triple bottom line by meeting environmental, economic, and social goals simultaneously.

Sustainability and the triple bottom line require that we limit our environmental impact while promoting economic well-being and social equity. These aims oblige us to make an ethical commitment to our fellow citizens and to future gen- erations. They also require that we apply knowledge from the sciences to help devise ways to limit our impact and maintain the environmental systems on which we depend.

The question “How can we develop in a sustainable way?” may be the single most important question we face. Environmental science holds the key to addressing this ques- tion. Because so much remains to be studied and done, and because it is so central to our modern world, environmental science will remain an exciting frontier for you to explore as a student today and as an informed citizen throughout your life.

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give us reason to hope—and identifying a problem is the first step toward devising a solution. Solving environmental problems can move us toward health, longevity, peace, and prosperity. Science in general, and environmental science in particular, can aid us in our efforts to develop balanced, workable, sustainable solutions and to create a better world for ourselves and our children.

➤ CONCLUSION Finding effective ways of living peacefully, healthfully, and sustainably on our diverse and complex planet requires a sol- id ethical grounding and a thorough scientific understanding of natural and social systems. Environmental science helps us understand our intricate relationship with our environment and informs our attempts to solve and prevent environmental problems. Many of today’s trends may worry us, but others

6. What does the study of ethics encompass? Differentiate two classic ethical standards, the categorical impera- tive and the principle of utility. What is environmental ethics?

7. Compare and contrast anthropocentrism, biocentrism, and ecocentrism.

8. Differentiate the preservation ethic from the conser- vation ethic. Explain the contributions of John Muir and Gifford Pinchot in the history of environmental ethics.

9. Describe Aldo Leopold’s land ethic. How did Leopold define the “community” to which ethical standards should be applied?

10. What is sustainable development, and why is it impor- tant? What is meant by the triple bottom line?

T E S T I N G Y O U R C O M P R E H E N S I O N

1. How and why did the agricultural revolution affect hu- man population size? How and why did the industrial revolution affect human population size? Explain some social consequences and some environmental impacts that have resulted.

2. What is the tragedy of the commons? Explain how the concept might apply to an unregulated industry that is a source of water pollution.

3. What is environmental science? Name several disciplines involved in environmental science.

4. Contrast the two meanings of science. Name three ap- plications of science.

5. Describe the scientific method. What is its typical se- quence of steps? What needs to occur before a research- er’s results are published? Why is this process important?

4. Describe your ethical perspective, or worldview, as it pertains to your relationship with the environment. How do you think your culture has influenced your world- view? How do you think your personal experience has influenced it? Do you feel that you fit into any particular category discussed in this chapter? Why or why not?

5. THINK IT THROUGH You have become head of a major funding agency that grants money to researchers pursuing work in environmental science. You must give your staff several priorities to determine what types of scientific research to fund. What environmental prob- lems would you most like to see addressed with re- search? Describe the research you think would need to be completed so that workable solutions to these prob- lems could be developed. What else besides scientific information would be needed to develop sustainable solutions?

S E E K I N G S O L U T I O N S

1. Resources such as soils, timber, fresh water, food crops, and biodiversity are renewable if we use them in mod- eration but can become nonrenewable if we overexploit them (see Figure 1.1, p. 3). For each of these five resourc- es, describe one way in which we sometimes overexploit them, and one thing we could do to conserve them. In supplying your answers, feel free to look ahead and pe- ruse coverage of these issues throughout this book.

2. Why do you think the Easter Islanders did not or could not stop themselves from stripping their island of its trees? What similarities do you perceive between Easter Island and the modern history of our society? What dif- ferences do you see between their predicament and ours?

3. What environmental problem do you feel most acutely yourself? Do you think there are people in the world who do not view your issue as a problem? Who might they be, and why might they take a different view?

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Nation Ecological footprint (hectares per person)

Proportion relative to world average footprint

Proportion relative to world area available

Bangladesh 0.6 0.2 (0.6 + 2.7) 0.3 (0.6 + 1.8) Tanzania 1.2 Colombia 1.9 Thailand 2.4 Mexico 3.0 Sweden 5.9 United States 8.0 World average 2.7 1.0 (2.7 + 2.7) 1.5 (2.7 + 1.8) Your personal footprint (see question 4)

Data from Living planet report 2010. WWF International, Zoological Society of London, and Global Footprint Network.

1. Why do you think the ecological footprint for people in Bangladesh is so small?

2. Why is it so large for people in the United States? 3. Based on the data in the table, how do you think average

per capita income affects ecological footprints? 4. Go to an online footprint calculator such as the

one at http://www.myfootprint.org or http://www. footprintnetwork.org/en/index.php/GFN/page/personal_

footprint, and take the test to determine your own personal ecological footprint. Enter the value you ob- tain in the table, and calculate the other values as you did for each nation. How does your footprint compare to those of the average person in the United States? How does it compare to that of people from other na- tions? Name three actions you could take to reduce your footprint.

Go to www.masteringenvironmentalscience.com for homework assignments, practice quizzes, Pearson eText, and more.

in the world, yet we use on average 2.7 ha (6.7 acres) per person, creating a global ecological deficit, or overshoot (p. 4), of 50%.

Compare the ecological footprints of each nation listed in the table. Calculate their proportional relationships to the world population’s average ecological footprint and to the area available globally to meet our ecological demands.

C A L C U L A T I N G E C O L O G I C A L F O O T P R I N T S

Mathis Wackernagel and his colleagues at the Global Foot- print Network have continued to refine the method of cal- culating ecological footprints—the amount of biologically productive land and water required to produce the energy and natural resources we consume and to absorb the wastes we generate. According to their most recent data, there are 1.8 hectares (4.4 acres) available for each person

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2 Environmental Systems: Matter, Energy, and Ecosystems Upon completing this chapter, you will be able to:

➤ Describe the nature of environmental systems ➤ Explain and apply the fundamentals of environmental chemistry ➤ Describe the molecular building blocks of organisms ➤ Differentiate among the types of energy and explain the basics of energy flow ➤ Distinguish photosynthesis from respiration and summarize their importance to living things ➤ Define ecosystem and evaluate how living and nonliving entities interact in ecosystem-level ecology ➤ Outline the fundamentals of landscape ecology and ecological modeling ➤ Assess ecosystem services and how they benefit our lives ➤ Describe how water, carbon, phosphorus, and nitrogen cycle through the environment

An oysterman unloads his catch on the shores of the Chesapeake Bay

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CENTRAL CASE STUDY

The Vanishing Oysters of the Chesapeake Bay “I’m 60. Danny’s 58. We’re the young ones.” —Grant Corbin, Oysterman in Deal Island, Maryland

“The Bay continues to be in serious trouble. And it’s really no question why this is occurring. We simply haven’t managed the Chesapeake Bay as a system the way science tells us we must.”

—Will Baker, President, Chesapeake Bay Foundation

Avisit to Deal Island, Maryland, on the Chesapeake Bay reveals a situation that unfortunately is all too common in modern America. The island, which was once bustling with productive industries and growing populations, is suffering. Eco- nomic opportunities in the community are few, and its populace is increasingly “graying” as

more and more young people leave to find work elsewhere. In 1930, Deal Island had a popu-

lation of 1,237 residents. In 2000 it held a mere 578—and only one household in five

included children.

Unlike other parts of the country with similar stories of economic decline, the demise of Deal Island and other bayside towns was not caused by the closing of a local factory, steel mill, or corporate headquarters. It was caused by people deci- mating the Chesapeake Bay oyster fishery.

The Chesapeake Bay was once a thriving system of interacting plants, animals, and microbes. Healthy populations of blue crabs, scallops, and fish such as giant sturgeon, striped bass, and shad inhabited the bay. Nutrients carried to the bay by thousands of streams in the bay’s roughly 168,000-km2 (64,000- mi2) watershed (the land area that funnels water to the bay through rivers) nourished fields of underwater grasses that provided food and refuge to juvenile fish, shellfish, and crabs. Hundreds of millions of oysters kept the bay’s water clear by filtering nutrients and phytoplankton (microscopic photosynthetic algae, protists, and cyanobacteria that drift near the surface) from the water column.

Although oysters had been eaten locally for some time, the intensive harvest of bay oysters for export began in the 1830s, and by the 1880s the bay boasted the world’s largest oyster fishery.

People flocked to the Ches- apeake to work on oystering ships or in canneries, shuck- ing houses (where oysters are separated from their shells), dockyards, and ship- yards. Bayside towns pros- pered along with the oys- ter industry and developed a unique maritime culture that defined the region.

But by 2010 the bay’s oysters had been reduced to a mere 1% of their historical abundance, and the oyster industry was all but ruined. Perpetual overhar- vesting, habitat destruction, virulent oyster diseases, and water pollution had nearly eradicated this eco- nomically and ecologically important species from bay waters. The monetary losses associated with this fishery collapse have been staggering, costing the economies of Maryland and Virginia an estimat- ed $4 billion in lost economic activity from 1980 to 2010 alone.

One of the biggest impacts in recent decades on oysters is the pollution of the bay with high lev- els of the nutrients nitrogen and phosphorus from agricultural fertilizers, animal manure, stormwater runoff, and atmospheric compounds produced by fossil fuel combustion. Elevated levels of these nu- trients cause the number of phytoplankton in the

UNITED STATES

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MEXICOPacific Ocean

Atlantic Ocean

Washington, D.C.

Chesapeake Bay

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bay to explode as they are no longer held in check by extensive oyster filtration of bay waters. When phytoplankton die, settle to the bay bottom, and are decomposed by bacteria, this depletes oxygen in the water (a condition called hypoxia) and creates “dead zones” in the bay. Grasses, oysters, and other immo- bile organisms perish in dead zones when deprived of oxygen. Crabs, fish, and other mobile organisms are forced to flee to habitats where oxygen levels are higher, but they face smaller food supplies and increased predation pressure. Collectively, hypoxia affects numerous components of the Chesapeake Bay system and contributes to the bay’s listing on the Environmental Protection Agency’s list of highly pol- luted waters.

Recent events have, at long last, given rea- son for hope for the recovery of the Chesapeake Bay system. The EPA agreed in 2010, for the first time in the region, to hold bay states to strict pol- lutant “budgets” whose aim is to reduce inputs of nitrogen and phosphorus into Chesapeake Bay by one-third of current levels by 2025. Further, oyster restoration efforts are finally showing promise (see THE SCIENCE BEHIND THE STORY, pp. 34–35), and aqua- culture shows potential for reinvigorating the oyster industry in the Chesapeake. If these efforts prove successful and we can begin to restore the bay to health, Deal Island and other communities may once again enjoy prosperity on the scenic shores of the Chesapeake. �

holistically, helping us to develop comprehensive solu- tions to complicated problems such as those faced in the Chesapeake Bay.

Systems involve feedback loops A system is a network of relationships among components that interact with and influence one another through the exchange of energy, matter, or information. Systems receive inputs of energy, matter, or information; process these in- puts; and produce outputs. For example, the Chesapeake Bay receives inputs of fresh water, sediments, nutrients, and pol- lutants from the rivers that empty into it. Oystermen, fisher- men, and crabbers harvest some of the Bay system’s output: matter and energy in the form of seafood. This output sub- sequently becomes input to the human economic system and to the digestive systems of the many people who consume the seafood.

Sometimes a system’s output can serve as input to that same system, a circular process known as a feedback loop. Feedback loops are of two types, negative and positive. In a negative feedback loop (FIGURE 2.1A) output that results from a system moving in one direction acts as input that moves the system in the other direction. Input and output essentially neutralize one another’s effects, stabilizing the system. A thermostat, for instance, stabilizes a room’s tem- perature by turning the furnace on when the room gets cold and shutting it off when the room gets hot. Similarly, nega- tive feedback regulates our body temperature. If we get too hot, our sweat glands pump out moisture that evaporates to cool us down, or we may move from sun to shade. If we get too cold, we shiver, creating heat, or we move into the sun or put on more clothing. Most systems in nature involve nega- tive feedback loops. Negative feedback loops enhance stabil- ity, and in the long run, only those systems that are stable will persist.

Positive feedback loops have the opposite effect. Rather than stabilizing a system, they drive it further toward an extreme. One positive feedback cycle of great concern to environmental scientists today involves the melting of gla- ciers and sea ice in the Arctic due to global warming (pp. 308–309). Ice and snow, being white, reflect sunlight and keep surfaces cool. But if the climate warms enough to melt the ice and snow, darker surfaces of land and water are ex- posed, and these darker surfaces absorb more sunlight. This absorption of light warms the surface, causing further melt- ing, which in turn exposes more dark surface area, leading to further warming (FIGURE 2.1B). Runaway cycles of positive feedback are rare in nature, but they are common in natural systems altered by human impact, and they can destabilize those systems.

Environmental systems interact Natural systems can be categorized in many differ- ent ways. For instance, scientists divide Earth’s major components into structural spheres. The lithosphere (p. 227) contains the rock and sediment beneath our feet, in the planet’s uppermost layers. The atmosphere (p. 279) is composed of the air surrounding our planet.

EARTH’S ENVIRONMENTAL SYSTEMS Understanding the rise and fall of the oyster industry in the Chesapeake Bay, like many other human impacts on the en- vironment, involves comprehending environmental systems and how they function. Our planet’s environment consists of complex networks of interlinked systems. These systems include processes that shape the land, air, water, and cli- mate; ecological webs of relationships among species; and the interaction of living organisms with the nonliving enti- ties around them. Earth’s systems also include the cycles of key chemical elements and compounds that support life and regulate climate. We depend on these systems for our very survival.

Taking a “systems approach” is helpful in environmen- tal science because so many issues are multifaceted and interconnected. This type of approach poses challenges, however, because systems often show behavior that is difficult to understand and predict. Even so, environmen- tal scientists are rising to the challenge of studying systems

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The hydrosphere (p. 250 ) encompasses all water—salt or fresh, liquid, ice, or vapor—in surface bodies, under- ground, and in the atmosphere. The biosphere (p. 52 ) consists of all the planet’s living organisms and the abi- otic (nonliving) portions of the environment with which they interact. Categorizing environmental systems in this manner can help make Earth’s dazzling complexity comprehensible, but it’s important to remember that most natural systems overlap or interact.

The Chesapeake Bay and the rivers that empty into it are an example of interacting systems. On a map, these riv- ers are a branched and braided network of water channels surrounded by farms, cities, and forests ( FIGURE 2.2 ). But where are this system’s boundaries? For a scientist inter- ested in runoff and the flow of water, sediment, or pollut- ants, it may make the most sense to view the Chesapeake Bay’s watershed as a system. However, for a scientist in- terested in the Bay’s dead zones, it may be best to view the watershed together with the Chesapeake Bay as the system of interest, because their interaction is central to the prob- lem. In environmental science, identifying the boundaries of systems depends on the questions being addressed.

The dead zones in the Chesapeake Bay are due to the extremely high levels of nitrogen and phosphorus delivered to its waters from the six states in its watershed and the 15 states in its airshed (the geographic area that produces air pollutants that are likely to end up in a waterway). In

(a) Negative feedback

Too hot

Brain (control center)

Too cold

Seek shade Sweat

Wear more clothes Shiver

Body cools

Body warms

FIGURE 2.1  Negative feedback loops (a) exert a stabilizing influence on systems and are common in nature. The human body’s response to heat and cold involves a nega- tive feedback loop. Positive feedback loops (b) have a destabilizing effect on systems and push them toward extremes. As Arctic glaciers and sea ice melt because of global warming, darker surfaces are exposed, which absorb more sunlight, causing fur- ther warming and further melting.

Light absorption speeds warming, exposing more dark surfaces

In cool climate, sunlight reflects off white surfaces

Solid surface of sea ice

Glacier completely covers land

Sea ice melting

Glacier melting

More water exposed

More land exposed

(b) Positive feedback

As climate warms, sunlight is absorbed where dark surfaces are exposed

1 32

Q: But isn’t positive feedback “good” and negative feedback “bad”? A: Understanding negative and positive feedback in systems can be difficult, because it goes against the the way we use those terms in everyday language. In daily life, positive feedback (such as a complimentary comment on a paper written for a course in school) is something that makes us feel good, whereas negative feedback (such as criticism on a paper) may make us feel bad. In essence, we have been trained to view positive feedback as a stabilizing force (“Keep up the good work, and you’ll get a good grade”) and negative feedback as a destabilizing force (“You need to change your approach if you’re going to succeed”).

In environmental systems, it’s the opposite! Negative feedback works against change in systems, and in doing so it enhances stability, typically keeping conditions within ranges beneficial to organisms. Positive feedback exerts destabilizing effects that push conditions in systems to extremes, threatening organisms adapted to the system’s normal conditions. Thus, negative feedback in environmental systems keeps conditions stable for living things whereas positive feedback can harm them.

FAQ

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2007, the Bay received an estimated 127 million kg (281 million lb) of nitrogen and 8.3 million kg (18.2 million lb) of phosphorus, with roughly one-third of nitrogen in- puts from atmospheric sources. Agriculture was a major source of these nutrients, contributing 38% of the nitrogen

Farms

Cities

Chesapeake Bay watershed

Industry

Forests

Washington DC

Baltimore

Richmond

Norfork

Philadelphia Harrisburg

Pittsburgh

Lake Erie

Binghamton

Richmond

Norfolk

Dissolved oxygen (mg/L)

J a m e s R i v e r

P o t o m

a c R i v e r

New York City

Boundaries of watershed

S u

s q u

e h

a n n a

R i v e

FIGURE 2.2  The Chesapeake Bay watershed encompasses 168,000 km2 (64,000 mi2) of land area in six states and the District of Columbia. Tens of thousands of streams carry water, sediment, and pollutants from a variety of sources downriver to the Chesapeake, where nutrient pollution has given rise to large areas of hypoxic waters. The zoomed-in map (at right) shows dissolved oxygen concentrations in the Chesapeake Bay in summer 2007. Oysters, crabs, and fish typically require a minimum of 3 mg/L of oxygen and are therefore excluded from large portions of the bay where oxygen levels are too low. Figure at right adapted from Chesapeake Bay Record Dead Zone Map, Chesapeake Bay Foundation.

(FIGURE 2.3A) and 45% of the phosphorus (FIGURE 2.3B) entering the bay.

Elevated nitrogen and phosphorus inputs cause phytoplankton—microscopic algae and other organisms drifting near the surface—to flourish. High densities lead

Agriculture—fertilizer and manure

(32%)

Agriculture—fertilizer and manure

(45%)

Municipal and industrial wastewater

(19%)

Municipal and industrial wastewater

(21%)

Atmospheric deposition—

mobile, utilities and industries

(24%)

Atmospheric deposition— agriculture

(8%) Natural sources (3%)

Urban/suburban fertilizer runoff

(10%)

Urban/suburban fertilizer runoff and

transported sediments (31%)

(a) Sources of nitrogen entering the Chesapeake Bay (b) Sources of phosphorus entering the Chesapeake Bay

Septic systems

(4%)

Atmospheric deposition—natural

(1%)

FIGURE 2.3  The Chesapeake Bay receives inputs of nitrogen (a) and phosphorus (b) from many sources in its watershed. Data from Chesapeake Bay Program Watershed Model Phase 4.3 (Chesapeake Bay Program Office, 2009).

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Freshwater river Nitrogen and phosphorus input

Warmer, less dense, fresh- water layer (oxygenated)

Colder, denser ocean water layer (hypoxic)

1 2

Dead phytoplankton and their waste drift to the bottom, providing more food for bacteria to decompose

Microbial decomposer population grows and consumes more oxygen

Insufficient oxygen suffocates oysters and grasses, fish and shrimp at the bottom; dead zone (hypoxic zone) forms

Phytoplankton flourish at the surface

3 4 5

FIGURE 2.4  Excess nitrogen and phosphorus causes eutrophication in aquatic systems such as the Chesa- peake Bay. Coupled with stratification (layering) of water, eutrophication can severely deplete dissolved oxygen. Nutrients from river water ➊ boost growth of phytoplankton ➋, which die and are decomposed at the bottom by bacteria ➌. Stability of the surface layer prevents deeper water from absorbing oxygen to replace oxygen con- sumed by decomposers ➍, and the oxygen depletion suffocates or drives away bottom-dwelling marine life ➎. This process gives rise to hypoxic zones like those in the bay.

Louisiana coast near the mouth of the Mississippi River (p. 267). Fisheries in these regions have seen reduced catches of seafood, and decreased economic activity, because of human-induced dead zones. The increase in the number of dead zones—there were 162 documented in the 1980s and 49 in the 1960s—re- flects how the activities of people are changing the chemistry of waters around the world. Let’s look more closely at how chem- istry is involved in important issues in environmental science.

CHEMISTRY AND THE ENVIRONMENT Chemistry plays a central role in the environmental challeng- es facing the Chesapeake Bay. Understanding how too much nitrogen or phosphorus in one part of a system can lead to too little oxygen in another requires a good working knowledge of chemistry.

Indeed, examine any environmental issue, and you will likely discover chemistry playing a key role. Chemistry is crucial to understanding how environmental chemicals af- fect the health of humans and wildlife, how air pollutants cause acid precipitation, and how synthetic chemicals thin the ozone layer. To appreciate the importance of chemistry in environmental science, we must begin with a grasp of the fundamentals.

to elevated mortality in phytoplankton populations, and dead phytoplankton settle to the bottom. The remains of dead phytoplankton are joined on the bottom by the waste products of zooplankton, tiny creatures that feed on phy- toplankton. This abundance of organic material causes an explosion in populations of bacterial decomposers, which deplete the oxygen in bottom waters while consuming this material. Deprived of oxygen, organisms will flee if they can or will suffocate if they cannot. Oxygen replenishes slowly at the bottom because fresh water entering the bay from rivers remains naturally stratified in a layer at the surface and mixes slowly with the denser, saltier bay water. This limits the amount of oxygenated surface water that reaches the bottom-dwelling life that needs it. This process of nutri- ent overenrichment, blooms of algae, increased production of organic matter, and subsequent ecosystem degradation is known as eutrophication (FIGURE 2.4). Eutrophication tends to be driven by increases in nitrogen in marine en- vironments and by increases in phosphorus in freshwater environments, though both contribute to eutrophication in all waters.

Increased nutrient pollution from farms, cities, and industries has led to the development of over 400 document- ed hypoxic dead zones globally as of 2008 (FIGURE 2.5), in- cluding that of the Chesapeake Bay as well as a large dead zone that forms each year in the Gulf of Mexico off the

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by organisms, such as carbon, nitrogen, and calcium, are called nutrients. Each element is assigned an abbreviation, or chemical symbol. The periodic table of the elements (see APPENDIX: Periodic Table of Elements) summarizes informa- tion on the elements in a comprehensive way.

Elements are composed of atoms, the smallest compo- nents that maintain the chemical properties of the element. An atom’s protons (positively charged particles) and neutrons (particles lacking electrical charge) are in its nucleus. The atoms of each element have a defined number of protons, called the atomic number. (Carbon, for instance, has six protons; thus, its atomic number is 6.) An atom’s nucleus is surrounded by electrons (negatively charged particles), which balance the positive charge of the protons (FIGURE 2.6).

Although all atoms of a given element contain the same number of protons, they do not necessarily contain the same number of neutrons. Atoms with differing num- bers of neutrons are referred to as isotopes (FIGURE 2.7A). Isotopes are denoted by their elemental symbol preceded by the mass number, or combined number of protons and neutrons in the atom. For example, 2H (deuterium) is an

Atoms and elements are chemical building blocks All material in the universe that has mass and occupies space is termed matter. Matter exists in the universe as a solid, liquid, or gas. Matter may be transformed from one type of substance into others, but it cannot be created or destroyed. This principle is referred to as the law of conservation of matter. In environmental science, this principle helps us understand that the amount of matter stays constant as it is recycled in nutrient cycles and ecosystems (pp. 31, 36–41). It also makes it clear that we cannot simply wish away “undesir- able” matter, such as nuclear waste or toxic pollutants.

The nitrogen, phosphorus, and oxygen that play key roles in the Chesapeake Bay’s predicament are each elements. An element is a fundamental type of matter, a chemical substance with a given set of properties, that cannot be bro- ken down into substances with other properties in chemi- cal reactions. Chemists currently recognize 92 elements oc- curring in nature, as well as more than 20 others that have been artificially created. Elements needed in large quantities

Hypoxic zone

Human footprint (%)

0 - 1 1 - 10 10 - 20 20 - 30 30 - 40 40 - 60

Dead zones are uncommon offshore from land with less human impact

Dead zones are common offshore from land with intensive impact

FIGURE 2.5  Over 400 marine dead zones have been recorded across the world. These dead zones (shown by dots in the map) occur mostly offshore from areas of land with the greatest human ecological footprints (here, expressed on a scale of 0 to 100, with higher numbers indicating bigger human footprints). Data from Diaz, R., and R. Rosenberg, 2008. Spreading dead zones and consequences for marine ecosystems. Science 321: 926–929. Reprinted with permission from AAAS.

Carbon (C) Atomic number = 6

Protons Neutrons Electrons

= 6 = 6 = 6

Nitrogen (N) Atomic number = 7

Protons Neutrons Electrons

= 7 = 7 = 7

Phosphorus (P) Atomic number = 15

Protons Neutrons Electrons

= 15 = 15 = 15

Proton

Neutron

Electron

Nucleus–

– –

–

– –

–

– –

–

– –

FIGURE 2.6  In an atom, protons and neutrons remain in the nucleus, and electrons move around the nucleus. Each chemical element has its own particular number of protons. Carbon possesses 6 protons, nitrogen 7, and phosphorus 15. These schematic diagrams are meant to clearly show and compare numbers of electrons for these three elements. In reality, however, electrons do not orbit the nucleus in rings as shown; they move through space in more complex ways.

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atoms in the molecule. A molecule composed of atoms of two or more different elements is called a compound. Wa- ter is a compound composed of two hydrogen atoms bonded to one oxygen atom (H2O). Another compound is carbon dioxide, consisting of one carbon atom bonded to two oxygen atoms (CO2).

Ions of differing charge bind with one another to form compounds with ionic bonds. A crystal of table salt, sodium chloride (NaCl), is held together by ionic bonds between the positively charged sodium ions (Na+) and the negatively charged chloride ions (Cl−). Atoms that lack an electrical charge combine by “sharing” electrons in covalent bonds. For example, two atoms of hydrogen share their electrons when they bind together to form hydrogen gas (H2).

Elements, molecules, and compounds can also come together in solutions without chemically bonding. Air in the atmosphere is a solution formed of constituents such as nitrogen, oxygen, water, carbon dioxide, methane (CH4), and ozone (O3). Human blood, ocean water, plant sap, and metal alloys (p. 236) such as brass are all solutions.

Hydrogen ions determine acidity In any aqueous solution, a small number of water molecules split apart, each forming a hydrogen ion (H+) and a hydroxide ion (OH−). The product of hydrogen and hydroxide ion concentra- tions is always 10−14; as one increases, the other decreases. Pure water contains equal numbers of these ions, each at a concentra- tion of 10−7, and we say that this water is neutral. Solutions in which the H+ concentration is greater than the OH− concentra- tion are acidic, whereas solutions in which the OH− concentration is greater than the H+ concentration are basic.

The pH scale (FIGURE 2.8) was devised to quantify the acidity or basicity of solutions. It runs from 0 to 14. Pure wa-

isotope of hydrogen with one neutron (and one proton) in the nucleus rather than the more common 1H (“normal” hydrogen), which contains zero neutrons and one proton. Because they differ slightly in mass, isotopes of an element differ slightly in their behavior.

Although elements cannot be broken down by chemical reactions, some isotopes are radioactive and “decay,” changing their chemical identity as they shed subatomic particles and emit high-energy radiation. Radioisotopes decay into lighter and lighter radioisotopes, until they become stable isotopes, isotopes that are not radioactive. Each radioisotope decays at a rate determined by that isotope’s half-life, the amount of time it takes for one-half the atoms in a given sample to give off radiation and decay. Different radioisotopes have very differ- ent half-lives, ranging from fractions of a second to billions of years. The radioisotope uranium-235 (235U) is our society’s source of energy for commercial nuclear power (p. 346). It decays into a series of daughter isotopes, eventually forming lead-207 (207Pb), and has a half-life of about 700 million years.

Atoms may also gain or lose electrons to become ions, electrically charged atoms or combinations of atoms (FIGURE 2.7B). Ions are denoted by their elemental sym- bol followed by their ionic charge. For instance, a common ion used by mussels and clams to form shells is Ca2+, a cal- cium atom that has lost two electrons and so has a charge of positive 2.

Atoms bond to form molecules and compounds Because of attractions between their electrons, atoms can bond together and form molecules, combinations of two or more atoms. Common molecules containing only a single element include those of oxygen gas (O2) and nitrogen gas (N2), both of which are abundant in air. As shown in these examples, scientists use a chemical formula (such as O2 and N2) as a shorthand way to indicate the type and number of

Addition of 1 neutron

(a) Hydrogen isotope, 2H Protons = 1 Neutrons = 1 Electrons = 1

Loss of 1 electron

(b) Hydrogen ion, H+

Protons = 1 Electrons = 0

Hydrogen atom, H Protons = 1 Electrons = 1

–

FIGURE 2.7  The mass number of hydrogen is 1, because a typical atom of this element contains 1 proton and 0 neutrons. Deu- terium (hydrogen-2, or 2H), a different isotope of hydrogen (a), con- tains a neutron as well as a proton and thus has greater mass than a typical hydrogen atom; its mass number is 2. Shown in (b) is the hydrogen ion, H+. By losing its electron, it gains a positive charge.

Ammonia

NaOH (sodium hydroxide)

Soft soap

Seawater

Pure water

Normal rainwater

Acid rain

Lemon juice

Car battery acid

Stomach acid

Basic

Acidic

NeutralpH

FIGURE 2.8  The pH scale measures how acidic or basic (alkaline) a solution is. The pH of pure water is 7, the midpoint of the scale. Acidic solutions have higher hydrogen ion concentrations and lower pH, whereas basic solutions have lower hydrogen ion concentrations and higher pH.

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nucleic acids, and carbohydrates. Along with lipids (which are not polymers), these four types of essential molecules are referred to as macromolecules because of their large size.

Proteins are made up of long chains of organic molecules called amino acids. Organisms combine up to 20 types of amino acids into long chains to build proteins. Proteins com- prise the majority of each organism’s matter and serve many functions in living things. Some help produce tissues and pro- vide structural support. For example, animals use proteins to generate skin, hair, muscles, and tendons. Some proteins help store energy, and others transport substances. Some function in the immune system, defending the organism against for- eign attackers. Others act as hormones, molecules that serve as chemical messengers within an organism. Proteins can also serve as enzymes, which are molecules that catalyze, or pro- mote, certain chemical reactions.

Nucleic acids direct the production of proteins. The two nucleic acids—deoxyribonucleic acid (DNA) and ribonucleic acid (RNA)—carry hereditary information for organisms and are responsible for passing traits from parents to offspring. Nucleic acids are composed of series of nucleotides, each of which con- tains a sugar molecule, a phosphate group, and a nitrogenous base. DNA includes four types of nucleotides and can be pictured as a ladder twisted into a spiral, giving the molecule a shape called a double helix (FIGURE 2.10). Regions of DNA coding for particu- lar proteins that perform particular functions are called genes.

Carbohydrates include simple sugars and large mole- cules comprised of chemically bonded simple sugars. Glucose (C6H12O6) fuels living cells and serves as a building block for complex carbohydrates, such as starch. Plants use starch to store energy, and animals eat plants to acquire starch. Plants and animals also use complex carbohydrates to build structure. Insects and crustaceans form hard shells from the

ter has a pH of 7 (neutral), solutions with pH less than 7 are acidic, and solutions with pH greater than 7 are basic. Each step on the scale represents a tenfold difference in hydrogen ion concentration. Thus, a substance with pH of 6 contains 10 times as many hydrogen ions as a substance with pH of 7, and 100 times as many hydrogen ions as a substance with pH of 8. Figure 2.8 shows pH for a number of common sub- stances. Most biological solutions have a pH between 6 and 8, and substances that are very acidic (battery acid) or very basic (sodium hydroxide) are harmful to living things. The acidifi- cation of soils and water from acid rain (pp. 291–294) in the northeastern and midwestern United States is just one exam- ple of how pH changes caused by human activities can affect organisms in the environment.

Matter is composed of organic and inorganic compounds Beyond their need for water, living things also depend on or- ganic compounds. Organic compounds consist of carbon atoms joined by bonds, and they may include other elements, such as hydrogen, nitrogen, oxygen, sulfur, and phosphorus. Carbon’s unusual ability to build elaborate molecules by linking carbon molecules to one another in chains, rings, and other structures has resulted in millions of different organic compounds. Because of the diversity of organic compounds and their importance in living organisms, chemists differentiate organic compounds from inorganic compounds, which lack carbon–carbon bonds.

One class of organic compound that is important in envi- ronmental science is hydrocarbons, which consist solely of at- oms of carbon and hydrogen. Hydrocarbons make up the fossil fuels we combust for so many of our energy needs (Chapter 15). The simplest hydrocarbon is methane (CH4), the key compo- nent of natural gas; it has one carbon atom covalently bonded to four hydrogen atoms (FIGURE 2.9). Crude oil is a complex mixture of hundreds of types of hydrocarbons.

Macromolecules are building blocks of life Just as the carbon atoms in hydrocarbons may be strung to- gether in chains, other organic compounds sometimes com- bine to form long chains of repeated molecules. Some of these chains, called polymers, play key roles as building blocks of life. Three types of polymers are essential to life: proteins,

H H

(a) Methane, CH4

(b) Ethane, C2H6

FIGURE 2.9  The simplest hydrocarbon is methane (a). Many hydrocarbons consist of linear chains of carbon atoms with hydro- gen atoms attached; the shortest of these is ethane (b). The air pollutant naphthalene (c) is a ringed hydrocarbon.

Phosphate group

Sugar Nitrogenous base

Nitrogenous base

Sugar-phosphate backbone

(a) DNA nucleotide

(b) DNA double helix

G C

FIGURE 2.10  Nucleic acids encode genetic information in the sequence of nucleotides (a), small molecules that pair together like rungs of a ladder. DNA includes four types of nucleotides, each with a different nitrogenous base: adenine (A), guanine (G), cytosine (C), and thymine (T). Adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G). In RNA, thymine is replaced by uracil (U). DNA twists into the shape of a double helix (b).

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this potential energy is converted to kinetic energy as the water rushes downstream.

Such energy transfers take place at the atomic level every time a chemical bond is broken or formed. Chemical energy is potential energy held in the bonds between atoms. Converting a molecule that has high-energy bonds (such as the carbon–carbon bonds of hydrocarbons in crude oil) into molecules with lower- energy bonds (such as the bonds in water or carbon dioxide) releases energy by changing potential energy into kinetic energy, and produces motion, action, or heat. Just as our automobile engines split the hydrocarbons of gasoline to release chemical energy and generate movement, our bodies split glucose mol- ecules from our food for the same purpose (FIGURE 2.11).

Energy is always conserved, but it changes in quality Although energy can change from one form to another, it cannot be created or destroyed. Just as matter is conserved, the total energy in the universe remains constant and thus is said to be conserved. Scientists have dubbed this principle the first law of thermodynamics. The potential energy of the water behind a dam will equal the kinetic energy of its eventual movement down the riverbed. Similarly, burning converts the chemical potential energy in a log of firewood to an equal amount of energy produced as heat and light.

Although the overall amount of energy is conserved in any process of energy transfer, the second law of thermo- dynamics states that the nature of energy will change from a more-ordered state to a less-ordered state, if no force coun- teracts this tendency. That is, systems tend to move toward increasing disorder, or entropy. For instance, a log of fire- wood—the highly organized and structurally complex prod- uct of many years of tree growth—transforms in a campfire to carbon ash, smoke, and gases such as carbon dioxide and water vapor, as well as the light and heat of the flame. With the help of oxygen, the complex biological polymers making up the wood are converted into a disorganized assortment of

carbohydrate chitin. Cellulose, the most abundant organic compound on Earth, is a complex carbohydrate found in the cell walls of leaves, bark, stems, and roots.

Lipids are a chemically diverse group of compounds, classi- fied together because they do not dissolve in water. Lipids include fats and oils (for energy storage), phospholipids (for membranes), waxes (for structure), and steroids (for hormone production).

Organisms use cells to compartmentalize macromolecules All living things are composed of cells, the most basic unit of organismal organization. Organisms range in complexity from single-celled bacteria to plants and animals that contain millions of cells. Cells vary greatly in size, shape, and function. Biologists classify organisms into two groups based on the structure of their cells. The cells of eukaryotes (plants, animals, fungi, and protists) contain a membrane-enclosed nucleus and various membrane- enclosed organelles that perform specific functions. Prokaryotes (bacteria and archaea) are generally single-celled, and their cells lack membrane-enclosed organelles and a nucleus.

ENERGY FUNDAMENTALS To create and maintain organized complexity, whether of a cell, an organism, or an ecological system, requires energy. Energy is needed to organize matter into complex forms, to build and maintain cellular structure, to power interactions among species, and to drive the geologic forces that shape our planet. Energy is somehow involved in nearly every biologi- cal, chemical, and physical phenomenon.

But what, exactly, is energy? Energy is an intangible phe- nomenon that can change the position, physical composition, or temperature of matter. Scientists differentiate two types of energy: potential energy, energy of position; and kinetic energy, energy of motion. Consider river water held behind a dam. Prevented from moving downstream, the water ac- cumulates potential energy. When the dam gates are opened,

Food molecules

CO2O2

Potential energy Kinetic energy

Glucose

+ + +C6H12O6 Oxygen Carbon dioxide Water

H2O

Heat

FIGURE 2.11 Energy is released when potential energy is converted to kinetic energy. Potential energy stored in sugars (such as glucose) in the food we eat, combined with oxygen, becomes kinetic energy when we exercise, releasing carbon dioxide, water, and heat as by-products.

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a by-product, they release the oxygen that we, and all other ani- mals, breathe.

Cellular respiration releases chemical energy Organisms make use of the chemical energy created by photosynthesis in a process known as cellular respiration. To release the chemical energy of glucose, cells use oxygen to break the high-energy chemical bonds in glucose (C6H12O6) and re- form its starting materials, water (H2O) and carbon dioxide (CO2). The energy released during this process is used to form

rudimentary molecules and heat and light energy. When energy transforms from a more-ordered to less-ordered state, it cannot accomplish tasks as efficiently. For example, the energy avail- able in ash (a less-ordered state of wood) is far lower than that available in a log of firewood (the more-ordered state of wood). Living organisms are able to resist entropy through regular inputs of energy from food sources and photosynthesis. Once death occurs and those energy inputs cease, an organism un- dergoes decomposition and loses its highly organized structure.

Light energy from the sun powers most living systems The energy that powers Earth’s ecological systems comes primarily from the sun. The sun releases radiation from large portions of the electromagnetic spectrum, although our at- mosphere filters much of this out and we see only some of this radiation as visible light (FIGURE 2.12).

Some organisms use the sun’s radiation directly to produce their own food. Such organisms, called autotrophs or producers, include green plants, algae, and bacteria called cyanobacteria. Autotrophs turn light energy from the sun into chemical energy through a process called photosynthesis (FIGURE 2.13). In pho- tosynthesis, sunlight powers a series of chemical reactions that convert carbon dioxide and water into sugars, transforming en- ergy from the sun into high-quality energy (sugars) the organism can use. It is an example of moving toward a state of lower en- tropy, and as such it requires a substantial input of outside energy.

Photosynthesis occurs within cell organelles called chloro- plasts, where the light-absorbing pigment chlorophyll (which is what makes plants green) uses solar energy to initiate a series of chemical reactions called light reactions. During these reactions, water molecules split and react to form hydrogen ions (H+) and molecular oxygen (O2), thus creating the oxygen that we breathe. The light reactions also produce small, high-energy molecules that are used to fuel reactions in the Calvin cycle, where carbon atoms from carbon dioxide are linked together to manufacture sugars. Photosynthesis is a complex process, but the overall reac- tion can be summarized in the following equation:

6CO2 + 6H2O + the sun’s energy C2H12O6 + 6O2 (sugar)

Thus in photosynthesis, green plants draw up water from the ground through their roots, absorb carbon dioxide from the air through their leaves, and harness the energy in sunlight to create sugars (such as C6H12O6) for their growth and maintenance. As

Visible light

High energy, shorter wavelength

Gamma rays X-rays

Ultra- violet Infrared

Wavelength (meters)

Radio waves

Low energy, longer wavelength

M ic

ro w

av es

10–14 10–12 10–10 10–8 10–6 10–4 10–2 1

Sun

FIGURE 2.12  The sun emits radiation from many portions of the electromagnetic spectrum. Visible light makes up only a small propor- tion of this energy. Some radiation that reaches our planet is reflected back; some is absorbed by air, land, and water; and a small amount powers photosynthesis (see Figure 14.1, p. 301).

Sunlight

Light reactions

Chloroplast

Calvin cycle

H2O

CO2

Sugars

ATP ADP

NADP+

Inorganic phosphate

NADPH

FIGURE 2.13  In photosynthesis, autotrophs such as plants, algae, and cyanobacteria use sunlight to convert carbon dioxide and water into sugars and oxygen. This schematic diagram summarizes the complex sets of chemical reactions that take place within chloroplasts. In the light reactions, water is converted to oxygen in the presence of sunlight, creating high-energy molecules (ATP and NADPH). These molecules help drive reactions in the Calvin cycle, in which carbon dioxide is used to produce sugars. Molecules of ADP, NADP+, and inorganic phosphate produced in the Calvin cycle in turn help power the light reactions, creating an endless loop.

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new chemical bonds or to perform other tasks within cells. The net equation for cellular respiration is the opposite of that for photosynthesis:

C6H12O6 + 6O2 6CO2 + 6H2O + energy (sugar)

However, the energy gained per glucose molecule in respira- tion is only two-thirds of the amount of energy required to synthesize a glucose molecule in photosynthesis—a prime example of the second law of thermodynamics in action.

Respiration occurs in autotrophs and also in heterotrophs, or consumers, organisms that gain their energy by feeding on the biomass of other organisms. Heterotrophs include most ani- mals, as well as the fungi and microbes that decompose organic matter. In most ecological systems, plants, algae, or cyanobacte- ria form the base of a food chain through which energy passes to heterotrophs (pp. 68–70).

ECOSYSTEMS Let’s now apply our knowledge of chemistry and energy to see how energy, matter, and nutrients move through the liv- ing and nonliving environment. An ecosystem consists of all organisms and nonliving entities that occur and interact in a particular area. Animals, plants, water, soil, nutrients—all these and more help comprise ecosystems.

(a) Energy flowing through an ecosystem

Energy is released as heat in one-way flow through system

Consumers

Sun

Detritus (non-living organic

matter)

Detritivores and decomposers

Producers Chemical energy

Heat

Earthworm

Soil bacteria

Twigs Leaves

Grass

Grasshopper

Rodent

Hawk

FIGURE 2.14  Energy enters, flows through, and exits an ecosystem. In (a), light energy from the sun (yellow ar- row) drives photosynthesis in producers, which begins the transfer of chemical energy (green arrows) among trophic levels (p. 68) and detritus. Energy exits the system through respiration in the form of heat (red arrows). In contrast, matter cycles within an ecosystem. In (b), blue arrows show the movement of nutrients among trophic levels and detritus. In both diagrams, box sizes represent relative magnitudes of energy or matter content, and arrow widths represent relative magnitudes of energy or matter transfer. Such magnitudes may vary tremendously from one ecosystem to another. For simplicity, various abiotic components (such as water, air, and inorganic soil content) of ecosystems are omitted from these schematic diagrams. (We will revisit the flow of energy and matter among organ- isms in greater detail in Chapter 4 (pp. 68–71).)

(b) Matter cycling within an ecosystem

Consumers

Detritus (non-living organic

matter)

Detritivores and decomposers

Producers

Earthworm

Soil bacteria

Twigs Leaves

Grass

Grasshopper

Rodent

Hawk

Matter is conserved and cycles within system

Nutrients

Energy flows and matter cycles through ecosystems The ecosystem concept originated with scientists who rec- ognized that biological entities are tightly intertwined with the chemical and physical aspects of their environment. For instance, in the Chesapeake Bay estuary (a water body where rivers flow into the ocean, mixing fresh water with salt water), aquatic organisms are intimately affected by the flow of water, sediment, and nutrients from the rivers that feed the bay and from the land that feeds those rivers. In turn, the photosynthesis, respiration, and decomposition of these organisms influence the chemical composition of the Chesa- peake’s waters.

Ecologists soon began analyzing ecosystems as an en- gineer might analyze the operation of a machine. In this view, ecosystems are systems that receive inputs of energy, process and transform that energy while cycling matter internally, and produce a variety of outputs (such as heat, water flow, and animal waste products) that can feed into other ecosystems.

Energy flows in one direction through ecosystems; it ar- rives mostly as radiation from the sun, powers the system, and exits in the form of heat (FIGURE 2.14A). Matter, in con- trast, is generally recycled within ecosystems (FIGURE 2.14B). Energy and matter are passed among organisms (producers,

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production is equal to gross primary production minus cel- lular respiration.

Ecosystems vary in the rate at which autotrophs con- vert energy to biomass. This rate is termed productivity, and ecosystems whose producers convert solar energy to biomass rapidly are said to have high net primary productivity. Fresh- water wetlands, tropical forests, and coral reefs tend to have the highest net primary productivities, whereas deserts, tun- dra, and open ocean tend to have the lowest (FIGURE 2.15A). These differences in net primary productivity among eco- system types result in distinct geographic patterns across the globe (FIGURE 2.15B). In terrestrial ecosystems, net primary productivity tends to increase with temperature and precipi- tation. In aquatic ecosystems, net primary productivity tends to rise with light and the availability of nutrients. The limiting nature of nutrients in waters such as the Chesapeake is why additions of nitrogen and phosphorus from people’s activities have such significant effects on the system.

consumers, and decomposers) through food web relationships (pp. 70–71). Matter is recycled because when organisms die and decay, their nutrients remain in the system. In contrast, most energy that organisms take in is eventually lost to the environment through respiration.

Energy is converted to biomass Energy f low in most ecosystems begins with the sun’s ra- diation. As autotrophs convert solar energy to the energy of chemical bonds in sugars during photosynthesis, they perform primary production. Specifically, the total amount of chemical energy produced by autotrophs is termed gross primary production. Autotrophs use most of this produc- tion to power their own metabolism by cellular respiration, however. The energy that remains after respiration and is used to generate biomass (such as leaves, stems, and roots; p. 69) ecologists call net primary production. Net primary

(b) Global map of net primary productivity

0–100 100–200 200–400 400–600 600–800 >800

(a) Net primary productivity for major ecosystem types

N et

p rim

ar y

pr od

uc tiv

ity (g

C /m

2 / yr

)

Tro pi

ca l r

ain fo

re st

Tro pi

ca l s

ea so

na l f

or es

Te m

pe ra

te e

ve rg

re en

fo re

Te m

pe ra

te d

ec id

uo us

fo re

Bo re

al fo

re st

Sa va

nn a

Te m

pe ra

te g

ra ss

lan d

Tu nd

ra a

nd a

lp in

De se

rt an

d se

m id

es er

t s hr

Cu lti

va te

d lan

Sw am

p an

d m

ar sh

La ke

a nd

st re

Co nt

in en

ta l s

he lf

Op en

o ce

Al ga

l b ed

s a nd

re ef

Es tu

ar ies

500

1,000

1,500

2,000

2,500 Terrestrial ecosystems Aquatic ecosystems

FIGURE 2.15  Freshwater wetlands, tropi- cal forests, coral reefs, and algal beds show high net primary productivities on average (a), whereas deserts, tundra, and the open ocean show low values. A world map created from satellite data (b) shows that on land, net primary productivity varies geographi- cally with temperature and precipitation. In the world’s oceans, net primary productivity is highest around the margins of continents, where nutrients (of both natural and human origin) run off from land. Data in (a) from Whit- taker, R.H., 1975. Communities and ecosystems, 2nd

ed. New York: MacMillan. Map in (b) from satellite

data presented by Field, C.B., et al., 1998. Primary

production of the biosphere: Integrating terrestrial

and oceanic components. Science 281: 237–240.

Reprinted with permission from AAAS.

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Because components of different ecosystems may in- termix, ecologists often find it useful to view these systems on larger geographic scales that encompass multiple eco- systems. In such a broad-scale approach, called landscape ecology, scientists study how landscape structure affects the abundance, distribution, and interaction of organisms. Taking a view across the landscape is important in study- ing birds that migrate long distances, mammals that move seasonally between mountains and valleys, and fish such as salmon that swim upriver from the ocean to reproduce. A landscape-level approach is also useful for planning sus- tainable cities and regional development (p. 404). These studies have been greatly aided by satellite imaging and geographic information systems (GIS)—computer software that takes multiple types of data (for instance, on geology, hydrology, vegetation, animal species, and human develop- ment) and layers them together on a common set of geo- graphic coordinates.

For a landscape ecologist, a landscape is made up of patches (of ecosystems, communities, or habitat) arrayed spa- tially in a mosaic (FIGURE 2.16). Landscape ecology is of great interest to conservation biologists (p. 174), scientists who study the loss, protection, and restoration of biodiversity (see The Science behind the Story, pp. 34–35). Populations of organisms have specific habitat requirements and so occupy

Ecosystems interact across landscapes Ecosystems vary widely in size. An ecosystem can be as small as a puddle of water containing algae and tadpoles or as large as forest, lake, or bay that supports a diversity of habitats and species. In general, the term refers to a system of moderate geographic extent that is somewhat self-contained. For exam- ple, the tidal marshes in the Chesapeake where river water empties into the bay are an ecosystem, as are the sections of the bay dominated by oyster reefs. However, ecosystems that border one another may interact extensively. For instance, rivers, tidal marshes, and open waters in estuaries all inter- act, as do forests and grasslands where they converge. Areas where ecosystems meet may consist of transitional zones called ecotones, in which elements of each ecosystem mix.

Montane coniferous forest Ecotone Patches of forest

and grassland

River

Freshwater marshLowland broadleaf forest Grassland

Corridor

FIGURE 2.16  Landscape ecology deals with spatial patterns above the ecosystem level. This generalized diagram of a landscape shows a mosaic of patches of five ecosystem types (three terrestrial types, a marsh, and a river). Thick red lines indicate ecotones. A stretch of lowland broadleaf forest running along the river serves as a corridor connecting the large region of forest on the left to the smaller patch of forest alongside the marsh (allow- ing forest animals to move between patches). The inset shows a magnified view of the forest-grassland ecotone and how it consists of patches on a smaller scale.

Ecosystems Where You Live Think about the area where you live. How would you describe this area’s ecosystems? How do these systems interact with one another? If one ecosystem were

greatly disturbed (say, if a wetland or forest were replaced by a shopping mall), what might be the impacts on nearby natural systems?

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In 2001, the Eastern oyster (Crassostrea virginica) was in dire trouble in the Chesapeake Bay. Populations of oysters in the Chesapeake (whose name is derived from an Algonquin word Chesepiook, meaning “great shellfish bay”) were a mere 1% of their historical abundance. Restoration efforts had largely failed. The bay oyster industry, once the largest in the world, had collapsed as a result of 200 years of overharvesting, poor water quality, virulent oyster diseases

David Schulte, U.S. Army Corps of Engineers

THE SCIENCE BEHIND THE STORY

“Turning the Tide” for Native Oysters in the Chesapeake Bay

spread throughout the bay by trans- planted oysters, and the destruction of the reefs that are preferred oyster habitat. Nonetheless, when scientists or resource managers proposed to help rebuild oyster populations by significantly restricting oyster harvests or establishing oyster reef “sanctuar- ies,” these initiatives were typically defeated by the politically powerful oyster industry.

With the collapse of the native oys- ter fishery and with political obstacles blocking restoration projects for native oysters, support grew among the oys- ter industry, state resource managers, and some scientists for the introduction of Suminoe oysters (Crassostrea ariak- ensis) from Asia. This species seemed well suited for conditions in the bay and showed resistance to the parasitic dis- eases that were ravaging native oysters. Proponents argued that Suminoe oyster introduction would reestablish thriving oyster populations in the bay, revital- izing the decimated oyster fishery and improving water quality (because as oysters feed, they filter phytoplankton and sediments from the water column).

Filter-feeding by oysters is an important ecological service in the bay because it reduces phytoplankton densities, clarifies waters, and supports the growth of underwater grasses that

provide food and refuge for waterfowl and young crabs. Because introduc- tions of invasive species can have profound ecological impacts (p. 75), the Army Corps of Engineers was directed to coordinate an environmental impact statement (EIS, p. 100) on oyster resto- ration approaches in the Chesapeake.

It was in this politically charged, high-stakes environment that Dave Schulte, a scientist with the Corps and doctoral student at the Virginia Institute of Marine Sciences, set out to determine whether there was a viable approach to

restoring native oyster populations. The work he and his team began would help turn the tide in favor of native oysters in the bay’s restoration efforts.

One of the biggest impacts on native oysters was the destruction of oyster reefs by a century of intensive oyster harvesting. Oysters settle and grow best on the shells of other oys- ters, and over long periods this process forms reefs (underwater outcrops of living oysters and oyster shells) that solidify and become as hard as stone. Throughout the bay, massive reefs that at one time had jutted out of the water at low tide had been reduced to rubble on the bottom from a century of repeated scouring by metal dredgers used by oyster harvesting ships. The key, Schulte realized, was to construct artificial reefs like those that once ex- isted, to get oysters off the bottom— away from smothering sediments and hypoxic waters—and up into the plankton-rich upper waters.

Armed with the resources avail- able to the Corps, he opted to take a landscape ecology approach (p. 33) and restore patches of reef habitat on nine complexes of reefs covering a total of 35.3 hectares (87 acres) in an oyster sanctuary near the mouth of the Great Wicomico River (see map) in the lower Chesapeake Bay. This was a very

Washington DC

Chesapeake Bay

P o t o m a c R i v e r

VIRGINIA

MARYLAND

Experiment conducted in the Great Wicomico River

Miles 25

Schulte’s study was conducted in the Great Wicomico River in Virginia in the lower Chesapeake Bay.

suitable patches across the landscape. If habitat patches are highly fragmented and isolated (pp. 169, 201–202), the popu- lations in those patches may perish. Accordingly, establish- ing corridors of habitat (p. 203; also see Figure 2.16) to link patches is one approach that conservation biologists pursue as they attempt to maintain biodiversity.

Modeling aids our understanding of ecosystems Another way in which ecologists seek to make sense of the natural systems they study is by working with models. In sci- ence, a model is a simplified representation of a complicated

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different approach from the smaller- scale restoration efforts of the past.

Artificial reefs of two heights were constructed in 2004 (see photograph), and oysters were allowed to colonize the reefs, safe from harvesting. Oyster popu- lations on the constructed reefs were sampled in 2007, and the results were stunning. The reef complex supported an estimated 185 million oysters, a number nearly as large as the wild popu- lation of 200 million oysters estimated to live on the remaining degraded habitat in all of Maryland’s waters.

Higher reefs supported an average of over 1,000 oysters per square meter— four times more than the lower reefs and 170 times more than unrestored bottom (see graph). Like natural reefs, the constructed reefs began to solidify, providing a firm foundation for the set- tlement of new oysters. In 2009, Schulte’s research made a splash when his team published its findings in the journal Science, bringing international attention to their study.

After reviewing eight alternative approaches to oyster restoration that

involved one or more oyster species, the Corps advocated an approach that avoided the introduction of non-native oysters. Instead it proposed a com- bination of native oyster restoration, a temporary moratorium on oyster harvests (accompanied by a compen- sation program for the oyster indus- try), and enhanced support for oyster aquaculture in the bay region.

Schulte’s restoration project cost roughly $3 million and will require substantial investments if it is to be repeated elsewhere in the bay. This is particularly true in upper portions of the bay, where oyster reproduction levels are lower (requiring restored reefs to be “seeded” with oysters), water conditions are poorer, and oysters are less resistant to disease. Many scientists contend that expand- ed reef restoration efforts are worth the cost because they enhance oys- ter populations and provide a vital service to the bay through water filter- ing. Some scientists also see value in promoting oyster farming, in which restoration efforts would be

supported by businesses instead of taxpayers.

Regardless of how they will be funded, protected sites for oyster restoration efforts are being estab- lished. In 2010, Maryland proposed creating 3,640 hectares (9,000 acres) of new oyster sanctuaries—25% of exist- ing oyster reefs in state waters—where restoration projects like Schulte’s could be replicated. This movement toward increased protection for oyster popula- tions, coupled with findings of growing resistance to disease in bay oysters, has given new hope that native oysters may once again thrive in the bay that bears their name.A water cannon blows oysters shells off a barge and onto the river

bottom to create an artificial oyster reef for the experiment.

Reef type

M ea

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(o ys

te rs

p er

m 2 )

200

400

600

800

1,000

Total Adults Spat

0 2 4 6 8

Hig h-r

elie f

Low -re

lief

Un res

tor ed

Reef height had a profound effect on the density of adult oysters and spat (newly settled oysters). Schulte’s work sug- gested that native oyster popula- tions could rebound in portions of the bay if they were provided elevated reefs and protected from harvest. Data from Schulte, D.M., R.P. Burke, and R.N Lipicus, 2009. Unprecedented restoration of a native oyster meta population. Science 325: 1124–1128. Reprinted with permission from AAAS.

natural process, designed to help us understand processes and make predictions.

Ecological modeling is the practice of constructing and testing models that aim to explain and predict how ecologi- cal systems function. These models are grounded in actual data and based on hypotheses about how system components

interact. Ecological modeling is extremely useful in large, intricate systems that are difficult to isolate and study. For example, ecological models are useful for understanding the flow of nutrients into the Chesapeake Bay and in predicting the responses of oysters and underwater grasses to changing water conditions.

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Ecosystems provide vital services All life on our planet, including human life, depends on healthy, functioning ecosystems. When Earth’s ecosystems function normally and undisturbed, they provide goods and services that we could not survive without. Examples of such ecosystem services include regulating atmospheric gases, precipitation, and temperature; providing people with food and natural resources; pollinating plants and controlling crop pests; preventing soil erosion; filtering wastes and pol- lutants; and providing recreational opportunities and aes- thetic, artistic, spiritual, and educational value (FIGURE 2.17).

When human activities impair ecosystem functioning, we must devote resources in an attempt to provide these services ourselves, requiring extra investments of time and money. For example, when we eliminate insect predators from agricultur- al fields by using intensive farming practices, we impair the ecological service of pest control that nature provides. Farm- ers must then apply synthetic pesticides to reduce pest popu- lations, costing them resources and increasing exposures to these chemicals among people and wildlife.

One of the most important ecosystem services is the cycling of nutrients. Through the processes that take place within and among ecosystems, the chemical elements and compounds that we need—water, carbon, nitrogen, phosphorus, and many more—cycle through our environment in intricate ways.

BIOGEOCHEMICAL CYCLES Just as nitrogen and phosphorus from fertilizer on Pennsyl- vania farm fields end up in Chesapeake Bay oysters on our dinner plates, all nutrients move through the environment in complex and fascinating ways. Whereas energy enters an ecosystem from the sun, flows from one organism to another, and is dissipated to the atmosphere as heat, the physical matter of an ecosystem is circulated over and over again.

Nutrients circulate in biogeochemical cycles Nutrients move through ecosystems in nutrient cycles (or biogeochemical cycles) that circulate elements or mol- ecules through the lithosphere, atmosphere, hydrosphere, and biosphere. A carbon atom in your fingernail today might have been part of the muscle of a cow a year earlier, may have resided in a blade of grass a month before that, and may have been part of a dinosaur’s tooth 100 million years ago. After we die, the nutrients in our bodies will spread widely through the environment, eventually being incorporated by an untold number of organisms far into the future.

Nutrients move from one pool, or reservoir, to another, remaining for varying amounts of time (residence time) in

Store water and regulate water flow Regulate

climate

Purify water

Provide timber and other resources

Dampen impacts from disturbance

Provide habitat

Control erosion

Pollinate plants

Provide pest control

Filter runoff and treat waste

Form soil

Cycle nutrients

Provide recreation

Provide food

Purify air

FIGURE 2.17  Ecological processes naturally provide countless ecosystem services. Our society, indeed our very survival, depends on these services.

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can change over time. As we will see in the following sections, human activities affect the cycling of nutrients by altering fluxes, residence times, and the relative amounts of nutrients in reservoirs.

The water cycle influences all other cycles Water is so integral to life that we frequently take it for grant- ed. The essential medium for many biochemical reactions, water plays key roles in nearly every environmental system, including each nutrient cycle. Water transports nutrients, sediments, and pollutants from the continents to the oceans via rivers, streams, and surface runoff. Nutrients can then be carried thousands of miles on ocean currents. Water also brings atmospheric pollutants from the air back down to the surface when they dissolve in falling rain or snow. These activities make the water cycle, or hydrologic cycle (FIGURE 2.19), an integral part of nutrient cycling on Earth.

The oceans are the largest reservoir in the hydrologic cycle, holding more than 97% of all water on Earth. The fresh water we depend on for our survival accounts for the remain- ing water, and two-thirds of this small amount is tied up in

each. The dinosaur, the grass, the cow, and your body are each reservoirs for carbon atoms. When a reservoir releases more materials than it accepts, it is called a source, and when a reservoir accepts more materials than it releases, it is called a sink. FIGURE 2.18 illustrates these concepts in a simple man- ner. The rate at which materials move between reservoirs is termed a flux, and the flux between any given pair of pools

Source

Large flux

Reservoirs

Small flux

Short residence time

Long residence time

Sink

FIGURE 2.18  The main components of a biogeochemical cycle are reservoirs (places where materials are stored) and fluxes (rates at which materials move among reservoirs). A source releases more materials than it accepts, and a sink accepts more materials than it releases.

Atmosphere 13,000

71,00071,000 Precipitation

111,000 Precipitation

111,000

Precipitation 385,000

Evaporation 425,000

Precipitation 385,000

Evaporation 425,000

EvaporationEvaporation

Human useHuman use

Oceans 1,350,000,000

Extraction

Infiltration <11,000

Extraction

Groundwater 15,300,000

TranspirationTranspiration

Land plants

Rivers Runoff 40,000 Runoff 40,000

ExtractionExtraction

Ice caps, glaciers, and snowfields

33,000,000

Uptake

Soil water 122,000

Infiltration <11,000

Aquifer

Water table

FIGURE 2.19  The water cycle, or hydrologic cycle, summarizes the many routes that water molecules take as they move through the environment. Gray arrows represent fluxes among reservoirs, or pools, for water. The water cycle is a system unto itself but also plays key roles in other biogeochemical cycles. Oceans hold 97% of our planet’s water, whereas most fresh water resides in groundwater and icecaps. Water vapor in the atmosphere con- denses and falls to the surface as precipitation; then it evaporates from land and transpires from plants to return to the atmosphere. Water flows downhill into rivers, eventually reaching the oceans. In the figure, pool names are printed in black type, and numbers in black type represent pool sizes expressed in units of cubic kilometers (km3). Processes, printed in italic red type, give rise to fluxes, printed in italic red type and expressed in km3 per year. Data from Schlesinger, W.H., 1997. Biogeochemistry: An analysis of global change, 2nd ed. London: Academic Press.

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detritus of marine organisms. Ocean waters are the second largest reservoir of carbon on Earth. The largest reservoir of carbon, sedimentary rock (p. 230), is formed in oceans and freshwater wetlands. When organisms in these habitats die, their remains can settle in sediments, and as layers of sedi- ment accumulate, the older layers are buried more deeply and experience high pressure for long periods. Over time, these conditions can convert the soft tissues of dead organisms into fossil fuels—coal, oil, and natural gas—deep underground (p. 329). The shells and skeletons of aquatic organisms, which are rich in calcium carbonate (CaCO3), are converted by these high pressures underground to sedimentary rock, such as lime- stone. Although any given carbon atom spends a relatively short time in the atmosphere, carbon trapped in sedimentary rock may reside there for hundreds of millions of years. Carbon trapped in sedimentary rocks and fossil fuel deposits may even- tually be released into the oceans or atmosphere by geologic processes such as uplift, erosion, and volcanic eruptions. It also reenters the atmosphere when we extract and burn fossil fuels.

The largest human impact on the carbon cycle is through our use of fossil fuels as an energy source. It is estimated that since the middle of the 18th century, people have added over 250 billion metric tons (276 billion tons) of carbon dioxide to the atmosphere through the combustion of coal, oil, and natural gas—greatly increasing the flux of carbon from the lithosphere to the atmosphere and shortening the residence time of carbon in fossil fuel deposits.

Moreover, when people burn forests and fields to clear land for agriculture, the carbon in wood and leaves is released to the atmosphere. Because these cleared sites have less veg- etation, photosynthesis removes less carbon dioxide from the atmosphere than before. As a result of vegetation clearing and fossil fuel combustion, scientists estimate that today’s at- mospheric carbon dioxide reservoir is the largest that Earth has experienced in the past 800,000 years—likely in the past 20 million years (p. 302)—and is a driving force behind glo- bal climate change (Chapter 14).

The nitrogen cycle involves specialized bacteria Nitrogen makes up 78% of our atmosphere by mass and is the sixth most abundant element on Earth. It is an essen- tial ingredient in the proteins, DNA, and RNA that build our bodies and, like phosphorus, is an essential nutrient for plant growth. Thus the nitrogen cycle (FIGURE 2.21) is of vital importance to us and to all other organisms. Despite its abundance in the air, nitrogen gas (N2) is chemically in- ert and cannot cycle out of the atmosphere and into living organisms without assistance from lightning, highly special- ized bacteria, or human intervention. However, once nitro- gen undergoes the right kind of chemical change, it becomes biologically active and available to the organisms that need it, and it can act as a potent fertilizer.

To become biologically available, inert nitrogen gas (N2) must be “fixed,” or combined with hydrogen in nature to form ammonia (NH3), whose water-soluble ions of ammonium (NH4

+) can be taken up by plants. Nitrogen fixation can be accomplished in two ways: by the intense energy of lightning

glaciers, snowfields, and ice caps (p. 250). Thus, considerably less than 1% of the planet’s water is in a form that we can read- ily use—groundwater, surface fresh water, and rain from at- mospheric water vapor.

Water moves from oceans, lakes, ponds, rivers, and moist soil into the atmosphere by evaporation, the conversion of a liquid to gaseous form. Water also enters the atmosphere by transpiration, the release of water vapor by plants through their leaves. Transpiration and evaporation act as natural processes of distillation, because water escaping into the air as a gas leaves behind its dissolved substances. Water returns from the atmosphere to Earth’s surface as precipitation when water vapor condenses and falls as rain or snow. Precipitation may be taken up by plants and used by animals, but much of it flows as runoff into streams, rivers, lakes, ponds, and oceans.

Some water soaks down through soil and rock through a process called infiltration to recharge underground res- ervoirs known as aquifers. Aquifers are spongelike regions of rock and soil that hold groundwater, water found un- derground beneath layers of soil. The uppermost level of groundwater held in an aquifer is referred to as the water table. Some aquifers hold groundwater for short periods of time, whereas others contain quite ancient water.

Human activity has affected nearly every flux, reservoir, and residence time in the water cycle. By damming rivers, we have slowed the movement of water from the land to the sea, and we increase evaporation by holding river waters in reser- voirs. We have removed natural vegetation by clear-cutting and developing land, which increases surface runoff, decreases infiltration and transpiration, and promotes soil erosion. Our withdrawals of surface water and groundwater for agricul- ture, industry, and domestic uses have depleted rivers, lakes, and streams and have lowered water tables. And by emitting into the atmosphere pollutants that dissolve in water drop- lets, we have changed the chemical nature of precipitation, in effect sabotaging the natural distillation process that evaporation and transpiration provide. (We will revisit the water cycle, water resources, and human impacts in more detail in Chapter 12).

The carbon cycle circulates a vital organic nutrient The carbon cycle describes the routes that carbon atoms take through the environment (FIGURE 2.20). Because car- bon forms the backbone of essential biological molecules, its cycling is of great importance. Producers, including ter- restrial and aquatic plants, algae, and cyanobacteria, pull carbon dioxide out of the atmosphere and out of surface water to use in photosynthesis. Autotrophs and the hetero- trophs that consume them use carbohydrates produced in photosynthesis to fuel their respiration and for structural growth, releasing carbon back into the atmosphere and waters as CO2. Because plants are a sizable reservoir of carbon and CO2 is a greenhouse gas (p. 300), researchers are attempting to meas- ure the amount of CO2 that various plants sequester to evaluate the potential of vegetation in combating global climate change.

Earth’s oceans function extensively in the carbon cycle. Oceans absorb carbon-containing compounds from the atmosphere, terrestrial runoff, undersea volcanoes, and the

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nitrogen-rich compounds, they release ammonium ions, mak- ing these available to nitrifying bacteria to convert again to ni- trates and nitrites. The next step in the nitrogen cycle occurs when denitrifying bacteria convert nitrates in soil or water to gaseous nitrogen. Denitrification thereby completes the cycle by releasing nitrogen back into the atmosphere as a gas.

We have greatly influenced the nitrogen cycle Historically, nitrogen fixation was a bottleneck, a step that limited the flux of nitrogen out of the atmosphere into water- soluble forms. Once people discovered how to fix nitrogen on massive scales, a process called industrial fixation, we

strikes, or by particular types of nitrogen-fixing bacteria that inhabit the top layer of soil. These bacteria live in a mutualistic relationship (p. 68) with many types of plants, including soy- beans and other legumes, providing them nutrients by con- verting nitrogen to a usable form. Other types of specialized bacteria then perform a process known as nitrification, con- verting ammonium ions first into nitrite ions (NO2

–), then into nitrate ions (NO3

–). Plants can take up these ions, which also become available after atmospheric deposition on soils or in water or after application of nitrate-based fertilizer.

Animals obtain the nitrogen they need by consuming plants or other animals. Decomposers obtain nitrogen from dead and decaying plant and animal matter and from the urine and feces of animals. Once decomposers process the

Atmosphere

Atmosphere 821 (+3.2/yr)

Ocean- atmosphere exchange

9090

9292

Runoff 0.8

Oceans 38,000

Burial 0.1Burial 0.1 Fossil fuel extraction Fossil fuel extraction

DecompositionDecomposition

Soil and soil biota

1,500

Anthropogenic sources

Net deforestation

Land plants

560 Consumers

Sedimentary rock 80,600,000

Fossil fuels (coal, oil, natural gas)

4,000

Oceans

Rivers

Volcanic and hydrothermal

emissions < 0.1

Volcanic and hydrothermal

emissions < 0.1

Reduced uptake by

plants 0.9

Reduced uptake by

plants 0.9

Fossil fuel combustion

Decomposers

Oceans

Respiration Respiration GPP

ProducersConsumers

Weathering < 0.1

Respiration 60

Respiration 60GPP

120

Respiration 60

Respiration 60GPP

120

FIGURE 2.20  The carbon (C) cycle summarizes the many routes that carbon atoms take as they move through the environment. Gray arrows represent fluxes among reservoirs, or pools, for carbon. In the carbon cycle, plants use carbon dioxide from the atmosphere for photosynthesis (gross primary production, or “GPP” in the figure). Carbon dioxide is returned to the atmosphere through respiration by plants, their consumers, and decomposers. The oceans sequester carbon in their water and in deep sediments. The vast majority of the planet’s carbon is stored in sedi- mentary rock. In the figure, pool names are printed in black type, and numbers in black type represent pool sizes expressed in petagrams (units of 1015 g) of C. Processes, printed in italic red type, give rise to fluxes, printed in italic red type and expressed in petagrams of C per year. Data from Schlesinger, W.H., 1997. Biogeochemistry: An analysis of global change, 2nd ed. London: Academic Press.

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The phosphorus cycle circulates a limited nutrient Sedimentary rocks are the largest reservoir in the phosphorus cy- cle (FIGURE 2.22). Environmental concentrations of phosphorus available to organisms tend to be very low, because weathering (p. 137), which releases phosphate ions (PO4

3–) into water, is the only process that makes phosphorus available for uptake. Phos- phorus is a key component of cell membranes, DNA, RNA, and a number of vital biochemical compounds, so its rarity in the en- vironment makes it a limiting factor for plant growth. Aquatic producers take up phosphates from surrounding waters, whereas terrestrial producers take up phosphorus from soil water through their roots. Phosphorus is incorporated into the tissues of pro- ducers, and consumers obtain their required phosphorus by

accelerated its flux into other reservoirs. Today, our species is fixing at least as much nitrogen artificially as is being fixed naturally, and we are overwhelming nature’s denitrification abilities. A 2008 study reported that people apply a staggering 150 million metric tons (165 million tons) of fixed nitrogen to Earth’s land area each year.

Human alteration of the nitrogen cycle has had profound impacts beyond the eutrophication of aquatic ecosystems. Oddly enough, the overapplication of nitrogen-based fertiliz- ers can strip the soil of other vital nutrients, such as calcium and potassium, reducing soil fertility. Additionally, burning fossil fuels, forests, or fields generates nitrogenous compounds in the atmosphere that act as greenhouse gases (Chapter 14), form acid precipitation (p. 291), and create photochemical smog (p. 288).

Groundwater

Oceans

Biotic cycling 8,000

Oceans

Atmosphere (N2)

Rivers

Industry and automobiles

Decomposers

Consumers Biotic

cycling 8,000

Denitrification 110

Precipitation 30

Volatilization

Fixation 15

Denitrification 110

Precipitation 30

Volatilization

Dust from land

Fixation 15

Producers

NH3 NH4 + NO2

–

NO3 –

Consumers

Atmosphere (N2) 3,870,000,000

Denitrification < – 200

Fixation by lightning

Natural biological fixation

100

Emissions (NOX) 20 Emissions (NOX) 20

Fixation by lightning

Natural biological fixation

100

Land plants

Fixation by crops (40) and fertilizer production (80)

Decomposition and waste Decomposition and waste

Assimilation 1,200

Runoff 36Runoff 36

Inorganic N 570,000 Extraction and

combustion Anthropogenic

additions 11

Deposition in precipitation

Extraction and combustion

Anthropogenic additions 11

Soil organic matter (NH3) 115,000

NH4 + NO2

NO3 _

Bacterial conversionBacterial conversion NitrificationNitrification

Deposition in precipitation

Sediments and sedimentary rock

Burial 10Burial 10 Fossil fuels

Denitrification < – 200

FIGURE 2.21  The nitrogen (N) cycle summarizes the many routes that nitrogen atoms take as they move through the environment. Gray arrows represent fluxes among reservoirs, or pools, for nitrogen. In the nitrogen cycle, spe- cialized bacteria play key roles in “fixing” atmospheric nitrogen and converting it to chemical forms that plants can use. Other types of bacteria convert nitrogen compounds back to the atmospheric gas N2. In the oceans, inorganic nitrogen is buried in sediments, whereas nitrogen compounds are cycled through food webs as they are on land. In the figure, pool names are printed in black type, and numbers in black type represent pool sizes expressed in teragrams (units of 1012 g) of N. Processes, printed in italic red type, give rise to fluxes, printed in italic red type and expressed in teragrams of N per year. Data from Schlesinger, W.H., 1997. Biogeochemistry: An analysis of global change, 2nd ed. London: Academic Press.

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Sediment and sedimentary rock 4,000,000,000

Burial 2

Burial 19

Burial 2

Burial 19

Geologic uplift

Uptake 60Geologic

uplift

Uptake 60

Soils 200,000

Fertilizers and detergents

Pollution

Erosion

Pollution

Erosion

Rivers

Runoff 21

Biotic cycling 1,110

Uptake 2

ProducersConsumers

Oceans 90,000

Decomposers

Biotic cycling 1,110

Uptake 2

WeatheringWeathering Mining 12Mining 12

Decomposers

Mineable rock 10,000

Atmosphere

Transport of dust and seaspray 1Transport of dust and seaspray 1

Consumers

Land plants 3,000

FIGURE 2.22  The phosphorus (P) cycle summarizes the many routes that phosphorus atoms take as they move through the environment. Gray arrows represent fluxes among reservoirs, or pools, for phosphorus. Most phosphorus resides underground in rock and sediment, but the phosphorus cycle moves this element through the soil, the oceans, and freshwater and terrestrial ecosystems. Rocks containing phosphorus are uplifted geologically and slowly weath- ered away. Small amounts of phosphorus cycle through food webs, where this nutrient is often a limiting factor for plant growth. In the figure, pool names are printed in black type, and numbers in black type represent pool sizes expressed in teragrams (units of 1012 g) of P. Processes, printed in italic red type, give rise to fluxes, printed in italic red type and expressed in teragrams of P per year. Data from Schlesinger, W.H., 1997. Biogeochemistry: An analysis of global change, 2nd ed. London: Academic Press.

But there are a number of approaches available to control nutrient pollution in the Chesapeake Bay watershed, Mis- sissippi River watershed, and other waterways affected by eutrophication:

▶ Reducing fertilizer use on farms and lawns ▶ Changing the timing of fertilizer application to minimize

rainy-season runoff ▶ More effectively managing manure applications to farm-

land to reduce nutrient runoff ▶ Planting and maintaining vegetation “buffers” around

streams that trap nutrient and sediment runoff ▶ Using artificial wetlands to filter stormwater and farm runoff ▶ Restoring nutrient-absorbing wetlands along waterways ▶ Improving technologies in sewage treatment plants to

enhance nitrogen and phosphorus capture ▶ Restoring frequently flooded lands to reduce runoff ▶ Reducing fossil fuel combustion to minimize atmos-

pheric inputs of nitrogen to waterways

eating the tissues of other organisms. Phosphorus from dead or- ganisms or waste products in the oceans can precipitate into solid form and settle to the bottom in sediments, which are eventually compressed into sedimentary rock.

Human activities increase phosphorus concentrations in surface waters through substantial runoff of the phosphorus- rich fertilizers we apply to lawns and farmlands. A 2008 study determined that an average hectare of land in the Chesa- peake Bay region received a net input of 4.52 kg (10 lb) of phosphorus per year, promoting phosphorus accumulation in soils, runoff into waterways, and phytoplankton blooms and hypoxia in the bay. People also add phosphorus to waterways through releases of treated wastewater rich in phosphates from domestic use of phosphate detergents.

Tackling nutrient enrichment requires diverse approaches With our reliance on synthetic fertilizers for food pro- duction and fossil fuels for energy, nutrient enrichment of ecosystems will be a challenge for many years to come.

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peake Bay Foundation (CBF), a nonprofit organization dedicated to conserving the bay, sued the Environmental Protection Agency in January 2009 for failing to use its avail- able powers under the Clean Water Act to clean up the bay. The CBF’s lawsuit focused media attention on the plight of the bay, its ongoing water quality issues, and its depleted fisheries—and spurred action. In May 2009, President Obama directed the EPA and other federal agencies to es- tablish a comprehensive plan for the Chesapeake, and in May 2010 the EPA and the CBF announced a settlement in which the EPA agreed to provide aggressive pollution regu- lation in the bay. Thanks to the efforts of concerned citizens and advocacy organizations like the CBF, the 17 million people living in the Chesapeake Bay watershed have reason to hope that the Chesapeake Bay tomorrow may be healthier than it is today.

After 25 years of failed pollution control agreements and nearly $6 billion spent on cleanup efforts, the Chesa-

Nutrient Pollution and Its Financial Impacts A sizable amount of the nitrogen and phosphorus that enters the Chesapeake Bay originates from farms and other sources far

from the bay, yet it is people living near the bay, such as oystermen, who bear many of the negative impacts. Who do you believe should be responsible for address- ing this problem? Should environmental policies on this issue be developed and enforced by state governments, the federal government, both, or neither? Explain the reasons for your answer.

chemistry are tied to nearly every significant process in envi- ronmental science. Moreover, applications of chemistry can provide solutions to environmental problems involving agri- cultural practices, water resources, air quality, energy policy, and environmental health.

Thinking in terms of systems is important in under- standing how Earth works, so that we may learn not to disrupt its processes and how to mitigate any disruptions we cause. By studying the environment from a systems per- spective and by integrating scientific findings with the pol- icy process, people who care about the Chesapeake Bay are working today to solve their pressing problems.

➤ CONCLUSION Earth hosts many interacting systems, and the way we per- ceive them depends on the questions we ask. Life interacts with its abiotic environment in ecosystems, systems through which energy flows and materials are recycled. Understand- ing the biogeochemical cycles that describe the movement of nutrients within and among ecosystems is crucial, because human activities are causing significant changes in the ways these cycles function.

Understanding energy, energy flow, and chemistry en- hances our comprehension of how organisms interact with one another, how they relate to their nonliving environ- ment, and how environmental systems function. Energy and

7. List five ecosystem services provided by functioning ecosystems, and rank them according to your perceived value of each.

8. What role do each of the following play in the carbon cycle? ▶ Cars ▶ Photosynthesis ▶ The oceans ▶ Earth’s crust

9. Distinguish the function performed by nitrogen-fixing bacteria from that performed by denitrifying bacteria.

10. How has human activity altered the hydrologic cycle? The carbon cycle? The phosphorus cycle? The nitro- gen cycle? To what environmental problems have these changes given rise?

T E S T I N G Y O U R C O M P R E H E N S I O N

1. Which type of feedback loop is most common in nature, and which more commonly results from human action? For either type of feedback loop, can you think of an example that was not mentioned in the text?

2. Describe how hypoxic conditions can develop in aquatic ecosystems such as the Chesapeake Bay.

3. Differentiate an ion from an isotope. 4. Describe the two major forms of energy, and give

examples of each. Compare and contrast the first law of thermodynamics and the second law of thermodynamics.

5. What substances are produced by photosynthesis? By cellular respiration?

6. Describe the typical movement of energy through an ecosystem. Describe the typical movement of matter through an ecosystem.

2. Consider the ecosystem(s) that surround(s) your campus. How is each affected by human activities?

3. For a conservation biologist interested in sustaining populations of each organism below, why would it be

S E E K I N G S O L U T I O N S

1. Can you think of an example of an environmental prob- lem not mentioned in this chapter that a good knowl- edge of chemistry could help us solve? Explain your answer.

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Humans are considered omnivores because we can eat both plants and animals. The choices we make about what to eat have significant ecological impacts. With this in mind, cal- culate the ecological energy requirements for four different diets, each of which provides a total of 2,000 dietary calories per day.

C A L C U L A T I N G E C O L O G I C A L F O O T P R I N T S

In ecological systems, a rough rule of thumb is that when en- ergy is transferred from plants to plant-eaters or from prey to predator, the efficiency is only about 10% (pp. 68–69). Much of this inefficiency is a consequence of the second law of ther- modynamics. Another way to think of this is that eating 1 calorie of material from an animal is the ecological equivalent of eating 10 calories of plant material.

Diet Source of calories Number of

calories consumed Ecologically equivalent

calories Total ecologically

equivalent calories 100% plant Plant

0% animal Animal 90% plant Plant 1,800 1,800 3,800 10% animal Animal 200 2,000 50% plant Plant 50% animal Animal

0% plant Plant 100% animal Animal

helpful to take a landscape ecology perspective? Explain your answer in each case.

▶ A forest-breeding warbler that suffers poor nesting success in small fragmented forest patches

▶ A bighorn sheep that must move seasonally between mountains and lowlands

▶ A toad that lives in upland areas but travels cross- country to breed in localized pools each spring

4. A simple change in the flux between just two reser- voirs in a single nutrient cycle can potentially have ma- jor consequences for ecosystems and, indeed, for the globe. Explain how this can be, using one example from the carbon cycle and one example from the nitrogen cycle.

5. THINK IT THROUGH You are an oysterman in the Chesapeake Bay, and your income is decreasing because

hypoxic zones are making it harder to harvest oysters. One day your senator comes to town, and you have a one- minute audience with her. What steps would you urge her to take in Washington, D.C., to try to help alleviate the bay’s water quality problems and bring back the oyster fishery?

Now suppose that you are a Pennsylvania farmer who has learned that the federal government is insisting that you use 30% less fertilizer on your crops each year to reduce nutrient inputs to the Chesapeake. You know that in good growing years you could do without that fertiliz- er, and you’d be glad not to have to pay for it. But in bad growing years, you need the fertilizer to ensure a harvest so that you can continue making a living. And you must apply the fertilizer each spring before you know whether it will be a good or bad year. What would you tell your senator when she comes to town?

1. How many ecologically equivalent calories would it take to support you for a year, for each of the four diets listed?

2. How does the ecological impact from a diet consisting strictly of animal products (meat, eggs, milk, and other dairy products) compare with that of a strictly vegetar- ian diet? How many additional ecologically equivalent calories do you consume each day by including as little as 10% of your calories from animal sources?

3. What percentages of the calories in your own diet do you think come from plant versus animal sources? Estimate the ecological impact of your diet, relative to a strictly vegetarian one.

4. Describe some challenges of providing food for the grow- ing human population, especially as people in many poor- er nations develop a taste for an American-style diet rich in animal protein and fat.

Go to www.masteringenvironmentalscience.com for homework assignments, practice quizzes, Pearson eText, and more.

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