env.science week 3 energy flow

M ost of us live our lives seemingly apart from nature. We make our homes in cities and towns, surround ourselves with con- crete and steel, and drown out the songs of birds with noise.

The closest many of us get to nature is a romp with the family dog on the grass in the backyard. A lucky few come in much closer con- tact with the great outdoors through hiking, camping, canoeing, and kayaking. For many of these people, though, nature is still viewed as something apart from humans—a thing to protect to preserve a few pristine places for people to enjoy.

Humans and Nature: The Vital Connections Hard as it may be for many people to accept, human beings are part of the fabric of life. We are a part of nature. We are dependent on the Earth and natural systems in thousands of ways and are an integral part of the cycles of nature. Consider our de-

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Principles of Ecology: How Ecosystems Work

Humans and Nature: The Vital Connections Ecology: The Study of Natural Systems The Structure of Natural Systems Ecosystem Function Spotlight on Sustainable Development 4-1: Sustainable Sewage Treatment: Mimicking Nature Spotlight on Sustainable Development 4-2: Colleges and Universities Go Green Point/Counterpoint: Controversy over Extinction

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4.3

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CHAPTER OUTLINE

CHAPTER 4

Never does nature say one thing, and wisdom another. —Juvenal

50

CRITICAL THINKING

Exercise The information gained from various fields of science such as ecology is often loosely trans- lated in the public arena. Terms are some- times misinterpreted. Facts are taken out of context. New findings are given more cre- dence than they deserve, and old, disproved ideas remain in the popular thinking for a long time. As you read this chapter, make a list of terms, ideas, concepts, and facts you encounter that contradict what you thought was true.

pendence first by taking a look around the room in which you are sitting. Everything in that room comes from the Earth or a natural system. The clothes you wear, your morning tea or coffee, and even the cornflakes you ate for breakfast are products of the Earth—the soil, water, air, and plants.

Like all other species, humans depend on the soil, air, water, sun, and a host of living organisms to survive. Each year, in fact, human beings (and other animals) consume enormous quantities of oxygen, which is used in the cells of our bodies to break down food molecules to generate en- ergy. Oxygen is produced by plants and algae. Without these organisms, humans and other animals could not survive. Trees, grasses, and other plants also provide a host of addi- tional free services. For example, plants protect the water- sheds near our homes, preventing flooding and erosion. Swamps purify the water in streams and lakes—water many of us drink. Birds help to control insect populations.

Clearly, nature serves us well. Although many of us have isolated ourselves from nature, we still depend on nature in many ways. We have not emancipated ourselves from it at all. We also influence natural cycles, and therefore, as Chap- ter 1 explained, not only do we depend on nature, the fate of natural systems depends on us.

KEY CONCEPTS

Ecology: The Study of Natural Systems

This chapter explores ecology, the study of living organisms and the web of relationships that binds all of us together in nature. Professor Garrett Hardin, a world-renowned ecologist, wrote that ecology takes as its domain the entire living world. Ecologists study how organisms interact with one another and how they interact with the abiotic, or nonliving, components of the environment (sunlight, for example).

4.2

Humans are a part of nature, dependent on natural systems for a variety of economically important resources and ecological services essential to our survival and long-term prosperity.

Throughout this chapter, we explore our connections to the living world. We examine the ways in which human sys- tems depend on natural systems and the ways in which hu- mans affect them. One of the goals of this chapter is to help you understand how nature works and how we can work bet- ter with nature to create a sustainable future. You will find that a great many of the lessons learned from the study of ecol- ogy can be applied to human society. Before we begin our journey, however, let me say a few words about the term ecology.

Ecology is probably one of the most misused words in the English language. Ban- ners proclaim, “Save Our Ecology.” Speakers argue that “our ecology is in dan- ger,” and others talk about the “ecological movement.” These common uses of the word ecology are incorrect. Why?

Ecology is a branch of science. It describes and quanti- fies the web of interactions in the environment. But ecol- ogy is not synonymous with the word environment. Thus, we can save our ecology department and ecology textbooks, but we cannot save our ecology. Our ecology is not in dan- ger, but our environment is. You cannot join the ecology movement, but you would be a welcome addition to the environmental movement.

KEY CONCEPTS

The Structure of Natural Systems

In order to understand ecology, you must study the structure of natural systems. We begin with the biosphere.

The Biosphere The science of ecology focuses much of its attention on bi- ological systems, examining their components and inter- actions. The largest biological system is the biosphere (BI-oh-sfere), the skin of life on planet Earth. As shown in FIGURE 4-1, the biosphere forms at the intersection of air, water, and land. In fact, all living organisms consist of com- ponents derived from these three realms. The carbon atoms in body proteins, for example, come from carbon dioxide in the atmosphere. Carbon dioxide is captured by plants and made into food molecules by a process known as photo- synthesis. Animals eat plants; the food molecules then be- come the building blocks of proteins and other important molecules in animals. The minerals in the bones of animals

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Ecology is a field of science that seeks to describe relationships between organisms and their chemical and physical environment.

51

GO GREEN

Check out current efforts to green your campus and become an active participant! If your college or university isn’t ac- tively pursuing ways to go green, consider organizing an effort with key student orga- nizations, faculty members, and administrators. Chances are there are plenty of environ- mentally active faculty and staff that would love to help out.

52 PART II. Natural Systems/Human Systems: Searching for a Sustainable Relationship

come from the soil, again through plants. Water comes di- rectly from streams and lakes and plant matter.

The biosphere extends from the bottom of the ocean, ap- proximately 11,000 meters (36,000 feet) below the surface, to the tops of the highest mountains, about 9,000 meters (30,000 feet) above sea level. Although that may seem like a long way, it’s really not. In fact, if the Earth were the size of an apple, the biosphere would be about the thickness of its skin. Although life exists throughout the biosphere, it is rare at the extremes, where conditions for survival are less than optimum. Most living things are concentrated in a nar- row band extending from less than 200 meters (600 feet) below the surface of the ocean to about 6,000 meters (20,000 feet) above sea level.

The biosphere is a closed system, much like a sealed terrarium. By definition, a closed system receives no materi- als from the outside. The only outside contribution is sunlight, which is vital to the health and well-being of virtually all life.

Sunlight powers almost all life on the planet. Even the en- ergy released by the combustion of coal, oil, and natural gas (which we use to power our homes and factories) comes from sunlight that fell on the Earth several hundred million years ago.

Because the Earth is a closed system, all materials neces- sary for life must be recycled. The carbon dioxide you exhale,

for instance, may be used by a rice plant during photosyn- thesis next month in Indonesia. Those carbon dioxide mole- cules will be incorporated into carbohydrate produced by the plant and stored in the seed. Consumed by an Indonesian boy, the carbohydrate will be broken back down during cel- lular energy production. The carbon dioxide molecules are re- leased into the atmosphere. Without this and dozens of other recycling processes, all life on the planet would grind to a halt. Protecting the environment, then, helps to protect global recycling systems on which we all depend.

KEY CONCEPTS

Biomes and Aquatic Life Zones The biosphere consists of terrestrial and aquatic systems. Viewed from outer space, the Earth—the terrestrial portion of the biosphere—resembles a giant jigsaw puzzle, consisting of large landmasses among vast expanses of ocean. The land-

The biosphere is an enormous biological system, spanning the entire planet. The materials within this closed system are recy- cled over and over in order for life to be sustained. The only out- side contribution to the biosphere is sunlight, which provides energy for all living things.

Lithosphere (earth) Hydrosphere (water)

Atmosphere (air)

Heat

Sun

Heat Heat

FIGURE 4-1 The biosphere. The biosphere exists at the intersection of land, air, and water. Organisms derive essential minerals and other substances from these three spheres.

CHAPTER 4: Principles of Ecology: How Ecosystems Work 53

masses, or continents, can be divided into fairly large regions called biomes (FIGURE 4-2). A biome is a terrestrial portion of the biosphere characterized by a distinct climate and a particular assemblage of plants and animals adapted to it.1 Chapter 5 describes the biomes in detail. This section provides an overview.

In a biome, abiotic conditions—such as soil type, tem- perature, and rainfall—determine the plant communities that can survive. These, in turn, determine which animals can subsist. As illustrated in Figure 4-2, the North American continent contains seven major biomes, five of which are discussed here. Starting in the north is the tundra (TON-dra), a region of long, cold winters and rather short growing sea- sons (FIGURE 4-3a). The rolling terrain of the tundra sup- ports grasses, mosses, lichens, wolves, musk oxen, and other animals adapted to the bitter winter cold. Trees cannot grow on the tundra because of the short growing season and be-

cause the subsoil (called permafrost) remains frozen year round, preventing the deep root growth necessary for trees.

Immediately south of the tundra lies the taiga (TIE-ga), also known as the northern coniferous or boreal forest. The taiga’s milder climate and longer growing season result in a greater diversity and abundance of plant and animal life than exists on the tundra. Evergreen trees, bears, wolverines, and moose are characteristic species (FIGURE 4-3b).

East of the Mississippi River lies the temperate decid- uous forest biome, characterized by an even warmer cli- mate and more abundant rainfall (FIGURE 4-3c). Broad-leaved trees make their home in this biome. Opossums, black bears, squirrels, and foxes are characteristic animal species.

West of the Mississippi lies the grassland biome (FIG- URE 4-3d). Inadequate rainfall and periodic drought prevent trees from growing on the grasslands, except near rivers, streams, and human habitation. Over the years, deep-rooted grasses have evolved on the plains. These grasses can with- stand fire, drought, and grazing even in arid grasslands. Coy- otes, hawks, and voles are characteristic animal species.

Ice

Tropic of Cancer

Tropic of Capricorn

Equator

Tundra

Mountains (complex zonation)

Taiga (northern coniferous forest)

Temperate forest

Temperate grassland

Arid grassland, semidesert

Desert

Chapparal/Mediterranean

Tropical scrub forest

Tropical rain forest, tropical evergreen forest

Tropical deciduous forest

Tropical savanna, thorn forest

FIGURE 4-2 Biomes. This map shows the world’s major biomes.

1Although each biome has its specific climate, there can be con- siderable climatic variation within a given biome.

54 PART II. Natural Systems/Human Systems: Searching for a Sustainable Relationship

In the Southwest, where even less rain falls, is the desert biome (FIGURE 4-3e). In contrast to what many people think, the desert often contains a rich assortment of plants and animals uniquely adapted to aridity and heat. Cacti, mesquite trees, rat- tlesnakes, and a variety of lizards all make their home in this seemingly inhospitable environment. The scorching hot deserts

(a)

(c)

(b)

(d)

(e)

FIGURE 4-3 North America’s five major biomes. (a) Tundra, (b) taiga, (c) temperate deciduous forest, (d) grassland, and (e) desert.

P o p u la

tio n

Environmental gradientLow High

Optimum rangeZone of physiological stress

Zone of intolerance

Zone of physiological stress

Zone of intolerance

Greatest abundance

Organisms infrequent

Organisms absent

Organisms infrequent

Organisms absent

Lower limit of tolerance Upper limit of tolerance

FIGURE 4-4 Range of tolerance. Organisms can live within a range of conditions, called the range of tolerance, but they thrive in the optimum range.

CHAPTER 4: Principles of Ecology: How Ecosystems Work 55

of Saudi Arabia receive less moisture than the desert around Tucson and therefore contain far fewer plants and animals.

The oceans can also be divided into distinct zones, known as aquatic life zones. The aquatic equivalent of biomes, each of these regions has a distinct environment and characteristic plant and animal life adapted to condi- tions of the zone. Both freshwater and saltwater (marine) systems exist. Freshwater systems include lakes, rivers, ponds, and marshes. The four major marine aquatic life zones are coral reefs, estuaries (the mouths of rivers, where fresh and salt water mix), the deep ocean, and the conti- nental shelf, all of which are discussed in the next chapter.

Humans inhabit all biomes on Earth but are concen- trated in those in which conditions are mildest and most conducive to growing food. Within many biomes is a wide assortment of natural resources such as minerals and fuel. Al- though they are vital to our economies and well-being, these resources are but a fraction of the benefit we gain from the biome. Plants and animals, for example, provide food and great enjoyment to many. Microorganisms in the soils of the biome detoxify wastes and recycle nutrients, keeping the soil rich and productive. The soil itself serves as a growing medium for all plants—crops as well as forests. Besides pro- viding timber, trees provide oxygen, remove air pollutants, and protect the soil from erosion, thus reducing sediment pol- lution in surface waters. Vegetation also reduces flooding.

KEY CONCEPTS The biosphere consists of distinct regions called biomes and aquatic life zones, each with its own chemical and physical con- ditions and unique assemblage of organisms. Humans inhabit all biomes, but are most prevalent in those with the mildest climates.

What Is an Ecosystem? The biosphere is a chemical, physical, and biological sys- tem that encompasses the entire surface of the planet. There- fore, the biosphere is often referred to as a global ecological system, or ecosystem. If the biosphere is a global ecosystem, biomes are regional ecosystems. For the sake of conven- ience, ecologists often limit their studies to smaller portions of a biome—individual forests, ponds, or meadows. All of these ecosystems, no matter how big or small, consist of two components: the living or biotic components and the non- living or abiotic components. Numerous interactions exist among these various components.

KEY CONCEPTS

Abiotic Components of Ecosystems and the Range of Tolerance The abiotic components of an ecosystem are the physical and chemical factors necessary for life—sunlight, precipitation, temperature, and nutrients. In most ecosystems, the abiotic conditions vary during the day and often shift from one season to the next. To live in most ecosystems, then, or- ganisms must be able to survive a range of conditions. The range of conditions to which an organism is adapted is called its range of tolerance. As FIGURE 4-4 shows, organisms do best in the optimum range. Outside of that are the zones of phys- iological stress, where survival and reproduction are possi- ble but not optimal. Outside of these zones are the zones of intolerance, where life for that organism is implausible. As a rule, organisms that have a wide range of tolerance are

Ecosystems are biological systems consisting of organisms and their environment.

56 PART II. Natural Systems/Human Systems: Searching for a Sustainable Relationship

more widely distributed than organisms with a narrow range of tolerance.

Many examples of the range of tolerance can be given. Think of your own tolerance for temperature. Most people are comfortable around 23°C (70°F). When the temperature be- comes much warmer, you enter the zone of physiological stress. If it increases even more, death occurs if you can’t find a way to cool yourself down. This is the zone of intolerance. The same occurs at the lower end of the temperature scale.

It is important to note that the tolerance of any given species for an environmental factor may vary with the age of the organism. Newly hatched salmon, for instance, may be much more vulnerable to pollutants in the water than adult salmon are. The tolerance of individuals in any population also varies. Thus, some humans are more tolerant than oth- ers to heat or cold.

Humans often alter the abiotic components of their en- vironment. Dams, for example, create lakes in streambeds, altering water temperature and water flow, with devastating effects on native fish populations. Dams also seriously alter downstream conditions. In the Colorado River, for instance, native razorback suckers and other species that once thrived in the warm waters of the river are now endangered species (in danger of extinction) because of the large dams that re- lease extremely cold water from the bottom of the reservoir (FIGURE 4-5a). This water is too cold for the sucker and other natives (FIGURE 4-5b). (The Point/Counterpoint in this chap- ter gives two opposing views on human-caused extinction.)

Natural events can also alter the biotic and abiotic con- ditions of the environment. Floods, tropical storms, and vol- canic eruptions all change conditions for varying periods, with sometimes dramatic effects on native species.

KEY CONCEPTS Organisms thrive within a range of abiotic conditions; altering those conditions can have severe consequences and can even cause extinction.

Limiting Factors Although species are sensitive to all of the abiotic factors in their environment, the one factor that is in short supply, called a limiting factor, tends to regulate pop- ulation size. In freshwater lakes and rivers, dissolved phos- phate is a limiting factor. Phosphate is needed by plants and algae for growth, but phosphate concentrations are natu- rally low. As a result, plant and algal growth is held in check. When phosphate is added to a body of water—say, by de- tergents released in the effluent of a sewage treatment plant— plants and algae proliferate. Algae often form dense surface mats, blocking sunlight (FIGURE 4-6).

Plants that are rooted on the bottom of the water body may perish for lack of sunlight. Because these plants produce oxygen, their demise often causes oxygen levels in the deeper waters to decline, killing fish and other aquatic organisms. On land, precipitation tends to be the limiting factor. At any

FIGURE 4-5 Upsetting the ecological balance. (a) Dams like the mighty Glen Canyon Dam in Arizona not only inundate upstream ar- eas, they often change the water temperature in the downstream section. (b) The cool water flowing out of the bottom of the reser- voir created by Glen Canyon Dam has endangered the razorback sucker and several other native fish species.

(a)

(b)

FIGURE 4-6 Algal bloom. This pond is choked with algae due to the abundance of plant nutrients from human sources.

CHAPTER 4: Principles of Ecology: How Ecosystems Work 57

given temperature, the more moisture that falls, the richer the plant and animal life.

KEY CONCEPTS

Biotic Components Ecosystems consist of numerous or- ganisms, including bacteria, plants, fungi, and animals. These are the biotic components of the ecosystem. Organisms of the same species within a biome or aquatic life zone usually oc- cupy a specific region. This group of organisms is called a pop- ulation. In any given ecosystem, populations of different organisms exist together in an interdependent biological community.

All organisms in a community, including humans, are part of the web of life. Organisms interact in many ways. Some organisms prey on others. Others are preyed upon. Still others compete with fellow members of the commu- nity of life.

KEY CONCEPTS

Niche and Competition If asked to give a brief description of yourself, you would probably begin by describing the place where you live. You might discuss the work you do, the friends you have, and other important relationships that de- scribe your place in human society. Now that you’re dis- covering your part in nature, you may even describe your place in the community of life.

A biologist would do much the same when describing an organism. He or she would start with a description of the place where an organism lives—that is, its habitat. Next, he or she would describe how the organism fits into the ecosys- tem, its ecological niche, or simply niche. An organism’s niche consists of its relationships with its environment— both the abiotic and biotic components. This niche includes what an organism eats, what eats it, its range of tolerance for various environmental factors, and other important ele- ments. The niche of an organism is its functional role com- pared with the habitat, which is its “address.”

Organisms in a community occupy the same habitat, but most of them have quite different niches, a phenomenon that minimizes competition. The fact that organisms occupy separate niches provides for a wider use of an ecosystem’s re- sources, especially food.

Niches do overlap somewhat. For example, two species may feed on some of the same foods. Coyotes and foxes, for example, both feed on rabbits and mice. Coyotes also feed on larger prey, however—even deer. Foxes tend to feed on smaller prey, including reptiles and amphibians.

The more two species’ niches overlap, the more they compete. When niches overlap considerably, competition becomes intense, and one species usually suffers. If two

Organisms are the biotic components of ecosystems; they form an interdependent community of life.

Organisms require many different abiotic factors to survive, but one factor—the limiting factor—tends to be critical to survival and growth of a population. Altering concentrations of limiting factors can result in dramatic fluctuations in populations.

species occupy identical niches, competition will eliminate one of them. As a result, two species cannot occupy the same niche for long. This rule is called the competitive exclusion principle.

The concept of the niche is very important to the sus- tainable management of natural resources. For example, successful control of an insect pest on crops is best achieved through an understanding of the species’ niche. An analysis of an insect’s niche might show that certain birds or insects feed on the pest. By encouraging these beneficial species— say, by providing trees for the birds—farmers can reduce pest populations naturally. They could also save enormous sums of money on chemical pesticides and help protect the environment.

KEY CONCEPTS

Humans: Competitors Extraordinaire Humans, like other organisms, compete with one another for a variety of re- sources. People also compete with the many other species that share this planet with us. For example, we compete with sea otters and seals over salmon. When we graze a cow on a pas- ture, our livestock compete with woodchucks and rabbits for food.

The competition between people and other species is fairly lopsided. As noted in Chapter 3, humans possess a marked advantage over most other species. This edge on the competition stems primarily from our technological prowess—high-powered rifles, bulldozers, and chain saws being three examples. Fishing nets and sonar give commer- cial fishers a marked advantage over their two major com- petitors, seals and sea otters.

As the human population grows, as our demand for food and resources climbs, and as our technological prowess increases, our competitive advantage will only increase, but the effect may not be advantageous to human civilization in the long run. Already, commercial overfishing has de- pleted dozens of the world’s fisheries (Figure 2-6). Many other fisheries are now in danger, as noted in Chapter 2. Overfishing has had a ripple effect on other species, reduc- ing populations of seals and other fish-eating animals.

On every continent and in every nation, humans are outcompeting other species. According to various estimates, 40 to 100 species become extinct every day, largely because of tropical deforestation. Unless we do something, hundreds of thousands of species will become extinct in the next decade. Many scientists warn that none of us will be im- mune to the impacts of such widespread biological impov- erishment. Cutting down the rain forests, for example, could alter global climate. Why? Rain forests absorb massive amounts of carbon dioxide and thus help control atmo- spheric levels of this important greenhouse gas. As forests are destroyed, carbon dioxide levels increase, and the Earth’s atmosphere could become warmer (Chapter 20). We could feel the effect in higher utility bills to cool our homes because

Competition occurs between species occupying the same habi- tat if their niches overlap; although competition is a naturally occurring process, natural systems have evolved to minimize it.

58 PART II. Natural Systems/Human Systems: Searching for a Sustainable Relationship

POINT

Evolution is the process of change in gene pools through time. When one gene pool becomes reproductively independent, a new species has formed. Such speciation generates species; extinction takes them away. Simply put, extinction is a fail- ure to adapt to change, the termination of a gene pool, the end of an evolutionary line.

Extinction is a natural process. Most species that ever lived are now extinct. The 3 to 30 million species on Earth today are no more than 1 to 10% of the species that have evolved since life began about 3.5 billion years ago. So why are thoughtful people concerned about endangered species? History makes it clear that, given enough time, each species will become extinct.

The concern is that today the natural process of ex- tinction proceeds at an unnatural rate. Let us estimate by how much human activity has accelerated rates of extinction. The average life span of species is 1 to 10 million years. As- sume (to be conservative) that the average longevity of species of higher vertebrates is 1 million years. In round numbers, there are 13,000 species of birds and mammals. So, on average, one species ought to go extinct each century. However, between 1,600 and 2,000, at least 36 species of mammals and 94 species of birds became extinct—32 species per year, or 32 times the natural rate.

What does it mean to increase a rate by 32 times? Ex- ceed the 55 mph speed limit by 32-fold, and you are mov- ing 1,760 miles per hour, over twice the speed of sound. The difference between natural rates of extinction and present, human-influenced rates is analogous to the difference be- tween a casual drive and Mach 2! Is that a problem? You de- cide: Concern is a moral construct, not a scientific one.

Several human activities have contributed—mostly in- advertently—to accelerating rates of extinction. The dodo and the passenger pigeon were extinguished by overhunt- ing. Wolves and grizzly bears were exterminated over much of their ranges as threats to livestock. The black-footed fer- ret was driven to the verge of extinction because prairie dogs, its staple food, were poisoned as an agricultural pest. The smallpox virus was exterminated in the “wild” (but sur-

vives in a half dozen laboratories). This is the closest that humans have come to deliberate elimination of an organism, and note that thoughtful scientists with the power to destroy smallpox chose not to do so, electing instead to manage it with care.

Habitat change is the most important cause of endan- germent and extinction. Clearing forests for agriculture has decimated the lemurs of Madagascar. Pesticides led to the decline of the peregrine falcon. Introducing exotic species (like goats on the Galapagos and mongooses in Hawaii) dis- places native animals and plants. Developing the Amazon Basin is a habitat alteration, and a cause of extinction, on an unprecedented scale.

Many urge saving species for their aesthetic value. Whooping cranes are beautiful, and part of their beauty is that they are products of a marvelous evolutionary process. Most concern about accelerated extinction, however, stresses economic value. A tiny fraction of seed plants are used com- mercially. An obscure plant like jojoba eventually could be a source of oil more reliable than the Middle East. Wild grasses have yielded genes that have improved disease re- sistance in wheat. Some animals like musk ox, kudu, and whales could contribute protein to our diet. Species may have medical value; penicillin, after all, was once an ob- scure mold on citrus fruit. Some sensitive species are mon- itors of environmental quality, and the presence of healthy populations of many species may promote greater stability or resilience of ecosystems. Naturalist Aldo Leopold noted that humans have a way of “tinkering” with the ecosphere. But he suggested that the first rule of tinkering ought to be that one never throws away any of the parts.

“Extinction is forever” and impoverishes both ecosys- tems and the potential richness of human life. Borrowing again from Aldo Leopold, I believe our concern about un- naturally rapid extinction is part of a “right relationship” be- tween people and the landscapes that nurture and inspire them.

Biologist Sir Julian Huxley noted that “we humans find ourselves, for better or worse, business agents for the cosmic process of evolution.” We hold power over the fu- ture of the biosphere, the power to destroy or preserve. German philosopher Georg Hegel noted that freedom im- plies responsibility. I agree. The power to destroy species implies a responsibility to preserve them. The question of human-accelerated extinction boils down to a simple eth- ical question, “Does posterity matter?” Some of us have ethics that are human centered. We ask simply, “Do my children deserve a life as rich, with as much opportunity, as mine?” If they do, then we have the responsibility to choose restraint. (Perhaps my life has been rich enough without the dodo, but I am reluctant to make that judgment for future generations.)

Humans Are Accelerating Extinction David M. Armstrong Dr. David M. Armstrong is a mammolo- gist who taught environmental biology at the University of Colorado at Boulder and has written several books on mam- mals and ecology of the Rocky Moun- tain region.

Controversy over Extinction

CHAPTER 4: Principles of Ecology: How Ecosystems Work 59

COUNTERPOINT

Evolution is the formation of new species from preexisting ones by a process of adaptation to the environment. Evolu- tion began long ago and is still going on. During evolution those species better adapted to the environment replaced the less well-adapted species. It is this process, repeated year after year for millennia, that has produced the present mix- ture of wild species. Perhaps 95% of the species that once existed no longer exist.

Human activities have eliminated many wild species. The dodo is gone, and so is the passenger pigeon. The whoop- ing crane, the California condor, and many other species are on the way out. The bison is still with us because it is pro- tected, and small herds are raised in semicaptivity. The Pa- cific salmon remains because we provide fish ladders around our dams so it can reach its breeding places. The mountain goat survives because it lives in inaccessible places. But some thousands of other animal species, to say nothing of plants, are extinct, or soon will be.

Some nature lovers weep at this passing and collect money to save species. They make lists of animals and plants that are in danger of extinction and sponsor legislation to save them.

I don’t. What the species preservers are trying to do is stop the clock. It cannot and should not be done. Extinction is an inevitable fact of evolution, and it is needed for progress. New species continually arise, and they are better adapted to their environment than those that have died out.

Extinction comes from failure to adapt to a changing en- vironment. The passenger pigeon did not disappear because of hunting alone, but because its food trees were destroyed by land clearing and farming. The prairie chicken cannot find enough of the proper food and nesting places in the cul- tivated fields that once were prairies.

And you cannot necessarily introduce a new species, even by breeding it in tremendous numbers and putting it out into the wild. Thousands of pheasants were bred and set out year after year in southern Illinois, but in the spring of each year there were none left. Another bird, the capercaillie, is a fine, large game bird in Scandinavia, but every attempt to introduce it into the United States has failed. An introduced species cannot survive unless it is preadapted to its new environment.

A few introduced species are preadapted, and some make spectacular gains. The United States has received the English sparrow, the starling, and the house mouse from Eu- rope, and also the gypsy moth, the European corn borer, the Mediterranean fruit fly, and the Japanese beetle. The United States gave Europe the gray squirrel and the muskrat, among others. The rabbit took over in Australia, at least for a time.

The rabbit and the squirrel were successful on new con- tinents because their requirements are not as narrow as those of other species that failed. Today, adjustment to human-made environments may be just as difficult as adjustment to new continents. The rabbit and the squirrel have succeeded in adjusting to the backyard habitat, but most wild animals have disappeared.

Human-made environments are artificial. People replace mixed grasses, shrubs, and trees with rows of clean-cultivated corn, soybeans, wheat, oats, or alfalfa. Variety has turned into uniform monotony, and the number of species of small vertebrates and invertebrates that can find the proper food to survive has become markedly reduced. But some species have multiplied in these environments and have assumed eco- nomic importance; the European corn borer in this country is an example.

Would it improve Earth if even half of the species that have died out were to return? A few starving, shipwrecked sailors might be better off if the dodo were to return, but I would not be. The smallpox virus has been eliminated, ex- cept for a few strains in medical laboratories. Should it be brought back? Should we bring panthers back into the east- ern states? Think of all the horses that the automobile and tractors have replaced, and of all the streets and roads that have been paved and the wild animals and plants killed as a consequence. Before people arrived in America about 10,000 years ago, the animal–plant situation was quite dif- ferent. What should we do? Should we all commit suicide?

Evolution exists, and it goes on continually. People are here because of it, but people may be replaced someday. It is neither possible nor desirable to stop it, and that is what we are trying to do when we try to preserve species on their way out. It can be done, I think, but should we do it to them all? Or to just a few, as we are doing now?

Critical Thinking Questions 1. Summarize and critically analyze the key points of

both authors. Do you see any flaws in their reasoning? 2. Which viewpoint do you adhere to? Why?

Extinction Is the Course of Nature Norman D. Levine Norman D. Levine was a professor emer- itus at the College of Veterinary Medi- cine and Agricultural Experiment Station, University of Illinois at Ur- bana. His research interests included parasitology, protozoology, and human ecology. This article is provided by the American Institute of Biological Sciences (© 1989).

You can link to web sites that represent both sides through Point/Counterpoint: Furthering the Debate at this book’s internet site, http://environment.jbpub.com/9e/. Evaluate each side’s argument more fully and clarify your own opinion.

60 PART II. Natural Systems/Human Systems: Searching for a Sustainable Relationship

of noticeably hotter summers. If predictions come true, we could face much higher food costs as hotter summers re- duce crop production. If conditions became bad enough, massive food shortages could occur. Although all this human intrusion is rather depressing, there is hope. We can find ways of coexisting with nature. Understanding how natural systems operate and patterning human systems after sus- tainable natural systems could help us redesign our infra-

structure. See Spotlight on Sustainable Development 4-1 for an example.

KEY CONCEPTS Humans are a major competitive force in nature. Our advanced technologies and massive population size permit us to out- compete many species. Destroying other species through com- petition, however, can be disadvantageous in the long run.

SPOTLIGHT ON SUSTAINABLE DEVELOPMENT

4-1 Sustainable Sewage Treatment: Mimicking Nature

Residents of eastern Mexico City produce more than 300 tons of feces every day, much of which is deposited on city streets, vacant lots, and alleys. The feces dry and are often pulverized by cars and trucks. They soon become entrained into the city’s dust, creating a monumental health hazard all too common in many poor countries.

Since the advent of cities and towns, dealing with hu- man waste has proved to be a huge challenge. Even in the rich, industrialized countries of the world, modern waste treatment practices leave something to be desired. The liq- uid waste is chemically treated and dumped into streams and lakes in most cases. The solid material or sludge that remains after treatment is trucked off to landfills.

Traditional waste treatment methods not only pollute our environment, they also waste valuable nutrients, which come from the foods we eat and ultimately the soil on which crops are grown. In nature, plant and animal waste is re- turned to the soil, where it nourishes new plant life.

Over the years, scientists and others have sought more environmentally compatible—and sustainable—ways of dealing with human waste. Biologist John Todd has led this effort through the invention of waste disposal systems that mimic nature’s ways. Todd designed and built his first solar- powered sewage treatment plant at Sugarbush ski resort in Vermont. In this system, known as an Eco Machine, raw sewage enters a solar-heated greenhouse and then flows into cylinders, where naturally occurring bacteria convert the ammonia in the sewage into nitrate, a plant nutrient. The effluent then enters special channels where algae consume the nitrates. The algae are part of an artificial ecosystem con- taining freshwater shrimp (that feed on algae) and fish (that feed on the shrimp). Snails in the system consume the sludge (organic wastes that fall to the bottom) and also serve as food for fish.

At the far end of the greenhouse is a small marsh con- taining organisms that remove additional impurities before the water is released into a nearby stream (FIGURE 1). In this artificial marsh are many plants that absorb toxic substances.

In Providence, Rhode Island, Todd installed a much larger system that handles up to 16,000 gallons of raw sewage per day flowing through a greenhouse containing 1,200 aquariums, which house a variety of organisms to pu- rify the water. This system, which costs one-third as much as an equivalent sewage treatment plant, has negligible environmental impact. To date, Todd’s company, which is now operated by his son, has installed dozens of systems in 11 countries. To learn more you can log on to Todd’s website, www.toddecological.com.

Another pioneer in ecologically sound waste disposal is environmental engineer Bill Wolverton. In the mid-1970s, while working at NASA, Wolverton began experiments to pu- rify wastewater from NASA facilities using lagoons filled with prolific, nutrient-hungry water hyacinths, which have be-

FIGURE 1 Proponents of this new technology point out that malfunctions at conventional plants can be serious and some- times require the evacuation of nearby residents to avoid poi- sonous chlorine gas emitted from them. A malfunction in a solar-aquatic plant may kill off some organisms, but it poses no threat to people.

CHAPTER 4: Principles of Ecology: How Ecosystems Work 61

Ecosystem Function Life on land and in the Earth’s waters is possible principally because of the existence of the producers, organisms such as the algae and plants. These organisms absorb sunlight and use its energy to synthesize organic foodstuffs from at- mospheric carbon dioxide and water via photosynthesis. These organic molecules are used by the producers them-

4.4 selves, but they also provide nourishment for all the other organisms. Producers, therefore, form the foundation of the living world.

Another large group of organisms is the consumers. Ecologists place consumers into four general categories, depending on the type of food they eat. Some, such as deer, elk, and cattle, feed directly on plants and are called her- bivores (ERB-ah-voors). Others, such as wolves, feed on

come a “pest” in lakes, streams, and rivers in the southern United States. One system he designed for a 4,000-person NASA facility in Mississippi has saved the agency millions of dollars in sewage fees. Since he began, Wolverton has designed more than a hundred systems in the southern United States. The effectiveness of these systems is illustrated in Figure 2. This graph shows permitted discharges of three pollutants from a conventional sewage treatment plant and levels achieved at a Walnut Cove wastewater treatment facility, designed by Wolverton (FIGURE 2).

Researchers are also experimenting with smaller con- structed wetlands for biological treatment that we can use in our own backyards. Many are designed so waste water perco- lates through a bed of gravel under a layer of soil and plants, and thus, there is no way sewage could reach the surface.

Another pioneer in biological waste treatment is Tom Watson, who lives in New Mexico. Watson specializes in household waste treatment centers using inexpensive plas- tic infiltrators in a pumice bed (FIGURE 3). Plants growing

in the overlying soil send their roots into pumice, where they feed on water and nutrients from household waste, includ- ing toilet water. Bacteria in the pumice decompose organic matter, releasing nutrients for plants.

Biological treatment systems have pros and cons. Those being built by John Todd can operate in virtually any cli- mate. These systems can also be scaled up to any size sim- ply by adding more greenhouses. Watson’s systems and constructed wetlands can also function well in a variety of climates, but because they’re not protected from the weather, they may have problems in extremely cold climates.

Biological treatment systems, both large and small, are well suited to developing countries because they offer a low- cost option for treating waste. In fact, some developing na- tions are currently considering installing biological systems to upgrade existing facilities. They’re finding that the nec- essary upgrades can be made at moderate cost. Moreover, the facilities do not require as much management or use as

Pipe to daylight

6" soil

18" pumice

Pipe from house

Roots

Infiltrator

Inspection pipe

FIGURE 3 The Watson Wick filter. In this simple, effective set up waste water enters the pumice bed, where bacteria go to work on waste. Plant roots absorb nutrients and water. Water coming out of the system is relatively free of nutrients. Tests so far have shown them to be more effective than conventional sewage treatment methods.

*Data compiled from Walnut Cove wastewater treatment monitoring data.

Walnut Cove, NC Monitoring Data* 30

0

Pe rm

it

M id 19

96 19

97 19

98 19

99 20

00 20

01 20

02 20

03 20

04 20

05 20

06 20

07

5

10

15

20

25

(mg/l)

BOD5 TSS NH3-N

FIGURE 2 Graph of pollutants released from a conventional water treatment facility (Permit) and Walnut Cove wastewater treatment. [BODS is biological oxygen demand, a measure of organic waste. TSS is suspended solids, and NH3-N is nitrogen in the form of ammonia.]

62 PART II. Natural Systems/Human Systems: Searching for a Sustainable Relationship

herbivores and other animals and are known as carnivores (CAR-neh-voors). Humans and a great many other animal species subsist on a mixed diet of plants and animals and are known as omnivores (OM- neh-voors). Another group feeds on animal waste or the remains of plants and ani- mals and are called detritivores (dee-TREH-teh-voors) or decomposers. This group includes many bacteria, fungi, and insects.

KEY CONCEPTS

Food Chains and Food Webs Biological communities consist of numerous food chains. A food chain is a series of organisms, each one feeding on the organism preceding it (FIGURE 4-7). All organisms in the community are members of one or more food chains.

Biologists recognize two general types of food chains: grazer and decomposer. Grazer food chains begin with plants and algae. These organisms are consumed by herbivores, or grazers. Herbivores, in turn, may be eaten by carnivores.

Decomposer food chains begin with dead material— either animal wastes (feces) or the remains of plants and animals. These are consumed by insects, worms, and a host of microorganisms such as bacteria. These organisms are responsible for the decomposition of the waste and the return of its nutrients to the environment for reuse (recycling).

In ecosystems, decomposer and grazer food chains are tightly linked (FIGURE 4-8). Thus, waste from the grazer food chain enters the decomposer food chain. Nutrients liber- ated by the decomposer food chain enter the soil and water and are reincorporated into plants at the base of the grazer food chain.

Food chains exist only on the pages of textbooks; in a community of living organisms, food chains are part of a much more complex network of feeding interactions, food

Photosynthetic organisms such as plants and algae produce food within ecosystems. Their well-being is essential to the survival and well-being of all other species.

webs (FIGURE 4-9). Food webs present a complete picture of the feeding relationships in any given ecosystem.

As with so many other topics in ecology, an under- standing of food chains and food webs is essential to living sustainably on the planet. For instance, efforts to protect the ozone layer, which shields the Earth from harmful ultra- violet radiation, are important to protect people from cancer, but they are also important because they protect phytoplankton in the world’s oceans. Phytoplankton, small microscopic pho- tosynthesizers, form the base of aquatic food chains. Ultraviolet radiation can kill phytoplankton and thus cause a collapse of aquatic food chains.

KEY CONCEPTS

The Flow of Energy and Nutrients Through Food Webs Energy and nutrients both flow through food webs, but in very different ways. Let us begin with energy. As you just learned, solar energy is first captured by plants and then used to produce organic food molecules. Energy from the sun is then stored in these molecules. In the food chain, organic molecules pass from plants to animals. In both plants and animals, the molecules are broken down. This process releases stored solar energy, which is used to power numerous cellular activities.

During cellular energy release, a good portion of the energy stored in organic food molecules is lost as heat. Heat escaping from plants and animals is radiated into the atmo- sphere and then into outer space. It cannot be recaptured and reused by plants or animals. Because all solar energy is even- tually converted to heat, energy is said to flow unidirection- ally through food chains and food webs. Put another way, energy cannot be recycled.

In contrast, nutrients flow cyclically; that is, they are recycled. Nutrients in the soil, air, and water are first in-

Food and energy flow through food chains that are part of much larger food webs in ecosystems.

much energy as conventional systems. They even produce cleaner treated water than their high-tech counterparts—and at a lower cost.

One concern is that in large biological treatment fa- cilities plants may absorb heavy metals such as mercury and lead, which would need to be disposed of in a way that was safe. Todd’s colleague, Alan Liss, notes, “It’s better to have a small amount of highly toxic plants than a huge amount of moderately toxic sludge.” Eliminating such wastes from the waste stream through preventive measures would easily solve this problem.

Larger biological systems such as those being built by John Todd and Bill Wolverton require a level of biological

knowledge that few sewage treatment engineers have. They represent a radically different approach for society, even though natural systems have been purifying water for mil- lennia. This appearance of being different makes it difficult to convince city officials to give them a try when they know that existing technology will do the job. Some facilities produce enormous amounts of plant waste, sometimes con- taining hazardous substances including heavy metals that must be safely disposed of.

Despite their drawbacks, biological systems that mimic nature can be adapted for most or all situations, and their use will undoubtedly rise in the years to come as cities and towns look for more sustainable ways of treating waste.

CHAPTER 4: Principles of Ecology: How Ecosystems Work 63

Terrestrial

Aquatic

Grasshopper

Insect- eating birds

Hawks

Grass

Diatoms

Zooplankton

Small fish

Larger fish

Humans

FIGURE 4-7 Simplified grazer food chains. These drawings show terrestrial (land-based) and aquatic grazer food chains.

corporated into plants and algae and are then passed from plants to animals in various food chains. Nutrients in the food chain eventually reenter the environment through waste or the decomposition of dead organisms.

Every time you exhale, for example, you release carbon dioxide, a waste product of cellular energy production. Carbon dioxide reenters the atmosphere for reuse. Thus, through the act of breathing you play an important role in the global recycling sys- tem that makes life possible. Other wastes must be decomposed before releasing their nutrients. The feces of a rhinoceros, for example, are broken down by bacteria, which liberate carbon dioxide, nitrogen, and minerals.

Nutrients also reenter the environment through the decomposition of dead organisms. When a plant or animal dies, bacteria and fungi devour its organic remains. This process is called decomposition.

Plant and animal remains and waste

Decomposer food chain

Bacteria and fungi

Detritus feeders

Remains and waste

Plants

Grazer food chain

Rabbits

Coyotes

FIGURE 4-8 A grazer food chain and a decomposer food chain, showing the connection between the two.

64 PART II. Natural Systems/Human Systems: Searching for a Sustainable Relationship

Although these microorganisms absorb many nutrients released during this process, some nutrients escape and enter the soil and water for reuse. (Of course, when a bac- terium dies, it also breaks down, releasing nutrients into its environment.)

One way or another, nutrients eventually make their way back to the environment for reuse. As a result, each new generation of organisms relies on the recycling of material in the biosphere. Every atom in your body has been recycled since the beginning of life on Earth. Perhaps some of those atoms were in the very first cells.

KEY CONCEPTS

Trophic Levels Ecologists classify the organisms in a food chain accord- ing to their position, or trophic level (literally, “feeding” level). The producers are the base of the grazer food chain and belong to the first trophic level. The grazers are part

Food chains are biological avenues for the flow of energy and the cycling of nutrients in the environment. Energy flows in one direction through food chains, but nutrients are recycled.

of the second trophic level. Carnivores that feed on grazers are in the third trophic level, and so on.

Most terrestrial food chains are limited to three or four trophic levels. Longer terrestrial food chains are rare because food chains generally do not have a large enough producer base to support many levels of consumers. Why not?

Plants absorb only a small portion of the sunlight that strikes the Earth (only 1 to 2%), which they use to produce organic matter, or biomass. Technically, biomass is the dry weight of living material in an ecosystem. The biomass at the first trophic level is the raw material for the second trophic level. The biomass at the second trophic level is the raw ma- terial for the third trophic level, and so on.

As shown at the top of FIGURE 4-10, not all of the bio- mass produced by plants is converted to grazer biomass. At least three reasons account for the incomplete transfer of biomass from one trophic level to the next. First, some of the plant material, such as the roots, is not eaten. Second, not all of the material that the grazers eat is digested. Third, some of the digested material is broken down to produce energy and heat and therefore cannot be used to build bio- mass in the grazers (Figure 4-10). As a rule, only 5 to 20% of the biomass at any trophic level is passed to the next

FIGURE 4-9 A food web. Food chains are actually threads in larger food webs.

CHAPTER 4: Principles of Ecology: How Ecosystems Work 65

level. (The amount varies depending on the organisms in- volved in the food chain.)

When plotted on graph paper, the biomass at the vari- ous trophic levels forms a pyramid, the biomass pyramid (FIGURE 4-11). Because biomass contains energy (stored in the bonds between atoms), the biomass pyramid can be converted into a graph of the chemical energy in the vari- ous trophic levels. This graph is called an energy pyramid.

In most food chains, the number of organisms also de- creases with each trophic level, forming a pyramid of numbers.

Knowledge of ecological pyramids helps us understand why people in many developing countries subsist on a diet of grains (corn, rice, or wheat) rather than meat. As shown in FIGURE 4-12, in the grain → human food chain on the right, 20,000 kilocalories of grain can feed 10 people for a day. (A kilocalorie is the same unit as a calorie.) If that amount of grain

SPOTLIGHT ON SUSTAINABLE DEVELOPMENT

4-2 Colleges and Universities Go Green

All across North America, colleges and universities are tak- ing steps to green their campuses. Campus green is not a new color. Many colleges and universities have been recy- cling waste and taking other measures to reduce their im- pact on the environment for many years.

Today’s green movement is much deeper and greener, however, and is often designed to create a sustainable future. Over 300 colleges and universities in the United States, for instance, have joined the Campus Climate Challenge, aimed at reducing their contribution to global warming. They’re buying renewable energy and implementing energy-efficiency measures that dramatically lower their carbon emissions.

Many colleges and universities are also building all new classrooms and other facilities to much higher, more energy-efficient standards using green building materials— often thanks to student insistence (FIGURE 1). The Univer- sity of Vermont and the University of Denver, for instance, built green buildings to house their law schools. The Col- orado College, where I teach, built new green science and

art buildings. Dozens of others universities and colleges have followed suit.

The challenge of greening college campus can be enor- mous. In addition to dorms and classroom buildings, insti- tutions of higher learning also maintain and operate office buildings, laundry service, food service, vehicle repair and maintenance, healthcare facilities, bookstores, restaurants, and even their own power plants. They’re like a small city!

Leadership varies too. At most schools, students have taken the lead. They can often create change that would be nearly impossible for faculty and administration. At other schools, the administration and faculty have led the charge. Some colleges and universities have even appointed sus- tainability specialists who coordinate all sustainability ac- tivities on campus. Sustainability specialists are helpful to ensure continuity because the most avid supporters of such movements, the students, are on campus for a rather short period—either two or four years—and the task of greening a campus can take many years to accomplish.

Some governing bodies, which are in charge of entire university systems, like the University of California’s Re- gents, have also taken a leadership role in campus green- ing. The UC Regents, for instance, established a Green Building Policy and Clean Energy Standard for all of the schools in the university system.

Financing green initiatives can be a challenge. With ris- ing tuition costs, the administrations of many schools are reluctant to raise fees. Students, however, have actively pursued this avenue, pushing for small increases in fees to fund renewable energy and energy efficiency. Such cam- paigns are almost always overwhelmingly supported by the student body.

Students and faculty interested in learning more can find direction through the National Wildlife Federation’s Campus Ecology Program. Another excellent resource is the Campus Sustainability Assessment Framework, which lists 170 so- cial, environmental, political, and economic indicators to assess the sustainability of a college or university campus.

Although greening a campus can be a challenge, it does help colleges walk their talk. And it provides good hands-on experience for students who will someday work for corpora- tions that are looking for ways to green their operations, too.

FIGURE 1 The Tutt Science building at Colorado College, where the author is a visiting professor, is LEED-certified with many green features, including energy-efficient design, energy- efficient lighting, water-efficient bathrooms, and a host of environmentally friendly building materials.

66 PART II. Natural Systems/Human Systems: Searching for a Sustainable Relationship

is fed to a steer and the beef is then fed to humans, however, only one person can subsist on the original 20,000 kilocalo- ries. Why? In the grain → steer → human food chain, the 20,000 kilocalories fed to the cow that day produce only 2,000 kilocalories of food, barely enough to feed one person for a day (assuming a 10% transfer of biomass). Although people don’t eat meat-only diets, this simplified example does illustrate a key point: the shorter the food chain, the more food is available to top-level consumers. (This is not a criticism of the cattle industry. Cattle often feed on grasses that grow on land too poor to support crops, although they are often fat- tened on grain for a year or so before being slaughtered.)

This simple rule has profound implications for the human race. The human population increases by about 73 to 74 mil- lion people a year. Feeding these people poses an enormous challenge. How can new residents be fed most efficiently?

The most efficient food source will be crops such as corn, rice, and wheat that are fed directly to people. It is far less efficient to feed corn and other grains to cattle and other livestock that are slaughtered for human consumption. Vegetarianism, say proponents, is not only good for your health but good for the environment because it requires less grain production than a nonvegetarian diet. It should be pointed out, however, that vegetarianism is not envi- ronmentally benign. Plowing former grasslands or forests

to grow food for people has a profound impact on the environment. Fertilizer and pesticide use can have an enormous negative effect as well. Many wild animals are killed to grow grains.

Eating lower on the food chain is not always possible or advisable. In the case of grain-fed cattle, it may be both; but range-fed beef or elk are quite different. They feed on plants that are, of course, not edible for humans, and they live on land that would be poor farmland. Consuming meat from these sources may be a better option from an environmental perspective.

KEY CONCEPTS The position of an organism in a food chain is called its trophic level. Producers are on the first trophic level. Herbivores are on the second level. Carnivores are on the third level. The length of a food chain is limited by the loss of energy from one trophic level to another. The largest number of organisms is generally supported by the base of the food chain, the producers.

GO GREEN

Consider becoming a vegetarian or at least increasing your con- sumption of vegetables and re- ducing your intake of meat. A high vegetable diet is not only healthy, it can be better for the environment, as explained in the text.

Biomass of producers

Biomass of producers

Consumed

Not consumed

Biomass of first-order consumers

Biomass of first-order consumers

Biomass of second-order consumers

Second-order consumers

HeatHeat

First-order consumers

Energy for body heat/activity

Body building/growth

Undigested

Decomposer food chain

Consumed

Not consumed

Digested

Undigested (fecal wastes)

D ig

e st

e d

Fecal wastes

Biomass of second-order consumers

FIGURE 4-10 Flow of energy and biomass through a food chain. Not all biomass from one trophic level ends up in the next level, for reasons shown in the diagram and discussed in the text. Note that all energy is eventually lost as heat.

CHAPTER 4: Principles of Ecology: How Ecosystems Work 67

Carnivore Herbivores

Third World inhabitants

Feeds 10 humans 2000 kilocalories each

Meat-based

Steer

Feeds one human

Grains Grains

Herbivore 2,000 kilocalories

Producer Producer

20,000 kilocalories 20,000 kilocalories(a) (b)

FIGURE 4-12 Energy pyramids in two food chains. (a) The typical meat-based diet. The 20,000 kilocalories of corn fed to cows produces only 2,000 kilocalories of meat. An adult needs only about 2,000 calories per day. (b) In a shorter food chain, 20,000 kilocalories can feed 10 people directly. This is the reason many people in developing nations subsist pri- marily on a vegetarian diet. Although few, if any, people eat a meat-only diet, this example does illustrate an important point: more food is available to those societies that eat lower on the food chain.

Secondary consumers

Primary consumers

Producers

Biomass pyramidBiomass

Biomass of third trophic level

= Total combined weight of all carnivores

Biomass of second trophic level

= Total combined weight of all herbivores

Biomass of first trophic level

= Total combined weight of all producers

Bars show the relative biomass at each trophic level.

FIGURE 4-11 Biomass pyramid. In most food chains, biomass decreases from one trophic level to the next higher one.

68 PART II. Natural Systems/Human Systems: Searching for a Sustainable Relationship

Nutrient Cycles The sustainability of natural systems results primarily from their dependence on the sun and their reliance on the recy- cling of nutrients, which ensures an adequate supply of life’s essential ingredients. The term nutrients is used here to refer to all ions (charged atoms) and molecules used by liv- ing organisms.

In ecosystems, nutrients flow from the environment through food webs but are eventually released back into the environment. This circular flow constitutes a nutrient cycle, also known as a biogeochemical cycle.

Nutrient cycles can be divided broadly into environ- mental and organismic phases (FIGURE 4-13). In the envi- ronmental phase, a nutrient exists in the air, water, or soil, or sometimes in two or more of them simultaneously. In the organismic phase, nutrients are found in the biota—the plants, animals, and microorganisms.

Dozens of global nutrient cycles operate continuously to ensure the availability of chemicals vital to all living things, pres- ent and future. Unfortunately, however, a great many human activities disrupt nutrient cycles. These activities can pro- foundly influence the survival of species, including our own.

This section examines two of the most important nutrient cycles, the carbon and nitrogen cycles, and the ways they are being altered.

KEY CONCEPTS

The Carbon Cycle The carbon cycle is shown in FIG- URE 4-14 in a slightly simplified form. To understand how it operates, we begin with carbon dioxide. In the envi- ronmental phase of the cycle, carbon dioxide resides in two reservoirs, or sinks: the atmosphere and the surface wa- ter (oceans, lakes, and rivers). As illustrated, atmospheric carbon dioxide is absorbed by plants and other photo- synthetic organisms in terrestrial ecosystems, thus enter- ing the organismic phase of the cycle. These organisms convert carbon dioxide into organic food materials, which are passed along the food chain. Carbon dioxide reenters the environmental phase via cellular energy production (cellular respiration) of the organisms in the grazer and decomposer food chains.

For tens of thousands of years, our ancestors lived in relative harmony with nature. Because their numbers were small and their technology fairly primitive, they had little impact on the environment. With the advent of the Industrial Revolution, however, human beings began

Nutrients are recycled in global nutrient cycles. In these cycles, nutrients alternate between organisms and the environment. Humans can disrupt nutrient cycles in many ways, with pro- found impacts on ecosystems and our own future.

Or gan

ismic Phase

Phase

Environmen tal

Producers

PO4

PO4

NO3

CO2

CO2

CH4

N2

Primary consumers

Secondary consumers

Waste and remains

Decay (organic molecules)

Nutrients in physical environment (air, water, and/or land)

NO3

N2

FIGURE 4-13 Nutrient cycle. Nutrients exist in organisms and their abiotic environment; they cycle back and forth between these two components of the ecosystem. CO2 � carbon dioxide, N2 � nitro- gen, PO4 � phosphate, NO3 � nitrate, and CH4 � methane.

CHAPTER 4: Principles of Ecology: How Ecosystems Work 69

to interfere with natural processes on a large scale. One of the victims has been the global carbon cycle. The wide- spread combustion of fossil fuels (which releases carbon dioxide) and rampant deforestation (which reduces car- bon dioxide uptake) have overloaded the cycle with carbon dioxide.

For many years before the Industrial Revolution, global carbon dioxide pro- duction equaled carbon dioxide absorption by plants and algae. Today, 7 billion tons of carbon dioxide are added to the atmosphere each year. Three-quarters of the in- crease results from the combustion of fossil fuels such as the gasoline in our cars; the remaining

quarter stems from deforestation, which reduces the amount of carbon dioxide absorbed by the planet’s plants. Making matters worse, many forests are burned after cut- ting, further adding to the carbon dioxide levels in the atmosphere.

In the past 100 years, global atmospheric carbon diox- ide levels have increased by around 30%. In the atmo- sphere, carbon dioxide traps heat escaping from Earth and reradiates it to the Earth’s surface. As carbon dioxide lev- els increase, global temperatures rise. Such a rise could shift rainfall patterns, destroy agricultural production in many regions, and wipe out thousands of species. A rising global temperature might cause glaciers and the polar ice caps to melt, raising the sea level and flooding many low- lying coastal regions. Fortunately, there are many cost- effective strategies for reducing our dependence on fossil fuel. The most notable are energy efficiency and the use of renewable fuel—solar energy and wind, for example. Tran- sitioning to these clean, reliable, and cost-effective fuels could help us ensure a healthy economy, continued pros- perity, and a better future.

Tertiary consumers

Carbon dioxide in atmosphere

Terrestrial and aquatic plants

Carbon dioxide returned

to atmosphere

Carbon dioxide returned

to atmosphere

Secondary consumers

Respiration (and fuel

consumption)

Waste and

remains

Grazer food chain Decomposer food chain

Primary consumers

FIGURE 4-14 A simplified view of the carbon cy- cle. Carbon dioxide in the atmosphere is absorbed by plants and passed through the food chain. It is released back into the environment as a result of the decomposition of the waste and dead remains of plants, animals, and other organisms. It is also released by cellular energy production and the com- bustion of organic materials such as coal, oil, gaso- line, and wood.

GO GREEN

In the summer, turn the ther- mostat setting up a little at night and when you are away from your home or apartment, for instance, from 78 to 82°F. In the winter, turn the heat down a little when you are sleeping or away, for instance, from 68 to 60–62°F. Doing so can make a substantial dent in your heating and cooling costs and reduce carbon dioxide emissions, helping to combat global warming.

70 PART II. Natural Systems/Human Systems: Searching for a Sustainable Relationship

KEY CONCEPTS

The Nitrogen Cycle Nitrogen is an element that is essen- tial to many important biological molecules, including amino acids, DNA, and RNA. The Earth’s atmosphere contains enormous amounts of it, but atmospheric nitrogen is in the form of nitrogen gas (N2), which is unusable to all but a few organisms. As a result, atmospheric nitrogen must first be converted to a usable form, either nitrate or ammonia.

The conversion of nitrogen to ammonia is known as ni- trogen fixation. It occurs in terrestrial and aquatic environ- ments. As FIGURE 4-15 shows, the roots of leguminous plants (peas, beans, clover, alfalfa, vetch, and others) contain small swellings called root nodules. Inside the nodules are bacteria that convert atmospheric nitrogen to ammonia (ammonium ions). Ammonia is also produced by bacteria called cyanobac- teria that live in the soil. Once ammonia is produced, other soil bacteria convert it to nitrite and then to nitrate. Nitrates

The carbon cycle is vital to the survival of the Earth’s many species. It is the basis of food and energy production in the living world. It is also vital to maintaining global temperature. The carbon cy- cle is currently being flooded with excess carbon dioxide as a re- sult of the combustion of fossil fuels and deforestation, which could have devastating effects on climate and ecosystems.

are incorporated by plants and used to make amino acids and nucleic acids. All consumers ultimately receive the nitro- gen they require from plants.

Nitrate in soil also comes indirectly from the decay of animal waste and the remains of plants and animals. As shown on the right side of Figure 4-15, decomposition returns am- monia to the soil for reuse. Ammonia is converted to nitrite, then to nitrate and reused. Some nitrate, however, may be converted to nitrite and then to nitrous oxide (N2O) by de- nitrifying bacteria, as illustrated in Figure 4-15. Nitrous oxide is converted to nitrogen and released into the atmosphere.

Humans alter the nitrogen cycle in at least four ways: (1) by applying excess nitrogen-containing fertilizer on farmland, much of which ends up in waterways; (2) by disposing of nitrogen-rich municipal sewage in water- ways; (3) by raising cattle in feedlots adjacent to water- ways; and (4) by burning fossil fuels, which release a class of chemicals known as nitrogen oxides into the atmosphere. The first three activities increase the con- centration of nitrogen in the

Consumers (primary, secondary,

and tertiary)

Free nitrogen gas (N2 in atmosphere)

Terrestrial and aquatic plants

Decay organisms (aerobic and anaerobic

bacteria and fungi)

Waste and remains

Proteins from dead cells

Ammonification Fixation Nitrification Denitrification

N2

Nitrates (NO3

–)

Nitrites (NO2

–)

NO2 –

N2O in topsoil

Denitrifying bacteria

Ammonium (NH4

+)

Nitrogen-fixing bacteria and

cyanobacteria

3

4

5 6

7

8

1

2

FIGURE 4-15 A simplified view of the nitrogen cycle. Nitrogen in the atmosphere is converted into ammonium ions by bacteria in the soil. Ammonia is converted to nitrates and taken up by plants.

GO GREEN

Buy your parents a program- mable thermostat and help them install it in their home. When used correctly it can cut heating and cooling costs by 10% per year.

CHAPTER 4: Principles of Ecology: How Ecosystems Work 71

soil or water, upsetting the ecological balance. Nitrogen ox- ides released into the atmosphere by power plants, automo- biles, and other sources are converted to nitric acid, which falls with rain or snow. Besides changing the pH (acidity) of soil and aquatic ecosystems, nitric acid also adds nitrogen to surface waters and may be responsible for 25% of the nitro- gen pollution in some coastal waters in the United States.

Nitrogen, like phosphorus, is a plant nutrient. It stim- ulates the growth of aquatic plants and causes rivers and lakes to become congested with dense mats of vegetation, making them unnavigable. Sunlight penetration to deeper lev-

els is also impaired by the growth of plants, causing oxygen levels in deeper waters to decline. In the autumn, when aquatic plants die and decay, oxygen levels may fall further, killing aquatic life.

Nothing can survive on the planet unless it is a cooperative part of a larger global life.

—Barry Commoner

CRITICAL THINKING AND CONCEPT REVIEW 1. “Humans are a part of nature.” Do you agree or dis-

agree with this statement? Support your answer. 2. Define the term ecology and give examples of its proper

and improper use. 3. The Earth is a closed system. What does this mean, and

what are the implications of this fact? 4. Define the following terms: biosphere, biome, aquatic

life zone, and ecosystem. 5. Define the term range of tolerance. Using your knowl-

edge of ecology, give some examples of ways in which humans alter the abiotic and biotic conditions of cer- tain organisms. Describe the potential consequences of such actions.

6. Describe ways in which humans alter conditions within their own range of tolerance.

7. What is a limiting factor? Give some examples. 8. Define the following terms: habitat, niche, producer,

consumer, trophic level, food chain, and food web. 9. A hunting advocate in your state is proposing the in-

troduction of a foreign species, one very similar to deer, that he encountered in Russia on a hunting expe- dition. He thinks the introduced species will provide additional hunting opportunities and additional tax

revenue for the state, which will be good for the econ- omy. The governor is in favor of the proposal. Write a letter to the governor explaining what needs to be known about this species before it should be consid- ered for introduction.

10. Explain why the biomass at one trophic level is less than the biomass at the next lower trophic level.

11. Outline the flow of carbon dioxide through the carbon cycle, and describe ways in which humans adversely in- fluence the carbon cycle.

12. Using what you have learned about ecology, describe why it is important to protect natural ecosystems and other species.

13. With the knowledge you have gained, explain why it is beneficial to set aside habitat to protect an endan- gered species.

14. Looking back over the principles you have learned in this chapter, write a set of guidelines for human society that would help us live sustainably on the Earth.

15. Reread the Point/Counterpoint in this chapter. Which view do you agree with? Why? Is your support of one view based on values or science, or both?

CRITICAL THINKING

Exercise Analysis It is difficult for me to know the list of misconceptions you came up with, but here are a few that might have arisen along the way. The exact meaning of the term ecology may have been one. Many people use it very loosely. You may not have understood the term niche, either. Like many people, you may have thought of it as a physical place an organism occupied, its habitat.

I suspect that your sense of the word environment has shifted. Now you can see that it includes biotic and abiotic components. The environment of an organism is quite complex.

I suspect that the discussion of the range of tolerance may have helped you to understand how our activities and natural events alter an organism’s chances of survival.

What about energy? Has your understanding of energy changed? Do you see food chains and food webs now in a different light? Rather than being a simple matter of one organism feeding on another, food chains and food webs are elaborate pathways for the flow of nutrients and energy.

You may have gained new perspective on nutrient cycles. You may already have known that they existed, but did you know that humans are altering them in major ways?

72 PART II. Natural Systems/Human Systems: Searching for a Sustainable Relationship

KEY TERMS abiotic aquatic life zones biogeochemical cycle biological community biomass biomass pyramid biome biosphere biotic carnivores closed system competitive exclusion principle consumers decomposer food chains decomposers decomposition desert biome detritivores

ecological niche ecological system ecology ecosystem endangered species energy pyramid environmental phase food chain food webs grassland biome grazer food chains habitat herbivores limiting factor niche nitrogen fixation northern boreal forest northern coniferous forest

nutrient cycle nutrients omnivores optimum range organismic phase photosynthesis population producers pyramid of numbers range of tolerance taiga temperate deciduous forest biome trophic level tundra zones of intolerance zones of physiological stress zones of tolerance

Connect to this book's website: http://environment.jbpub.com/9e/ The site features eLearning, an online review area that provides quizzes, chapter outlines, and other tools to help you study for your class. You can also follow useful links for in-depth information, research the differing views in the Point/Counterpoints, or keep up on the latest environmental news.

REFERENCES AND FURTHER READING To save on paper and allow for updates, additional reading recommendations and the list of sources for the information discussed in this chapter are available at http://environment .jbpub.com/9e/.