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Coastal marsh. 'Wetlands in the Atchafalaya National Wildlife Refuge in Louisiana provide valuable ecosystem services.

2 Communities and Ecosystems

Campbell Essential Biology with Physiology, Fourth Edition, by Eric J. Simon, Jean L. Dickey, and Jane B. Reece. Published by Benjamin Cummings. Copyright CD, 2013 by Pearson Education, Inc.

CHAPTER CONTENTS

The Loss of Biodiversity 426

Community Ecology 428

Ecosystem Ecology 437

Conservation and Restoration Biology 444

CHAPTER THREAD Biodiversity in Decline

BIOLOGY AND SOCIETY Why Biodiversity Matters 425

THE PROCESS OF SCIENCE How Does Tropical Forest Fragmentation

Affect Biodiversity? 446

EVOLUTION CONNECTION Can Biophilia Save Biodiversity? 448

Biodiversity in Decline BIOLOGY AND SOCIETY

Why Biodiversity Matters As the human population has expanded, hundreds of species have become extinct and thou- sands more are threatened with extinction. These changes represent a loss in biological di- versity, or biodiversity. Biodiversity loss goes hand in hand with the disappearance of natural ecosystems. Only about a quarter of Earth's land surfaces remain untouched by human altera- tions. We see the evidence of our impact on natural ecosystems every day. We live and work in altered landscapes. And though we may be less aware of it, our impact on the oceans is also extensive.

What is the value of biodiversity? Most people appreciate the direct benefits provided by certain ecosystems. For example, you probably know that we use resources—such as water, wood, and fish—that come from natural or near-natural ecosystems. These resources have economic value, as the massive 2010 oil spill in the Gulf of Mexico dramatically demonstrated. Billions of dollars were lost by fishing, recreation, and other industries as a result of the di- saster. But human well-being also depends on less obvious services that healthy ecosystems provide. The coastal wetlands affected by the Gulf oil spill normally act as a buffer against hur- ricanes, reduce the impact of flooding, and filter pollutants. The wetlands also furnish nesting sites for birds and marine turtles and breeding areas and nurseries for a wide variety of fish and shellfish. Natural ecosystems provide other services as well—such as recycling nutrients, preventing erosion and mudslides, controlling agricultural pests, and pollinating crops. Some scientists have attempted to assign an economic value to these benefits. They arrived at an average annual value of ecosystem services of $33 trillion, almost twice the global gross national product for the year they published their results. Although rough, these estimates make the important point that we cannot afford to take biodiversity for granted.

In this chapter, we'll examine the interactions among organisms and how those relation- ships determine the features of communities. On a larger scale, we'll explore the dynamics of ecosystems. Finally, we'll consider how scientists are working to save biodiversity. And throughout the chapter, you'll learn how an understanding of ecology can help us manage Earth's resources wisely.

Campbell Essential Biology with Physiology, Fourth Edition, by Eric J. Simon, Jean L. Dickey, and Jane B. Reece. Published by Benjamin Cummings. Copyright C 2013 by Pearson Education, Inc.

CHAPTER 20 COMMUNITIES AND ECOSYSTEMS

V Figure 20.1 Einkorn wheat, one of the wild relatives of modern cultivated varieties.

HI CHECKPOINT

How does the loss of genetic diversity endanger a population?

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The Loss of Biodiversity Species Diversity As mentioned previously, biodiversity is short for biological diversity, the variety of living things that you

learned about in Unit 3. It includes genetic diversity, species diversity, and ecosystem diversity. Thus, the loss of biodiversity encompasses more than just the fate of individual species.

Genetic Diversity When species are lost, so are their unique genes. The genetic diversity within a population is the raw material that makes microevolution and adaptation to the environment possible. If local populations are lost, then the number of individuals in the species declines, and so do the genetic resources for that species. Severe reduc- tion in genetic variation threatens the survival of a species. The enormous genetic diversity of all the organisms on Earth has great potential benefit for people, too. Many researchers and biotechnology leaders are enthusiastic about the potential that genetic "bioprospecting" holds for future development of new medicines, industrial chemicals, and other products. Bioprospecting may also hold the key to the world's food supply. For example, researchers are currently scrambling to stop the spread of a deadly new strain of wheat stem rust, a fungal pathogen that has devastated harvests in eastern Africa and central Asia. At least 75% of the wheat varieties planted worldwide are susceptible to this pathogen, but researchers hope to find a resis- tance gene in the wild relatives of wheat (Figure 20.1). F4

In view of the damage we are doing to the biosphere, ecologists believe that we are pushing species toward extinction at an alarming rate. The present rate of spe- cies loss may be as much as 100 times higher than at any time in the past 100,000 years. Some researchers esti- mate that at the current rate of destruction, over half of all currently living plant and animal species will be gone by the end of this century. Figure 20.2 shows two recent victims. Here are some examples of where things stand:

• Approximately 12% of the 10,027 known bird species and 21% of the 5,490 known mammalian species in the world are threatened with extinction.

• More than 20% of the known freshwater fishes in the world have either become extinct during human history or are seriously threatened.

• Roughly 30% of all known amphibian species are either near extinction or endangered.

• Of the approximately 20,000 known plant species in the United States, 200 species have become ex- tinct since dependable records have been kept, and 790 species are endangered or threatened.

T Figure 20.2 Recent additions to the list of human-caused extinctions.

A Chinese river dolphin. Last sighted in 2002, this former resident of the Yangtze River is presumed extinct after a thorough 2006 search failed to find any.

Golden toads. These small frogs, unique to high-altitude Costa Rican cloud forests, have not been seen since 1989. The males shown here are gathered near a puddle to wait for mates.

426

THE LOSS OF BIODIVERSITY Ecosystem Diversity Ecosystem diversity is the third component of biological diversity. Recall that an ecosystem includes both the organisms and the abiotic factors in a particular area. Because of the network of interactions among popula- tions of different species within an ecosystem, the loss of one species can have a negative effect on the entire ecosystem. The disappearance of natural ecosystems results in the loss of ecosystem services, functions performed by an ecosystem that directly or indirectly benefit people. These vital services include air and water purification, climate regulation, and erosion control. For example, forests absorb and store carbon from the atmosphere, a service that vanishes when forests are destroyed or degraded (see Figure 18.39). Coral reefs not only are rich in species diversity (Figure 20.3) but also provide a wealth of benefits to people, including food, storm protection, and recreation. An estimated 20% of the world's coral reefs have already been de- stroyed by human activities. A study published in 2011 found that 75% of the remaining reefs are threatened, a percentage expected to top 90% by 2030 if current abuses continue. IM

V Figure 20.3 A coral reef, a colorful display of biodiversity.

Causes of Declining Biodiversity Ecologists have identified four main factors responsible for the loss of biodiversity: habitat destruction and fragmenta- tion, invasive species, overexploitation, and pollution. The ever-expanding size and dominance of the human popula- tion are at the root of all four factors. In addition, scientists expect global climate change to become a leading cause of extinctions in the near future (see Chapter 18).

Habitat Destruction The massive destruction and fragmentation of habitats caused by agriculture, urban development, forestry, and mining pose the single greatest threat to biodiversity (Figure 20.4). According to the International Union for the Conservation of Nature, which compiles informa- tion on the conservation status of species worldwide, habitat destruction affects more than 85% of all birds, mammals, and amphibians that are threatened with extinction. We'll take a closer look at the consequences of habitat fragmentation later in this chapter.

Invasive Species Ranking second behind habitat loss as a cause of biodi- versity loss is the introduction of invasive species. Uncon-

trolled population growth of human-introduced species to non-native habitats has caused

havoc when the introduced species have competed with, preyed on, or

parasitized native species (see Chapter 19). The lack of n -

teractio n s with other species that could keep the newcom-

ers in check is often a key factor in a non-native spe- cies becoming invasive.

GI CHECKPOINT When ecosystems are destroyed, the services they provide are lost. What are some examples of ecosystem services?

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Figure 20.4 Habitat destruction. In a contro- versial method known as mountaintop removal, mining companies blast the tops off of mountains and then scoop out coal. The earth removed from the mountain is dumped into a neighboring valley.

427

Campbell Essential Biology with Physiology, Fourth Edition, by Eric J. Simon, Jean L. Dickey, and Jane B. Reece. Published by Benjamin Cummings. Copyright tilD 2013 by Pearson Education, Inc.

CHAPTER 20 COMMUNITIES AND ECOSYSTEMS

Si CHECKPOINT What are the four main causes of declining biodiversity?

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Figure 20.6 Diverse species interacting in a Kenyan savanna community.

Overexploitation Unsustainable marine fisheries (see Figure 19.13) demon- strate how people can overexploit wildlife by harvesting at rates that exceed the ability of populations to rebound. Among the many terrestrial species whose numbers have been drastically reduced by excessive commercial harvesting, poaching, or sport hunting are the American bison, Galapagos tortoises, and tigers. Overharvesting also threatens some plants, including rare trees such as mahogany and rosewood that produce valuable wood.

Pollution Air and water pollution (Figure 20.5) is a contributing factor in declining populations of hundreds of species worldwide. The global water cycle can transport pollut- ants from terrestrial to aquatic ecosystems hundreds of miles away. Pollutants that are emitted into the atmo- sphere may be carried aloft for thousands of miles before falling to earth in the form of acid precipitation. 12

Figure 20.5 A pelican stuck in oil from the 2010 Gulf of Mexico disaster. Wildlife is often the most visible casualty of pollution, but the impact extends throughout the ecosystem.

Community Ecology On your next walk through a field or woodland, or even across campus or your own backyard, observe the variety of species present. You may see birds in trees, butterflies on flowers, dandelions in the grass of a lawn, or lizards darting for cover as you approach. Each of these organisms interacts with other organisms as it goes about looking for food, nesting sites, living space, or shelter. An organism's biotic environment includes not just individuals from its own popula- tion, but also populations of other species living in the

same area. Ecologists call such an assemblage of species living close enough together for potential interaction a community. In Figure 20.6, the lion, the zebra, the hyena, the vultures, the plants, and the unseen microbes are all members of an ecological community in Kenya.

Interspecific Interactions Our study of communities begins with interspecific interactions—that is, interactions between species. Interspecific interactions can be classified according to the effect on the populations concerned, which may be helpful (+) or harmful (—). In some cases, two popula- tions in a community vie for a resource such as food or space. The effect of this interaction is generally negative for both species ( —/--)—neither species has access to the full range of resources offered by the habitat. On the other hand, some interspecific interactions benefit both parties (+1+). For example, the interactions between flowers and their pollinators are mutually beneficial. In a third type of interspecific interaction, one species exploits another species as a source of food. The effect of this interaction is clearly beneficial to one popula- tion and harmful to the other (+/—). In the next several pages, you will learn more about these interspecific interactions and how they affect communities. We will also look at some examples of interspecific interactions as powerful agents of natural selection.

Campbell Essential Biology with Physiology, Fourth Edition, by Eric J. Simon, Jean L. Dickey, and Jane B. Reece. Published by Benjamin Cummings. Copyright 0 2013 by Pearson Education, Inc.

V Figure 20.8 Competitive exclusion in laboratory populations of Paramecium.

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51 CHECKPOINT

In another experiment, Gause found that Paramecium caudatum and P. bursorio could coexist in the same habitat, but both populations were smaller than when each species occupied the habitat alone. How would you explain these results?

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V Figure 20.7 Species that use similar resources.

If; (a) Virginia's warbler (b) Orange-crowned warbler

COMMUNITY ECOLOGY Interspecific Competition (—/—) In the logistic model of population growth (see Figure 19.6), increasing population density reduces the amount of resources available for each individual. This intraspecific (within-species) competition for limited resources ultimately limits population growth. In interspecific competition (between-species competi- tion), the population growth of a species may be limited by the population densities of competing species as well as by the density of its own population.

What determines whether populations in a com- munity compete with each other? Each species has an ecological niche, defined as its total use of the biotic and abiotic resources in its environment. For example, the ecological niche of a small bird called the Virginia's warbler (Figure 20.7a) includes its nest sites and nest- building materials, the insects it eats, and climatic conditions such as the amount of precipitation and the temperature and humidity that enable it to survive. In other words, the ecological niche encompasses every- thing the Virginia's warbler needs for its existence. The ecological niche of the orange-crowned warbler (Figure 20.7b) includes some of the same resources used by the Virginia's warbler. Consequently, when these two species inhabit the same area, they are competitors.

Ecologists investigated the effects of interspecific competition between populations of these two birds in a community in central Arizona. When they removed either Virginia's warblers or orange-crowned warblers from the study site, members of the remaining species were significantly more successful in raising their off- spring. This study showed that interspecific competition can have a direct, negative effect on reproductive fitness.

If the ecological niches of two species are too similar, they cannot coexist in the same place. Ecologists call this

the competitive exclusion principle, a concept introduced by Russian ecologist G. F. Gause, who demonstrated this effect with an elegant series of experiments. Gause used two closely related species of protists, Paramecium caudatum and P. aurelia. First, he established the carrying capacity for each species separately under the conditions used to culture (grow) them in the laboratory (Figure 20.8, top graph). Then he cultured the two species in the same habitat. Within two weeks, the P. caudatum population had crashed (bottom graph). Gause concluded that the requirements of these two species were so similar that the superior competitor—in this case, P. aurelia— deprived P. caudatum of essential resources. II

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Campbell Essential Biology with Physiology, Fourth Edition, by Eric J. Simon, Jean L. Dickey, and Jane B. Reece. Published by Benjamin Cummings.

Figure 20.9 Mutualism. Coral polyps are inhabited by unicellular algae. Why aren't

popular with show-offs predators?

CHAPTER 20 COMMUNITIES AND ECOSYSTEMS

Y Figure 20.10 Cryptic coloration. Camouflage conceals the pygmy seahorse from predators.

Mutualism (+1+) In mutualism, both species benefit from an interac- tion. Some mutualisms occur between symbiotic species—those in which the organisms have a close physical association with each other. For example, in the

symbiotic root-fungus associations known as mycorrhizae (see the Biology and Society essay in Chapter 16), the fungus delivers mineral nu- trients to the plant and receives organic nutri- ents in return. Coral reef ecosystems depend on the mutualism between certain species of coral animals and millions of unicellular algae that live in the cells of each coral polyp (Figure 20.9). Reefs are constructed by successive generations of colonial corals that secrete an external cal- cium carbonate skeleton. The formation of the skeleton must outpace ero- sion and competition for space from fast-growing seaweeds. The sugars that the algae produce by photosynthesis provide at least half of the energy used by

the coral animals. In return, the algae gain a secure shelter that allows access to light. They also use the coral's waste products, includ- ing CO, and ammonia, a valuable source of nitrogen. Mutualism can also occur between species that are not symbiotic, such as flowers and their pollinators (see Figure 13.28).

Predation (+/—) Predation refers to an interaction in which one spe- cies (the predator) kills and eats another (the prey). Because predation has such a negative impact on the reproductive success of the prey, numerous adaptations for predator avoidance have evolved in prey popula- tions through natural selection. For example, some prey species, like the pronghorn antelope, run fast enough to escape their predators (see the Evolution Connection section in Chapter 19). Others, like rabbits, flee into shelters. Still other prey species rely on mechanical de- fenses, such as the porcupine's sharp quills or the hard shells of clams and oysters.

Adaptive coloration is a type of defense that has evolved in many species of animals. Camouflage, called

cryptic coloration, makes potential prey difficult to spot against its background

(Figure 20.10). Warning coloration, bright patterns of yellow, red, or orange

in combination with black, often marks ani- mals with effective chemical defenses. Predators learn to associate these color patterns with undesirable consequences, such as a noxious taste or painful sting,

and avoid potential prey with similar markings. The vivid colors of the poison dart frog (Figure 20.11), an inhabitant of Costa Rican rain forests, warn of noxious chemicals in the frog's skin.

A prey species may also gain significant protection through mimicry, a "copycat" adaptation in which one species looks like another. For example, the pattern of alternating red, yellow, and black rings of the harmless scarlet king snake resembles the bold color pattern of the venomous eastern coral snake (Figure 20.12). Some insects have combined protective coloration with adaptations of body structures in elaborate disguises. For instance, there are insects that resemble twigs, leaves, and bird droppings. Some

► Figure 20.11 Warning coloration of a poison dart frog.

430

♦ Figure 20.13 An insect mimicking a snake. When disturbed, this sphinx moth larva flips over (left) and resembles a snake (right).

T Figure 20.14 Flavorful plants.

Peppermint Parts of the

Cloves. The cloves used in peppermint plant yield a

cooking are the flower buds

pungent oil. of this plant.

Cinnamon. Cinnamon comes from the inner bark of this tree.

A Figure 20.12 Mimicry in snakes. The color pattern of the nonvenomous scarlet king snake (left) is similar to that of the venomous eastern coral snake (right).

81 CHECKPOINT People find most bitter- tasting foods objectionable. Why do you suppose we have taste receptors for bitter-tasting chemicals?

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even do a passable imitation of a vertebrate. For example, the colors on the dorsal side of certain cater- pillars are an effective camouflage, but when disturbed, the caterpillar flips over to reveal the snakelike eyes of its ventral side (Figure 20.13). Eyespots that resemble vertebrate eyes are common in several groups of moths and butterflies. A flash of these large "eyes" startles would-be predators. In other species, an eyespot may deflect a predator's attack away from vital body parts.

Herbivory (+/—) Herbivory is the consumption of plant parts or algae by an animal. Although herbivory is not usually fatal to plants, a plant whose body parts have been partially eaten by an animal must expend energy to replace the loss. Consequently, numerous defenses against herbi- vores have evolved in plants. Spines and thorns are obvious anti-herbivore devices, as anyone who has plucked a rose from a thorny rosebush or brushed against a spiky cactus knows. Chemical toxins are also very common in plants. Like the chemical defenses of

animals, toxins in plants are distasteful, and herbivores learn to avoid them. Among such chemical weapons are the poison strychnine, produced by a tropical vine called Strychnos toxifera; morphine, from the opium poppy; nicotine, produced by the tobacco plant; mesca- line, from peyote cactus; and tannins, from a variety of plant species. Other defensive compounds that are not toxic to humans but may be distasteful to herbivores are responsible for the familiar flavors of peppermint, cloves, and cinnamon (Figure 20.14). Some plants even produce chemicals that cause abnormal develop- ment in insects that eat them. Chemical compa- nies have taken advan- tage of the poisonous properties of certain plants to produce pesticides. For example, nicotine is used as an insecticide. El

431

IS CHECKPOINT

How is the interaction between a parasite and its host similar to predator-prey and herbivore-plant interactions?

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A terrestrial food chain

An aquatic food chain

CHAPTER 20 COMMUNITIES AND ECOSYSTEMS

Parasites and Pathogens (+/—) Both plants and animals may be victimized by parasites or pathogens. These interactions are beneficial to one species (the parasite or pathogen) and harmful to the other, known as the host. A parasite lives on or in a host from which it obtains nourishment. Pathogens are disease-causing bacteria, viruses, fungi, or protists that can be thought of as microscopic parasites. Invertebrate parasites include flatworms, such as flukes and tape- worms, and a variety of roundworms, which live inside a host organism's body. External parasites, such as ticks, lice, mites, and mosquitoes, attach to their victims temporarily to feed on blood or other body fluids. Plants are also attacked by parasites, including roundworms and aphids, tiny insects that tap into the phloem to suck plant sap (see Figure 19.10). In any parasite population, reproductive success is greatest for individuals that are best at locating and feeding on their hosts. For example,

some aquatic leeches first locate a host by detecting movement in the water and then confirm its identity based on the host's body temperature and chemical cues on its skin.

Non-native pathogens, whose impact can be rapid and dramatic, have provided some opportunities to investigate the effects of pathogens on communities. In one example, ecologists studied the consequences of the epidemic of chestnut blight, a disease caused by a protist. The loss of chestnuts, massive canopy trees that once dominated many forest communities in North America, had a significant impact on community composition and structure. Trees such as oaks and hickories that had formerly competed with chestnuts became more numerous; overall, the diversity of tree species increased. Dead chestnut trees also furnished niches for other organisms, such as insects, cavity- nesting birds, and, eventually, decomposers. I

Trophic Structure Now that we have looked at how populations in a

community interact with one another, let's consider the community as a whole. The feeding relationships among the various species in a community are referred to as its trophic structure. A community's trophic structure determines the passage of energy and nutri- ents from plants and other photosynthetic organisms to herbivores and then to predators. The sequence of food transfer between trophic levels is called a food chain.

Figure 20.15 shows two food chains, one terrestrial and one aquatic. At the bottom of both chains is the trophic level that supports all others. This level consists of auto- trophs, which ecologists call producers. Photosynthetic producers transform light energy to chemical energy stored in the bonds of organic compounds. Plants are the main producers on land. In water, the producers are mainly photosynthetic protists and cyanobacteria, collec- tively called phytoplankton. Multicellular algae and aquatic plants are also important producers in shallow waters. In a few communities, such as those surrounding hydrothermal vents, the producers are chemosynthetic prokaryotes.

All organisms in trophic levels above the produc- ers are heterotrophs, or consumers, and all consumers depend directly or indirectly on the output of produc- ers. Herbivores, which eat plants, algae, or phytoplank- ton, are the primary consumers. Primary consumers on land include grasshoppers and many other insects, snails, and certain vertebrates, such as grazing mam- mals and birds that eat seeds and fruits. In aquatic environments, primary consumers include a variety of

V Figure 20.15 Examples of food chains. The arrows trace the transfer of food from one trophic level to the next in terrestrial and aquatic communities.

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Campbell Essential Biology with Physiology, Fourth Edition, by Eric J. Simon, Jean L. Dickey, and Jane B. Reece. Published by Benjamin Cummings. Copyright 0 2013 by Pearson Education, Inc.

COMMUNE TY f COLOGY zooplankton (mainly protists and microscopic animals such as small shrimps) that eat phytoplankton.

Above the primary consumers, the trophic levels are made up of carnivores, which eat the consumers from the level below. On land, secondary consumers include many small mammals, such as the mouse shown in Figure 20.15 eating an herbivorous insect, and a great variety of birds, frogs, and spiders, as well as lions and other large carnivores that eat grazers. In aquatic com- munities, secondary consumers are mainly small fishes that eat zooplankton. Higher trophic levels include tertiary consumers (third-level consumers), such as snakes that eat mice and other secondary consumers. Most communities have secondary and tertiary con- sumers. As the figure indicates, some also have a higher level—quaternary consumers (fourth-level consumers). These include hawks in terrestrial communities and killer whales in the marine environment. There are no fifth-level consumers—you'll find out why shortly.

Figure 20.15 shows only those consumers that eat living organisms. Some consumers derive their energy from detritus, the dead material left by all trophic levels, including animal wastes, plant litter, and dead bodies. Different organisms consume detritus in different stages of decay. Scavengers, which are large animals such as crows and vultures, feast on carcasses left behind by pred- ators or speeding cars. The diet of detritivores is made up primarily of decaying organic material. Earthworms and millipedes are examples of detritivores. Decomposers, mainly prokaryotes and fungi, secrete enzymes that digest molecules in organic material and convert them to inorganic forms. Enormous numbers of microscopic decomposers in the soil and in the mud at the bottom of lakes and oceans break down organic materials to inor- ganic compounds—the raw materials used by plants and phytoplankton to make new organic materials that may eventually become food for consumers. Many garden- ers keep a compost pile, using the services of detritivores

Y Figure 20.16 Adding compost to a garden. As it decays, the rich organic material of compost provides a slow-release source of inorganic nutrients for plants.

and decomposers to break down organic material from kitchen scraps and yard trimmings (Figure 20.16).

Biological Magnification Organisms can't metabolize many of the toxins produced by industrial wastes or applied as pesticides; after con- sumption, the chemicals remain in the body. These toxins become concentrated as they pass through a food chain, a process called biological magnification. Figure 20.17 shows the biological magnification of chemicals called PCBs (organic compounds used in electrical equipment until 1977) in a Great Lakes food chain. Zooplankton- represented by the base of the pyramid—feed on phyto- plankton contaminated by PCBs in the water. Smelt (small fish) feed on contaminated zooplankton. Because each smelt consumes many zooplankton, the concentration of PCBs is higher in smelt than in zooplankton. For the same reason, the concentration of PCBs in trout is higher than in smelt. The top-level predators—herring gulls in this example—have the highest concentrations of PCBs in the food chain and are the organisms most severely affected by any toxic compounds in the environment. In this case, fewer of the contaminated eggs hatched, resulting in a decline in the reproductive success of herring gulls. Many other synthetic chemicals that cannot be degraded by microorganisms, including DDT and mercury, can also become concentrated through biological magnification. Since people are top-level predators, too, eating fish from contaminated waters can be dangerous.

Concentration of PCBs

Herring gut eggs 124 ppm

Lake trout 4.83 ppm

-4 Figure 20.17 Biological magnification of PCBs in a Great Lakes food chain in the early 1960s.

Smelt 1.04 ppm

Zooplankton 0.123 ppm Phytoplankton

0.025 ppm 433

Campbell Essential Biology with Physiology, Fourth Edition, by Eric J. Simon, Jean L. Dickey, and Jane B. Reece. Published by Benjamin Cummings.

CHAPTER 20 COMMUNITIES AND ECOSYSTEMS

Food Webs Few, if any, communities are so simple that they are characterized by a single unbranched food chain. Several types of primary consumers usually feed on the same plant species, and one species of primary con- sumer may eat several different plants. Such branching of food chains occurs at the other trophic levels as well. Thus, the feeding relationships in a community are woven into elaborate food webs.

Consider the grasshopper mouse in Figure 20.18, the secondary consumer shown crunching on a grasshopper in Figure 20.15. Its diet also includes plants, making it a

primary consumer, too. It is an omnivore, an animal that eats producers as well as consumers of different levels. The rattlesnake that eats the mouse also feeds on more than one trophic level. The blue arrow leading to the rattlesnake indicates that it eats primary consumers—it is a secondary consumer. The purple arrow shows that it eats secondary consumers, so it is also a tertiary consumer. Sound complicated? Actually, Figure 20.18 shows a simplified food web. An actual food web would involve many more organisms at each trophic level, and most of the animals would have a more diverse diet than shown in the figure. 12

s Figure 20.18 A simplified food web for a Sonoran desert community. As in the food chains of Figure 20.15, the arrows in this web indicate "who eats whom," the direction of nutrient transfers. We also continue the color coding introduced in Figure 20.15 for the trophic levels and food transfers.

S CHECKPOINT

You're eating a pizza. At which trophic level(s) are you feeding?

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Campbell Essential Biology with Physiology, Fourth Edition, by Eric J. Simon, Jean L. Dickey, and Jane B. Reece. Published by Benjamin Cummings. Copyright 6> 2013 by Pearson Education, Inc.

COMMUNITY ECOLOGY

Figure 20.20 Relative abundance of tree species in woodlands A and B.

V Figure 20.21 A Pisaster sea star.

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How could a community appear to have relatively little diversity even though it is rich in species?

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435

Species Diversity in Communities The species diversity of a community—the variety of species that make up the community—has two com- ponents. The first component is species richness, or the number of different species in the community. The other component is the relative abundance of the dif- ferent species, the proportional representation of each species in a community. To understand why both components are important for describing species diversity, imagine walking through the woodlands shown in Figure 20.19. On the path through woodland A, you would pass by four different species of trees, but most of the trees you encounter would be the same species. Now imagine walking on a path through woodland B. You would see the same four species of trees that you saw in woodland A—the species richness of the two woodlands is the same. However, woodland B might seem more diverse to you because no single species predominates. As Figure 20.20 shows, the relative abundance of one species in woodland A is much higher than the relative abundances of the other three species. In woodland B, all four species are equally abundant. As a result, species diversity is greater in woodland B. Because plants pro- vide food and shelter for many animals, a diverse plant community promotes animal diversity.

Although the abundance of a dominant species such as a forest tree can have an impact on the diversity of

V Figure 20.19 Which woodland is more diverse?

Woodland A

Woodland B

other species in the community, a nondominant species may also exert control over community composition. A keystone species is a species whose impact on its com- munity is much larger than its total mass or abundance indicates. The term keystone species was derived from the wedge-shaped stone at the top of an arch that locks the other pieces in place. If the keystone is removed, the arch collapses. A keystone species occupies an ecological niche that holds the rest of its community in place.

To investigate the role of a potential keystone species in a community, ecologists compare diversity when the species is present or absent. Experiments by Robert Paine in the 1960s were among the first to provide evidence of the key- stone species effect. Paine manually removed a predator, a sea star of the genus Pisaster (Figure 20.21), from experimental areas within the intertidal zone of the Washington coast. The result was that Pisaster's main prey, a mus- sel, outcompeted many of the other shoreline organisms (algae, barnacles, and snails, for instance) for the important resource of space on the rocks. The number of different organisms present in experimental areas dropped from more than 15 species to fewer than 5 species.

Ecologists have identified other species that play a key role in ecosystem structure. For instance, the decline of sea otters off the western coast of Alaska allowed populations of sea urchins, their main prey, to increase. The abundance of urchins, which consume seaweeds such as kelp, has resulted in the loss of many of the kelp "forests" (see Figure 15.26) and the diversity of marine life that they support. In many ecosystems, however, ecologists are just beginning to understand the complex relationships among species; the value of an individual species may not be apparent until it is gone.151

CHAPTER 20 COMMUNITIES AND ECOSYSTEMS

Disturbances in Communities Most communities are constantly changing in response to disturbances. Disturbances are episodes that damage biological communities, at least temporarily, by destroy- ing organisms and altering the availability of resources such as mineral nutrients and water. Examples of natu- ral disturbances are storms, fire, floods, and droughts, but people are by far the most significant agents of disturbance today.

Understanding the effects of disturbance is espe- cially important because of the potential impact on the human population. For instance, one consequence of human-caused disturbance is the emergence of previ-

ously unknown infectious diseases. Three- quarters of emerging diseases have jumped to humans from another vertebrate species. In many cases, people come into contact with the

pathogens through activities such as clearing land for ag- riculture, road building, or hunting in previously isolated ecosystems. HIV, which may have been transmitted to people from the blood of primates butchered for food, is probably the best-known example. Others include po- tentially fatal hemorrhagic fevers, such as Ebola. Habitat destruction may also cause pathogen-carrying animals to venture closer to human dwellings in search of food.

Small-scale natural disturbances often have positive effects on a biological community. For example, when a large tree falls in a windstorm, it creates new habitats

(Figure 20.22). More light may now reach the forest floor, giving small seedlings the opportunity to grow; or the depression left by the tree's roots may fill with water and be used as egg-laying sites by frogs, salamanders, and numerous insects. Eil

Ecological Succession Communities change drastically following a severe dis- turbance that strips away vegetation and even soil. The disturbed area may be colonized by a variety of species, which are gradually replaced by a succession of other species, in a process called ecological succession.

When ecological succession begins in a virtually lifeless area with no soil, it is called primary succession (Figure 20.23). Examples of such areas are cooled lava flows on volcanic islands and the rubble left by a retreat- ing glacier. Often the only life-forms initially present are autotrophic bacteria. Lichens and mosses, which grow from windblown spores, are commonly the first multicellular photosynthesizers to colonize the area. Soil develops gradually as rocks weather and organic matter accumulates from the decomposed remains of the early colonizers. Lichens and mosses are eventually overgrown by grasses and shrubs that sprout from seeds blown in from nearby areas or carried in by animals. Finally, the area is colonized by plants that become the community's prevalent form of vegetation. Primary succession can take hundreds or thousands of years.

CHECKPOINT

Why might the effect of a small-scale natural distur- bance, such as a forest tree felled by wind, be consid- ered positive?

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Where do deadly new diseases like Ebola come from?

V Figure 20.22 A small-scale disturbance. When this tree fell during a windstorm, its root system and the surrounding soil uplifted, resulting in a depression that filled with water. The dead tree, the root mound, and the water-filled depression are new habitats.

V Figure 20.23 Primary succession under way on a lava flow in Volcanoes National Park, Hawaii.

436

Campbell Essential Biology with Physiology, Fourth Edition, by Eric J. Simon, Jean L. Dickey, and Jane B. Reece. Published by Benjamin Cummings. Copyright @ 2013 by Pearson Education, Inc.

ECOSYSTEM ECOLOGY

CI CHECKPOINT

What is the main abiotic fac- tor that distinguishes primary from secondary succession?

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♦ Figure 20.25 A terrarium ecosystem. Though it is small and artificial, this sealed terrarium illustrates the two major ecosystem processes: energy flow and chemical cycling.

Heat energy

Bacteria, protists, and fungi

437

Secondary succession occurs where a disturbance has destroyed an existing community but left the soil intact. For example, secondary succession occurs as areas recover from floods or fires (Figure 20.24). Disturbances that lead to secondary succession are also caused by human activities. Even before colonial times, people were clearing the temperate deciduous forests of eastern North America for agriculture and settle- ments. Some of this land was later abandoned as the soil was depleted of its chemical nutrients or the residents moved west to new territories. Whenever human intervention stops, secondary succession begins. IS

V Figure 20.24 Secondary succession after a fire.

Ecosystem Ecology In addition to the community of species in a given area, an ecosystem includes all the abiotic factors, such as energy, soil characteristics, and water. Let's look at a small-scale ecosystem—a terrarium—to see how the community interacts with these abiotic fac- tors (Figure 20.25). A terrarium microcosm exhibits the two major processes that sustain all ecosystems: energy flow and chemical cycling. Energy flow is the passage of energy through the components of the ecosystem. Chemical cycling is the use and reuse of chemical ele- ments such as carbon and nitrogen within the ecosystem.

Energy enters the terrarium in the form of sunlight (yellow arrows). Plants (producers) convert light energy to chemical energy through the process of photosynthe- sis. Animals (consumers) take in some of this chemical energy in the form of organic compounds when they eat the plants. Detritivores and decomposers in the soil obtain chemical energy when they feed on the dead remains of plants and animals. Every use of chemical energy by organisms involves a loss of some energy to the surroundings in the form of heat (red arrows). Be- cause so much of the energy captured by photosynthesis is lost as heat, this ecosystem would run out of energy if it were not powered by a continuous inflow of energy from the sun.

In contrast to energy flow, chemical cycling (blue arrows in Figure 20.25) involves the transfer of materi- als within the ecosystem. While most ecosystems have a constant input of energy from sunlight or another source, the supply of the chemical elements used to construct molecules is limited. Chemical elements such as carbon and nitrogen are cycled between the abiotic components of the ecosystem, including air, water, and soil, and the biotic components of the ecosystem (the community). Plants acquire these elements in inorganic

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form from the air and soil and fix them into organic molecules. Animals, such as the snail in Figure 20.25, consume some of these organic molecules. When the plants and animals become detritus, decomposers return most of the elements to the soil and air in inorganic form. Some elements are also returned to the air and soil as the by-products of plant and animal metabolism.

In summary, both energy flow and chemical cycling involve the transfer of substances through the trophic levels of the ecosystem. However, energy flows through, and ultimately out of, ecosystems, whereas chemicals are recycled within and between ecosystems.

Campbell Essential Biology with Physiology, Fourth Edition, by Eric J. Simon, Jean L. Dickey, and Jane B. Reece. Published by Benjamin Cummings. Copyrighte 2013 by Pearson Education, Inc.

V Figure 20.26 Primary production of different ecosystems. Primary production is the amount of biomass created by the producers of an ecosystem over a unit of time, in this case a year. Aquatic ecosystems are color-coded blue in these histograms; terrestrial ecosystems are green.

Open ocean

Estuaiy

Algal beds and coral reefs

Desert and semidesert scrub

Tundra

Temperate grassland

Cultivated land

em coniferous forest (taiga)

Savanna

Temperate broadleaf forest

Tropical rain forest

0 500 1,000 1,500 2,000 2,500

Average Ornery production (g/m 2/yr)

438

Cellular respiration

Is. Figure 20.27 What becomes of a caterpillar's food? Only about 15% of the calories of plant material this herbivore consumes will be stored as biomass available to the next link in the food chain.

at material by caterpillar

15 kcal

CHAPTER 20 COMMUNITIES AND ECOSYSTEMS

Energy Flow in Ecosystems All organisms require energy for growth, maintenance, reproduction, and, in many species, locomotion. In this section, we take a closer look at energy flow through eco- systems. Along the way, we'll answer two key questions: What limits the length of food chains? and How do les- sons about energy flow apply to people's use of resources?

Primary Production and the Energy Budgets of Ecosystems Each day, Earth receives about 10 19 kcal of solar en- ergy, the energy equivalent of about 100 million atomic bombs. Most of this energy is absorbed, scattered, or reflected by the atmosphere or by Earth's surface. Of the visible light that reaches plants, algae, and cyanobacte- ria, only about 1% is converted to chemical energy by photosynthesis.

Ecologists call the amount, or mass, of living organic material in an ecosystem the biomass. The rate at which an ecosystem's producers convert solar energy to the chemical energy stored in biomass is called primary production. The primary production of the entire bio- sphere is roughly 165 billion tons of biomass per year.

Different ecosystems vary considerably in their primary production (Figure 20.26) as well as in their contribution to the total production of the biosphere. Tropical rain forests are among the most productive terrestrial ecosystems and contribute a large portion of the planet's overall production of biomass. Coral reefs

also have very high production, but their contribution to global production is small because they cover such a small area. Interestingly, even though the open ocean has very low production, it contributes the most to Earth's total net primary production because of its huge size—it covers 65% of Earth's surface area. Whatever the ecosystem, primary production sets the spending limit for the energy budget of the entire ecosystem because consumers must acquire their organic fuels from producers. Now let's see how this energy budget is divided among the different trophic levels in an eco- system's food web. S

Ecological Pyramids When energy flows as organic matter through the trophic levels of an ecosystem, much of it is lost at each link in the food chain. Consider the transfer of organic matter from plants (producers) to herbivores (primary consumers). In most ecosystems, herbivores manage to eat only a fraction of the plant material produced, and they can't digest all of what they do consume. For example, a caterpillar feeding on leaves passes about half the energy in the leaves as feces (Figure 20.27). Another 35% of the energy is expended in cellular respiration. Only about 15% of the energy in the caterpillar's food is transformed into caterpillar biomass. Only this biomass (and the energy it contains) is available to the consumer that eats the caterpillar.

Figure 20.28, called a pyramid of production, illus- trates the cumulative loss of energy with each transfer in a food chain. Each tier of the pyramid represents all of the organisms in one trophic level, and the width of each tier indicates how much of the chemical energy of the tier below is actually incorporated into the organic matter of that trophic level. Note that producers convert

Growth

El CHECKPOINT Deserts and semidesert scrub cover about the same amount of surface area as tropical rain forests but contribute less than toio of Earth's primary production, while rain forests contribute 220/0. Explain this difference.

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10 kcal

100 kcal

1,000 kcal

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Vegetarians

Meat-eaters

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only about 1% of the energy in the sunlight available to them to primary production. In this generalized pyra- mid, 10% of the energy available at each trophic level becomes incorporated into the next higher level. Actual efficiencies of energy transfer are usually in the range of 5-20%. In other words, 80-95% of the energy at one trophic level never reaches the next.

An important implication of this stepwise decline of energy in a trophic structure is that the amount of energy available to top-level consumers is small compared with that available to lower-level consumers. Only a tiny frac- tion of the energy stored by photosynthesis flows through a food chain to a tertiary consumer, such as a snake feed- ing on a mouse. This explains why top-level consumers such as lions and hawks require so much geographic ter- ritory: It takes a lot of vegetation to support trophic levels so many steps removed from photosynthetic produc- tion. You can also understand why most food chains are

Ecosystem Energetics and Human Resource Use The dynamics of energy flow apply to the human population as much as to other organisms. The two production pyramids in Figure 20.29 are based on the same general- ized model used to construct Figure 20.28, with roughly 10% of the energy in each trophic level available for consumption by

the next trophic level. The pyramid on the left shows energy flow from producers (represented by corn) to people as primary consumers—vegetarians. The pyramid on the right illustrates energy flow from the same corn crop, with people as secondary consumers, eating beef. Clearly, the human population has less energy available to it when people eat at higher trophic levels than as primary consumers.

Eating meat of any kind is both economically and environmentally expensive. Compared with growing plants for direct human consumption, producing Are vegetarians greener? meat usually requires that more land be cultivated, more water be used for irrigation, more fossil fuels be burned, and more chemical fertilizers and pesti- cides be applied to croplands used for growing grain. In many countries, people cannot afford to buy meat and are vegetarians by necessity. As countries become more affluent, the demand for meat increases—and so do the environmental costs of food production. E

limited to three to five levels; there is sim- ply not enough energy at the very top of an ecological pyramid to support another trophic level. There are, for example, no nonhuman predators of lions, eagles, and killer whales; the biomass in populations of these top-level consumers is insufficient to supply yet another trophic level with a reliable source of nutrition.

ECOSYSTEM ECOLOGY

♦ Figure 20.29 Food energy available to the human population at different trophic levels.

GI CHECKPOINT

Why is the energy at different trophic levels represented as a pyramid in Figure 20.28?

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439

Campbell Essential Biology with Physiology, Fourth Edition, by Eric J. Simon, Jean L. Dickey, and Jane B. Reece. Published by Benjamin Cummings. Copyright CD 2013 by Pearson Education, Inc.

Nutrients available to producers

CHAPTER 20 COM M AND LC.0'.3YST

Chemical Cycling in Ecosystems

V Figure 20.30 Plant growth on fallen tree. In the temperate rain forest of Olympic National Park, in Washington State, plants— including other trees—quickly take advantage of the min- eral nutrients supplied by decomposing "nurse logs."

The sun (or in some cases Earth's interior) supplies ecosystems with a continual input of energy, but aside from an occasional meteorite, there are no extrater- restrial sources of chemical elements. Life, therefore, depends on the recycling of chemicals. While an organ- ism is alive, much of its chemical stock changes con- tinuously, as nutrients are acquired and waste products are released. Atoms present in the complex molecules of an organism at the time of its death are returned to the environment by the action of decomposers, re- plenishing the pool of inorganic nutrients that plants and other producers use to build new organic matter (Figure 20.30). In a sense, each living thing only bor- rows an ecosystem's chemical elements, returning what is left in its body after it dies. Let's take a closer look at how chemicals cycle between organisms and the abiotic components of ecosystems.

The General Scheme of Chemical Cycling Because chemical cycles in an ecosystem involve both biotic components (organisms and nonliving organic material) and abiotic (geologic and atmospheric) components, they are called biogeochemical cycles. Figure 20.31 is a general scheme for the cycling of a mineral nutrient within an ecosystem. Note that the cycle has an abiotic reservoir (white box) where a chemical accumulates or is stockpiled outside of living organisms. The atmosphere, for example, is an abiotic reservoir for carbon. Phosphorus, on the other hand, is available only from the soil. The water of aquatic ecosystems contains dissolved carbon, nitrogen, and phosphorus compounds.

Let's trace our way around our general biogeochemi- cal cycle. () Producers incorporate chemicals from the abiotic reservoir into organic compounds. 0 Consum- ers feed on the producers, incorporating some of the chemicals into their own bodies. 0 Both producers and consumers release some chemicals back to the environ-

ment in waste products. 0 Decomposers play a central role by break-

ing down the complex organic molecules in

detritus such as plant litter, ani- mal wastes, and

dead organisms. The products of this metabolism are inorganic molecules that replenish the abiotic reservoirs. Geologic processes such as erosion and the weathering of rock also contribute to the abiotic reservoirs. Produc- ers use the inorganic molecules from abiotic reservoirs as raw materials for synthesizing new organic molecules (carbohydrates and proteins, for example), and the cycle continues.

Biogeochemical cycles can be local or global. Soil is the main reservoir for nutrients in a local cycle, such as phosphorus. In contrast, for those chemicals that spend part of their time in gaseous form—carbon and nitro- gen are examples—the cycling is essentially global. For instance, some of the carbon a plant acquires from the air may have been released into the atmosphere by the respiration of a plant or animal on another continent.

Now let's examine three important biogeochemical cycles more closely: the cycles for carbon, phosphorus, and nitrogen. As you study the cycles, look for the four basic steps we described, as well as the geologic pro- cesses that may move chemicals around and between ecosystems. In all the diagrams, the main abiotic reservoirs appear in white boxes. 19

V Figure 20.31 General scheme for biogeochemical cycles.

4 CHECKPOINT

What is the role of abiotic reservoirs in biogeochemical cycles? What are the three major abiotic reservoirs?

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Plants, algae, cyanobacteria

Higher-level consumers

Primary consumers

ECOSYST EM ECOLOGY

V Figure 20.32 The carbon cycle.

V Figure 20.33 The phosphorus cycle.

441

Phosphates in soil

(inorganic)

Weathering of rock

The Carbon Cycle Carbon, the major ingredient of all organic molecules, has an atmospheric reservoir and cycles globally. Other abiotic reservoirs of carbon include fossil fuels and dissolved carbon compounds in the oceans. The reciprocal metabolic processes of photosynthe- sis and cellular respiration, which you may recall from previous chapters, are mainly responsible for the cycling of carbon between the biotic and abiotic worlds (Figure 20.32). 0 Photosynthesis removes CO, from the atmosphere and incorporates it into organic molecules, which are 0 passed along the food chain by consumers. 0 Cellular respiration returns CO, to the atmosphere. 0 Decomposers break down the car- bon compounds in detritus; that carbon, too, is even- tually released as CO,. On a global scale, the return of CO, to the atmosphere by respiration closely balances its removal by photosynthesis. However, increasing levels of CO, caused by 0 the burning of wood and fossil fuels (coal and petroleum) are contributing to global climate change (see Figure 18.46).

The Phosphorus Cycle Organisms require phosphorus as an ingredient of nucleic acids, phospholipids, and ATP and (in verte- brates) as a mineral component of bones and teeth. In contrast to the carbon cycle and the other major biogeo- chemical cycles, the phosphorus cycle does not have an atmospheric component. Rocks are the only source of phosphorus for terrestrial ecosystems; in fact, rocks that have high phosphorus content are mined for fertilizer.

At the center of Figure 20.33,0 the weathering (breakdown) of rock gradually adds inorganic phosphate (P043- ) to the soil. 0 Plants absorb dissolved phosphate from the soil and assimilate it by building the phos- phorus atoms into organic compounds. 0 Consumers obtain phosphorus in organic form by eating plants. 0 Phosphates are returned to the soil by the action of decomposers on animal waste and the remains of dead plants and animals. 0 Some of the phosphates drain from terrestrial ecosystems into the sea, where they may settle and eventually become part of new rocks. Phos- phorus removed from the cycle in this way will not be available to living organisms until 0 geologic processes uplift the rocks and expose them to weathering.

Phosphates move from land to aquatic ecosystems much more rapidly than they are replaced, and soil characteristics may also decrease the amount of phos- phate available to plants. As a result, phosphate is a limiting factor in many terrestrial ecosystems. Farmers and gardeners often use phosphate fertilizer, such as crushed phosphate rock or bone meal (finely ground bones from

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IS CHECKPOINT

What are the abiotic reser- voirs of nitrogen? In what form does nitrogen occur in each reservoir?

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Nitrogen letaion

Assimilation by plants

Nitrogen-fixing bacteria in

root nodules

Free-living nitrogen-fixing

bacteria

CHAPTER 20 COMMUNITIES AND ECOSYSTEMS

The Nitrogen Cycle As an ingredient of proteins and nucleic acids, nitrogen is essential to the structure and functioning of all organisms. Nitrogen has two abiotic reservoirs, the atmosphere and the soil. The atmospheric reservoir is huge; almost 80% of the atmosphere is nitrogen gas (NO. However, plants cannot use nitrogen gas. The process of nitrogen fixation converts gaseous N2 to nitrogen compounds that plants can assimilate. Most of the nitrogen available in natural ecosystems comes from biological fixation performed by certain bacteria. Without these organisms, the reservoir of usable soil nitrogen would be extremely limited.

Figure 20.34 illustrates the actions of two types of nitrogen-fixing bacteria. 0 Some bacteria live symbiot- ically in the roots of certain species of plants, supplying their hosts with a direct source of usable nitrogen. The largest group of plants with this mutualistic relationship is the legumes, a family that includes peanuts and soy- beans. Many farmers improve soil fertility by alternating crops of legumes, which add nitrogen to the soil, with plants such as corn that require nitrogen fertilizer. 0 Free-living nitrogen-fixing bacteria in soil or water convert N2 to ammonia (NH 3), which then picks up another H± to become ammonium (NH 41.

0 After nitrogen is "fixed," some of the ammonium is taken up and used by plants. 0 Nitrifying bacteria in the soil also convert some of the ammonium to nitrate (NO3- ), 0 which is more readily assimilated by plants. Plants use this nitrogen to make molecules such as amino acids, which are then incorporated into proteins.

When an herbivore (represented here by a rabbit) eats a plant, it digests the proteins into amino acids and then uses the amino acids to build the proteins it needs. Higher-order consumers get nitrogen from the organic molecules of their prey. Because animals form nitrogen- containing waste products during protein metabolism, consumers excrete some nitrogen into the soil or water. The urine that rabbits and other mammals excrete contains urea, a nitrogen compound that is widely used as fertilizer.

Organisms that are not consumed eventually die and become detritus, which is decomposed by bacteria and fungi. 0 Decomposition releases ammonium from organic compounds back into the soil, replenishing that abiotic reservoir. Under low-oxygen conditions, how- ever, 0 soil bacteria known as denitrifying bacteria strip the oxygen atoms from nitrates, releasing N2 back into the atmosphere and depleting the soil of usable nitrogen.

Human activities are disrupting the nitrogen cycle by adding more nitrogen to the biosphere each year than natural processes do. Combustion of fossil fuels and modern agricultural practices are two major sources of nitrogen. For example, many farmers use enormous amounts of synthetic nitrogen fertilizer to supplement natural nitrogen. However, less than half the fertilizer applied is actually used by the crop plants. Some nitrogen escapes to the atmosphere, where it forms nitrous oxide (N20), a gas that contributes to global warming. And as you'll learn next, nitrogen fertilizers also pollute aquatic systems. GI

Figure 20.34 The nitrogen cycle.

442

ECOSYSTEM ECOLOGY

IS CHECKPOINT

How does the excessive addition of mineral nutrients to a pond eventually result, in the loss of most fish in the pond?

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V Figure 20.36 The Gulf of Mexico dead zone.

Nutrient Pollution Low nutrient levels, especially of phosphorus and nitro- gen, often limit the growth of algae and cyanobacteria in aquatic ecosystems. Nutrient pollution occurs when human activities add excess amounts of these chemicals to aquatic ecosystems.

In many areas, phosphate pollution comes from agri- cultural fertilizers and runoff of animal waste from live- stock feedlots (where hundreds of animals are penned together). Phosphates are also a common ingredient in dishwasher detergents, making the outflow from sewage treatment facilities—which also contains phosphorus from human waste—a major source of phosphate pollution. Phosphate pollu- tion of lakes and rivers results in heavy growth of algae and cyanobacteria (Figure 20.35). Microbes consume a great deal of oxygen as they decompose the extra bio- mass, a process that depletes the water of oxygen. These changes lead to reduced diversity of aquatic species and a much less appealing body of water.

Agricultural sources of nitrogen include feedlots and the large amounts of inorganic nitrogen fertiliz- ers that are routinely applied to crops. Lawns and golf courses also receive sizable applications of nitrogen fertilizers. Crop and lawn plants take up some of the nitrogen compounds, and denitrifiers convert some to atmospheric N 2, but nitrate is not bound tightly by soil particles and is easily washed out of the soil by rain or irrigation. As a result, chemical fertilizers often exceed

T Figure 20.35 Algal growth resulting from nutrient pollution. The flat green area in this photo is not a lawn, but rather the surface of a polluted pond.

h-phosphate detergents any states?

the soil's natural recycling capacity. Nitrogen pollu- tion may also come from sewage treatment facilities when extreme conditions (such as unusual storms) or malfunctioning equipment prevent them from meeting water quality standards.

In an example of how far-reaching this problem can be, nitrogen runoff from midwestern farm fields has been linked to an annual summer "dead zone" in the Gulf of Mexico (Figure 20.36). Vast algal blooms extend outward from where the Mississippi River deposits its nutrient-laden waters. As the algae die, decomposition of the huge quantities of biomass diminishes the supply

of dissolved oxygen over an area that ranges from 13,000 km 2 (roughly the area of Connecticut) to 22,000 km 2 (about the area of Michigan). Oxygen depletion disrupts benthic communi-

ties, displacing fish and invertebrates that can move and killing organisms that are attached to the substrate. More than 400 recurring and permanent coastal dead zones totaling approximately 245,000 km 2 have been documented worldwide. 151

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Light blue lines represent rivers draining into the Mississippi River (shown in dark blue). Nitrogen runoff carried by these rivers ends up in the Gulf of Mexico. In the images below, red and orange indicate high concentrations of phytoplankton. Bacteria feeding on dead phytoplankton deplete the water of oxygen, creating a "dead zone!'

Summer

Winter

443

CHAPTER 20 COMMUNITIES AND ECOSYSTEMS Conservation and Restoration Biology

As we have seen in this unit, many of the environmental problems facing us today have been caused by human enterprises. But the science of ecology is not just useful for telling us how things have gone wrong. Ecological research is also the foundation for finding solutions to these problems and for reversing the negative conse- quences of ecosystem alteration. Thus, we end the ecol- ogy unit by highlighting these beneficial applications of ecological research.

Conservation biology is a goal-oriented science that seeks to understand and counter the loss of biodiversity. Conservation biologists recognize that biodiversity can be sustained only if the evolutionary mechanisms that have given rise to species and communities of organ- isms continue to operate. Thus, the goal is not simply to preserve individual species but to sustain ecosystems, where natural selection can continue to function, and to maintain the genetic variability on which natural selec- tion acts. The expanding field of restoration ecology uses ecological principles to develop methods of return- ing degraded areas to their natural state.

Biodiversity "Hot Spots" Conservation biologists are applying their understand- ing of population, community, and ecosystem dynamics in establishing parks, wilderness areas, and other legally protected nature reserves. Choosing locations for these protected zones often focuses on biodiversity hot spots. These relatively small areas have a large number of endangered and threatened species and an exceptional

concentration of endemic species, species that are found nowhere else. Together, the "hottest" of Earth's biodiversity hot spots, shown in Figure 2037, total less than 1.5% of Earth's land surface but are home to a third of all species of plants and vertebrates. For example, all lemurs are endemic to Madagascar, a large island off the eastern coast of Africa that is home to more than 50 species of lemurs. In fact, almost all of the mammals, reptiles, amphibians, and plants that inhabit Mada- gascar are endemic. There are also hot spots in aquatic ecosystems, such as certain river systems and coral reefs. Because biodiversity hot spots can also be hot spots of extinction, they rank high on the list of areas demand- ing strong global conservation efforts.

Concentrations of species provide an opportunity to protect many species in very limited areas. However, the "hot spot" designation tends to favor the most notice- able organisms, especially vertebrates and plants. Invertebrates and microorganisms are often overlooked. Furthermore, species endangerment is a global problem, and focusing on hot spots should not detract from efforts to conserve habitats and species diversity in other areas. Finally, even the protection of a nature reserve does not shield organisms from the effects of climate change or other threats, such as invasive species or infectious disease. The golden toad you saw in Figure 20.2 went extinct despite living in a protected reserve—a victim of changing weather patterns, airborne pollution, and disease. To stem the tide of biodiversity loss, we will have to address environmental problems globally as well as locally. IS

T Figure 20.37 Earth's terrestrial biodiversity hot spots (purple).

IS CHECKPOINT What is a biodiversity hot spot?

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CONSERVATION AND RESTORATION BIOLOGY Conservation at the Ecosystem Level

Most conservation efforts in the past have focused on saving individual species, and this work continues. (You have already learned about one example, the red-cockaded woodpecker, see Figure 19.12.) More and more, however, conservation biology aims at sustaining the biodiversity of entire communities and ecosystems. On an even broader scale, conservation biology considers the biodiversity of whole landscapes. Ecologically, a landscape is a regional assemblage of interacting ecosystems, such as an area with forest, adjacent fields, wetlands, streams, and streamside habitats. Landscape ecology is the application of ecologi- cal principles to the study of land-use patterns. Its goal is to make ecosystem conservation a functional part of the planning for land use.

V Figure 20.38 Edges between ecosystems within a landscape.

Natural edges. Forests border grassland ecosystems in Lake Clark National Park, Alaska.

Edges created by human activity. Pronounced edges (roads) surround J> clear-cuts in this photograph of a heavily logged rain forest in Malaysia. rt.

Edges between ecosystems are prominent features of landscapes, whether natural or altered by people (Figure 20.38). Such edges have their own sets of physical conditions—such as soil type and surface features—that differ from the ecosystems on either side of them. Edges also may have their own type and amount of distur- bance. For instance, the edge of a forest often has more blown-down trees than a forest interior because the edge is less protected from strong winds. Because of their spe- cific physical features, edges also have their own communities of organisms. Some or- ganisms thrive in edges because they require resources found only there. For instance, whitetail deer browse on woody shrubs found in edge areas between woods and fields, and their populations often expand when forests are logged or interrupted by development.

Edges can have both positive and negative effects on biodiversity. A recent study in a tropical rain forest in western Africa indicated that natural edge communities are important sites of speciation. On the other hand, landscapes where human activities have produced edges often have fewer species.

Another important landscape feature, especially where habitats have been severely fragmented, is the movement corridor, a narrow strip or series of small clumps of suitable habitat connecting otherwise isolated patches. In places where there is extremely heavy human impact, artificial corridors are sometime constructed (Figure 20.39). Corridors can promote dispersal and help sustain populations, and they are especially important to species that migrate between different habitats seasonally. But a corridor can also be harmful—as, for example, in the spread of disease, especially among small subpopulations in closely situated habitat patches. II

Figure 20.39 An artificial corridor. This bridge over a road provides an artificial corridor for animals in Banff National Park, Canada.

CHECKPOINT

How is a landscape different from an ecosystem?

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Why do some animals prefer living on the edge?

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CHAPTER 20 COMMUNITIES AND ECOSYSTEMS

Biodiversity In Decline THE PROCESS OF SCIENCE

How Does Tropical Forest Fragmentation Affect Biodiversity? Long-term studies are essential for learning how we might best conserve biodiversity and other natural resources. One site for such studies is the Biological Dynamics of Forest Fragmentation Project (BDFFP), a 1,000-Ian' ecological "laboratory" located deep in the Amazonian forest of Brazil. When the project was begun in 1979, laws required landowners who cleared forest for ranching or other agricultural operations to leave scattered tracts of untouched forest. Biologists recruited some of these landowners to create reserves in isolated fragments of 1 ha (roughly 2.5 acres), 10 ha, and 100 ha. Figure 20.40 shows some of these forest "islands." Be- fore each area was isolated from the main forest, a small army of specialists inventoried the organisms present and measured the trees.

Hundreds of researchers have used the BDFFP sites to investigate the effects of forest fragmentation on all levels of ecological study. The initial observations for these investigations were gleaned from the results

V Figure 20.40 Fragments of forest in the Amazon that were created as part of the Biological Dynamics of Forest Fragmentation Project. The patch of forest on the right is a 1-ha fragment.

of other ecological studies—for example, the effects of fragmentation in temperate forests or differences in the biodiversity of small islands compared with mainland ecosystems. These observations led many researchers to ask the question, How does fragmentation of tropical forests affect species diversity within the fragments? Based on previous research on numerous species, a rea- sonable hypothesis might be: Species diversity declines as the size of the forest fragment decreases. An ecologist studying large predators such as jaguars and pumas, which require large hunting territories, might therefore make the prediction that predators will only be found in the largest areas. The BDFFP is unique because it allows researchers to test their predictions with new observations that compare species diversity in forest fragments with a comparable control: species diversity in the same area when it was intact. An undisturbed area of 25,400 acres is also available for comparison with fragmented areas. In addition, data can be collected over a period of years or even decades.

Since the BDFFP was established, scientists have studied many different groups of plants and animals. In general, the results have shown that fragmentation of forest into smaller pieces leads to a decline in spe- cies diversity. Species richness decreases as a result of

local extinctions of many species of large mammals, insects, and insectivorous birds. The popula-

tion density of remaining species often declines. Researchers also documented

edge effects, such as those described in the previous section. Changes in

abiotic factors along the fragment edges, including increased wind disturbance, higher temperature, and decreased soil moisture, played a role in community alterations. For example, ecolo- gists found that tree mortality was higher than normal at frag- ment edges. They also observed changes in the composition of communities of invertebrates that inhabit the soil and leaf litter.

Campbell Essential Biology with Physiology, Fourth Edition, by Eric J. Simon, Jean L Dickey, and Jane B. Reece. Published by Benjamin Cummings. Copyright 0 2013 by Pearson Education, Inc.

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CONSERVATION AND RESTORATION BIOLOGY

151 CHECKPOINT The water in the Kissimmee River eventually flows into the Everglades. How will the Kissimmee River Restoration Project affect water quality in the Everglades ecosystem?

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V Figure 20.42 The Kissimmee River Restoration Project.

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Restoring Ecosystems One of the major strategies in restoration ecology is bioremediation, the use of living organisms to detoxify polluted ecosystems. For example, bacteria have been used to clean up old mining sites and oil spills (see Figure 15.18). Researchers are also investigating the po- tential of using plants to remove toxic substances such as heavy metals and organic pollutants (for example, PCBs) from contaminated soil (Figure 20.41).

Some restoration projects have the broader goal of returning ecosystems to their natural state, which may involve replanting vegetation, fencing out non-native animals, or removing dams that restrict water flow. Hundreds of restoration projects are currently under way in the United States. One of the most ambitious endeavors is the Kissimmee River Restoration Project in south-central Florida.

The Kissimmee River was once a meandering shallow river that wound its way from Lake Kissimmee south- ward into Lake Okeechobee. Periodic flooding of the river covered a wide floodplain during about half of the year, creating wetlands that provided habitat for large numbers of birds, fishes, and invertebrates. And as the floods deposited the river's load of nutrient-rich silt on the floodplain, they boosted soil fertility and maintained the water quality of the river.

V Figure 20.41 Bioremediation using plants. A researcher from the U.S. Department of Agriculture investigates the use of canola plants to reduce toxic levels of selenium in contaminated soil.

Between 1962 and 1971, the U.S. Army Corps of Engineers converted the 166-km wandering river to a straight canal 9 m deep, 100 m wide, and 90 km long. This project, designed to allow development on the floodplain, drained approximately 31,000 acres of wetlands, with significant negative impacts on fish and wetland bird populations. Without the marshes to help filter and reduce agricultural runoff, the river trans- ported phosphates and other excess nutrients from Lake Okeechobee to the Everglades ecosystem to the south.

The restoration project involves removing water control structures such as dams, reservoirs, and channel modifications and filling in about 35 km of the canal (Figure 20.42). The first phase of the project was completed in 2004. The photo shows a section of the Kissimmee canal that has been plugged, diverting flow into the remnant river channels. Birds and other wildlife have returned in unexpected numbers to the 11,000 acres of wetlands that have been restored. The marshes are filled with native vegetation, and game fishes again swim in the river channels. GI

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CHAPTER 20 COMMUNITIES AND ECOSYSTEMS

HI CHECKPOINT

What is meant by sustainable development?

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The Goal of Sustainable Development The demand for the "provisioning" services of ecosys- tems, such as food, wood, and water, is increasing as the world population grows and becomes more affluent. Although these demands are currently being met, they are satisfied at the expense of other critical ecosystem services, such as climate regulation and protection against natural disasters. Clearly, we have set ourselves and the rest of the biosphere on a precarious path into the future. How can we best manage Earth's resources to ensure that all generations inherit an adequate supply of natural and economic resources and a relatively stable environment?

Many nations, scientific associations, corporations, and private foundations have embraced the concept of sustainable development. The Ecological Society of America, the world's largest organization of ecologists, endorses a research agenda called the Sustainable Bio- sphere Initiative. The goal of this initiative is to acquire the ecological information necessary for the responsible development, management, and conservation of Earth's resources. The research agenda includes the search for ways to sustain the productivity of natural and artifi- cial ecosystems and studies of the relationship between

biological diversity, global climate change, and ecologi- cal processes.

Sustainable development depends on more than continued research and application of ecological knowl- edge. It also requires that we connect the life sciences with the social sciences, economics, and humanities. Conservation of biodiversity is only one side of sustain- able development; the other key factor is improving the human condition. Public education and the political commitment and cooperation of nations are essential to the success of this endeavor.

An awareness of our unique ability to alter the biosphere and jeopardize the existence of other species, as well as our own, may help us choose a path toward a sustainable future. The risk of a world without ad- equate natural resources for all its people is not a vision of the distant future. It is a prospect for your children's lifetime, or perhaps even your own. But although the current state of the biosphere is grim, the situation is far from hopeless. Now is the time to take action by aggressively pursuing greater knowledge about the diversity of life on our planet and by working toward long-term sustainability. El

Biodiversity in Decline EVOLUTION CONNECTION

Can Biophilia Save Biodiversity? For millions of years, the diversity of life has flourished via evolutionary adaptation in response to environ- mental change. For many species, however, the pace of evolution can't match the breakneck speed at which hu- mans are changing the environment (see the Evolution Connection sections in Chapters 18 and 19). Perhaps those species are doomed to extinction. Or perhaps they can be saved by one human characteristic that works in their favor: biophilia.

Biophilia, which literally means "love of life," is a term that Edward 0. Wilson, one of the world's fore-

most experts on biodiversity and conserva- tion, uses for the human desire to affiliate with

other life in its many forms (Figure 20.43). People de- velop close relationships with pets, nurture houseplants,

r. Figure 20.43 Doing what comes naturally. Edward 0. Wilson has helped teach scientists and the general public a greater respect for Earth's biodiversity. This photograph finds biophiliac Wilson in the woods near Massachusetts's Walden Pond, a landscape immortalized by another great naturalist and writer, Henry David Thoreau.

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Are you a biophiliac?

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CONSERVATION AND RESTORATION BIOLOGY

invite avian visitors with backyard feeders, and flock to zoos, gardens, and nature parks (Figure 20.44). Our at- traction to pristine landscapes with clean water and lush vegetation is also testimony to our biophilia. Wilson proposes that our biophilia is innate, an evolutionary product of natural selection acting on a brainy species whose survival depended on a close connection to the environment and a practical appreciation of plants and animals. We evolved in natural environments rich in biodiversity, and we still have an affinity for such set- tings. Our behavior reflects remnants of our ancestral attachment to nature and the diversity of life.

It will come as no surprise that many biologists have embraced the concept of biophilia. After all, these are people who have turned their passion for nature into ca- reers. But biophilia strikes a chord with biologists for an- other reason. If biophilia is evolutionarily embedded in our genome, then there is hope that we can become bet- ter custodians of the biosphere. If we all pay more atten- tion to our biophilia, a new environmental ethic could catch on among individuals and societies. And that ethic is a resolve never to knowingly allow a single species to

become extinct as a result of our actions or any ecosys- tem to be destroyed as long as there are reasonable ways to prevent it. Yes, we should be motivated to preserve biodiversity because we depend on it for food, medicine, building materials, fertile soil, flood control, habitable climate, drinkable water, and breathable air. But maybe we can also work harder to prevent the extinction of other forms of life just because it is the ethical thing for us to do. Again, Wilson sounds the call: "Right now, we're pushing the species of the world through a bottle- neck. We've got to make it a major moral principle to get as many of them through this as possible. It's the chal- lenge now and for the rest of the century. And there's one good thing about our species: We like a challenge!"

Biophilia is a fitting capstone for this unit. Modern biology is the scientific extension of our human ten- dency to feel connected to and curious about all forms of life. We are most likely to save what we appreciate, and we are most likely to appreciate what we understand. We hope that our discussion of biodiversity has deepened your biophilia and broadened your education.

V Figure 20.44 Biophilia. Whether we seek other organisms in their own habitats or invite them into ours, we clearly find pleasure in the diversity of life.

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THE COMPONENTS OF BIODIVERSITY

Ecosystem Diversity

The current rate of species Loss of genetic diversity extinctions is extremely high threatens the survival of a compared to the rate of species and eliminates natural extinctions over the potential benefits to people. past 100,000 years.

Destruction of ecosystems results in the loss of essential ecosystem services.

Genetic Diversity Species Diversity

III SUMMARY OF KEY CONCEPTS Go to www.masteringbicdogy.com for homework assignments, practice quizzes, MR Pearson eText, and more.

The Loss of Biodiversity

Causes of Declining Biodiversity Habitat destruction is the leading cause of extinctions. Invasive species, overexploitation, and pollution are also significant factors.

Community Ecology

Interspecific Interactions Populations in a community interact in a variety of ways that can be gener- ally categorized as being beneficial (+) or harmful (—) to the populations. Because +/— interactions (exploitation of one species by another species) may have such a negative impact on the individual that is harmed, defensive evolutionary adaptations are common.

INTERACTIONS BETWEEN SPECIES IN A COMMUNITY

Interspecific Interaction Effect on Species 1

Effect on Interspecific Interaction Effect on Effect on Species 2 Species 1 Species 2

CHAPTER 20 COMMUNITIES AND ECOSYSTEMS Chapter Review

Trophic Structure The trophic structure of a community defines the feeding relationships among organisms. These relationships are sometimes organized into food chains or food webs. Toxins may accumulate by the process of biological magnification as they are passed up a food chain to the top predators.

Increasing PCB concentration

Species Diversity in Communities Diversity within a community includes species richness and relative abundance of different species. A keystone species is a species that has a great impact on the composition of the community despite relatively low abundance or biomass.

Disturbances in Communities Disturbances are episodes that damage communities, at least temporarily, by destroying organisms or altering the availability of resources such as mineral nutrients and water. People are the most significant cause of disturbances today.

Ecological Succession The sequence of changes in a community after a disturbance is called ecological succession. Primary succession occurs where a community arises in a virtually lifeless area with no soil. Secondary succession occurs where a disturbance has destroyed an existing community but left the soil intact.

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Approximately 90% loss of energy at each trophic level

Autotrophs

Producer

Heterotrophs

Herbivore (primary

consumer)

Carnivore (secondary consumer)

CHAPTER REVIEW

Ecosystem Ecology

Energy Flow in Ecosystems An ecosystem is a biological community and the abiotic factors with which the community interacts. Energy must flow continuously through an ecosystem, from producers to consumers and decomposers. Chemical elements can be recycled between an ecosystem's living community and the abiotic environment. Trophic relationships determine an ecosystem's routes of energy flow and chemical cycling.

Light

Primary production is the rate at which plants and other producers build biomass. Ecosystems vary considerably in their productivity. Primary production sets the spending limit for the energy budget of the entire ecosystem because consumers must acquire their organic fuels from producers. In a food chain, only about 10% of the biomass at one trophic level is available to the next, resulting in a pyramid of production.

When people eat producers instead of consumers, less photosynthetic production is required, which reduces the impact on the environment

Z. Z.

Chemical Cycling in Ecosystems Biogeochemical cycles involve biotic and abiotic components. Each circuit has an abiotic reservoir through which the chemical cycles. Some chemical elements require "processing" by certain microorganisms before they are available to plants as inorganic nutrients. A chemical's specific route through an ecosystem varies with the element and the trophic structure of the ecosystem. Phosphorus is not very mobile and is cycled locally. Carbon and nitrogen spend part of their time in gaseous form and are cycled globally. Runoff of nitrogen and phosphorus, especially from agricultural land, causes algal blooms in aquatic ecosystems, lowering water quality and sometimes depleting the water of oxygen.

Conservation and Restoration Biology

Biodiversity "Hot Spots" Conservation biology is a goal-oriented science that seeks to counter the loss of biodiversity. The front lines for conservation biology are biodiversity "hot spots," relatively small geographic areas that are especially rich in endangered species.

Conservation at the Ecosystem Level Increasingly, conservation biology aims at sustaining the biodiversity of entire communities, ecosystems, and landscapes. Edges between ecosystems are prominent features of landscapes, with positive and negative effects on biodiversity. Corridors can promote dispersal and help sustain populations.

Restoring Ecosystems In some cases, toxic substances such as heavy metals can be removed from an ecosystem by microbes or plants. Ecologists are working to revitalize some ecosystems by planting native vegetation, removing barriers to wildlife, and other means. The Kissimmee River Restoration Project is an attempt to undo the ecological damage done when the river was engineered into straight channels.

The Goal of Sustainable Development Balancing the needs of people with the health of the biosphere, sustainable development has the goal of long-term prosperity of human societies and the ecosystems that support them.

Evolution Connection: Biophilia and an Environmental Ethic Biophilia is a term used by biologist E. 0. Wilson to describe the innate affinity people have for nature and for other organisms. Wilson—and many other biologists—hope that our biophilia will help us develop an environmental ethic of preserving biodiversity.

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CHAPTER 20 COMMUNITIES AND ECOSYSTEMS

THE PROCESS OF SCIENCE 11. An ecologist studying desert plants performed the following experiment.

She staked out two identical plots that included a few sagebrush plants and numerous small annual wildflowers. She found the same five wildflower species in similar numbers in both plots. Then she enclosed one of the plots with a fence to keep out kangaroo rats, the most common herbivores in the area. After two years, four species of wildflowers were no longer present in the fenced plot, but one wildflower species had increased dramatically. The unfenced control plot had not changed significantly in species composition. Using the concepts discussed in the chapter, what do you think happened?

12. Imagine that you have been chosen as the biologist for the design team implementing a self-contained space station to be assembled in orbit. It will be stocked with the organisms you choose, creating an ecosystem that will support you and five other people for two years. Describe the main functions you expect the organisms to perform. List the types of organisms you would select, and explain why you chose them.

13. Biologists in the United States are concerned that populations of many migratory songbirds, such as warblers, are declining. Evidence suggests that some of these birds might be victims of pesticides. Most of the pesticides implicated in songbird mortality have not been used in the United States since the 1970s. Suggest a hypothesis to explain the current decline in songbird numbers. Design an experiment that could test your hypothesis.

FIBIOLOGY AND SOCIETY 14. Some organizations are starting to envision a sustainable society—one

in which each generation inherits sufficient natural and economic resources and a relatively stable environment. The Worldwatch Institute, an environmental policy organization, estimates that we must reach sustainability by the year 2030 to avoid economic and environmental collapse. In what ways is our current system not sustainable? What might we do to work toward sustainability, and what are the major roadblocks to achieving it? How would your life be different in a sustainable society?

15. The Biological Dynamics of Forest Fragmentation Project, which has contributed so much to our understanding of threats to biodiversity, is itself endangered. Encouraged by Brazilian government agencies, urban sprawl and intensive forest settlement are closing in on the study site. Activities such as clear-cutting, burning, hunting, and logging threaten the integrity of the surrounding forest. Researchers at the BDFFP, which is jointly operated by a Brazilian research agency and the Smithsonian Tropical Institute, hope that they can bring attention to the problem through the Brazilian media and pressure the government into

protecting the project. How would you argue the importance of protecting the BDFFP against ecologically destructive activities in a letter to a newspaper editor or to a government official?

SELF-QUIZ 1. Currently, the number one cause of biodiversity loss is

2. According to the concept of competitive exclusion, a. two species cannot coexist in the same habitat b. extinction or emigration is the only possible result of competitive

interactions. c. intraspecific competition results in the success of the best-adapted

individuals. d. two species cannot share the same niche in a community.

3. The concept of trophic structure emphasizes the a. prevalent form of vegetation. b. keystone species concept c. feeding relationships within a community. d. species richness of the community.

4. Match each organism with its trophic level (you may choose a level more than once). a. alga 1. decomposer b. grasshopper 2. producer c. zooplankton 3. tertiary consumer d. eagle 4. secondary consumer e. fungus 5. primary consumer

5. Why are the top predators in food chains most severely affected by pesticides such as DDT?

6. Over a period of many years, grass grows on a sand dune, then shrubs grow, and then eventually trees grow. This is an example of ecological

7. According to the pyramid of production, why is eating grain-fed beef a relatively inefficient means of obtaining the energy trapped by photosynthesis?

8. Local conditions, such as heavy rainfall or the removal of plants, may limit the amount of nitrogen, phosphorus, or calcium available to a particular terrestrial ecosystem, but the amount of carbon available to the ecosystem is seldom a problem. Why?

9. A is a local grouping of interacting ecosystems with several adjacent habitats.

10. Movement corridors are a. the routes taken by migratory animals. b. strips or clumps of habitat that connect isolated fragments. c. landscapes that include several different ecosystems. d. edges or boundaries between ecosystems. e. buffer zones that protect the long-term viability of protected areas.

Answers to these questions can be found in Appendix: Self-Quiz Answers.

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