Communities and Ecosystems
Chapter
31
Outline
	 31.1	Ecology of Communities
	•	Communities are assemblages of interacting populations (species).552
	•	In an ecosystem, species interact with one another and with the physical environment.552
	•	Communities are characterized and compared in terms of species richness and diversity.553
	•	Ecological succession is a change in community composition and diversity over time.554
	•	Interactions between species include competition, predation, parasitism, commensalism, and mutualism.556
	•	An ecological niche is the role a species plays in its community.556
	•	Competition leads to resource partitioning, which reduces competition between species.557
	•	The introduction of exotic species can disrupt ecosystems.559
	 31.2	Ecology of Ecosystems
	•	In an ecosystem, species are categorized by their food source. Autotrophs are producers, and heterotrophs are consumers. 560
	•	Ecosystems are characterized by energy flow and chemical cycling among populations.561
	•	Biogeochemical cycles are sedimentary (phosphorus) or gaseous (nitrogen, carbon). Human activity can alter these cycles.564
	 31.3	Ecology of Major Ecosystems
	•	The Earth’s major aquatic ecosystems are of two types: freshwater or saltwater (marine).568
	•	The Earth’s major terrestrial ecosystems are the tundra, the taiga, temperate forests, temperate grasslands, deserts, tropical grasslands, and tropical rain forests.569
Nitrogen is an essential element for all living things. Unfortunately, the nitrogen gas in the atmosphere is not biologically available to plants until it has been “fixed.” This fixing process is part of the nitrogen cycle, and much of the fixing is performed by soil bacteria. Once this process is complete, plants can absorb the nitrogen in the form of ammonium and nitrate ions, and other organisms receive this nitrogen when they eat plants. Humans fix nitrogen when they make fertilizer, and this provides more than double the amount of nitrogen produced by natural nitrogen fixation. While commercial fertilizers benefit agriculture by causing increased growth of crops, they can also have negative consequences. When excess water runs off agricultural fields, it carries fertilizer into nearby bodies of water. The nitrogen in the fertilizer causes aquatic algae and plants to undergo a huge population increase and then die off. Their subsequent decomposition by organisms of decay leads to a lack of oxygen that can cause a major fish kill. 
Excessive use of fertilizer contributes to other ecological problems as well. The nitrates in fertilizer become nitric acid (the primary component of acid rain); cause increasing levels of nitrous oxide gas, which produces smog; and lead to the depletion of certain nutrients in the soil. The question that arises is: Do the benefits of artificial nitrogen fixation outweigh its risks to the environment?
In this chapter, you will learn about communities, aquatic and terrestrial ecosystems, and chemical cycling, including the nitrogen cycle.
31.1		Ecology of Communities 
A community is an assemblage of populations, each a different species, interacting with one another within a single environment. For example, the various species living on and within a fallen log, such as plants, fungi, worms, and insects, interact with one another and form a community. The fungi break down the log and provide food for the earthworms and insects living in and on the log. Those insects may feed on one another, too. If birds flying throughout the forest feed on the insects and worms living in and on the log, they are also part of the larger forest community.
Communities come in different sizes, and it is sometimes difficult to decide where one community ends and another one begins. The relationships and interactions between species in a community form over time. Some of the relationships between species are products of coevolution, by which an evolutionary change in one species results in an evolutionary change in another. As discussed on page 558, flowering plants and their pollinators are co-adapted (Fig. 31.1). A striking example is the flower of the Australian orchid, Chiloglottis trapeziformis. This flower resembles the body of a wasp, and its odor mimics the pheromones of a female wasp. Therefore, male wasps are attracted to the flower, and when they attempt to mate with it, they become covered with pollen, which they transfer to the next orchid flower (Fig. 31.1b). The orchid is dependent upon wasps for pollination because neither wind nor other insects pollinate these flowers.
All species in a community possess adaptations suitable to the conditions of the particular physical environment. An ecosystem consists of these species interacting with each other and with the physical environment. If the physical environment of an ecosystem changes, corresponding changes will most likely occur in the species comprising the community and in the relationships between these species. Extinction of species can occur when environmental change is too rapid for suitable adaptations to evolve. 
Rapid environmental changes can be detrimental to humans too, even though technology increases our ability to adapt. Sometimes the economy of an area is dependent upon the climate and, in some cases, the species composition, of an aquatic or terrestrial ecosystem. Therefore, human activities that negatively impact the community of the area can also negatively affect the economy of that area. Knowledge of community and ecosystem ecology will help you better understand how human activities resulting in, say, climate change can in the end be detrimental to ourselves.
Community Composition and Diversity
Two characteristics of communities—species composition and diversity—allow us to compare communities. The species composition of a community, also known as species richness, is simply a listing of the various species found in that community. The diversity includes both species richness and species evenness, or the relative abundance of individuals of different species.
Species Composition
It is apparent by comparing the photographs in Figure 31.2 that a coniferous forest has a different species composition than a tropical rain forest. Narrow-leaved evergreen tree species are prominent in the coniferous forest, whereas broad-leaved evergreen tree species are numerous in the tropical rain forest. As the list of mammals demonstrates, a coniferous community and a tropical rain forest community contain different types of mammals. Ecologists comparing these two communities would go on to find differences in other plants and animals too. In the end, ecologists would conclude that not only are the species compositions of these two communities different, but the tropical rain forest has more species and therefore higher species richness.
Diversity
The diversity of a community goes beyond species richness to include abundance, the number of individuals of each species per unit area. For example, suppose a deciduous forest in West Virginia has, among individuals of other species, 76 yellow poplar trees but only one American elm. If you were simply walking through this forest, you might miss the lone American elm. If, instead, the forest had 36 yellow poplar trees and 41 American elm trees, the forest would seem more diverse to you, and indeed would have a higher diversity value. The greater the species richness and the more even the distribution of the species in the community, the greater the diversity.
Ecological Succession
Community species composition and diversity do change over time, although they may seem to remain static because it can take decades—even longer than the human life span—for noticeable changes to occur. Natural forces, such as glaciers, volcanic eruptions, lightning-ignited forest fires, hurricanes, tornadoes, and floods, bring about community changes. Communities also change because of human activities, such as logging, road building, sedimentation, and farming. A more or less orderly process of community change is known as ecological succession.
Ecologists have developed models to explain why succession occurs and to predict patterns of succession following a natural or human-made disturbance. The climax-pattern model says that the climate of an area always leads to the same stable climax community, a specific assemblage of bacterial, fungal, plant, and animal species (Fig. 31.3). For example, a coniferous forest community is expected in northern latitudes, a deciduous forest in temperate zones, and a tropical forest in areas with a tropical climate. Now that we know that disturbances influence community composition and diversity, and that despite the climate, the composition of a community is not always the same, the climax model of succession is being modified.
Two Types of Succession
Ecologists define two types of ecological succession: primary and secondary  (Fig. 31.4). Primary succession occurs where soil has not yet formed. For example, hardened lava flows or the scraped bedrock that remains following a glacial retreat are subject to primary succession. Secondary succession begins, for example, in a cultivated field that is no longer farmed, where soil is already present. With both primary and secondary succession, a progression of species occurs over time as first spores of fungi and then seeds of nonvascular plants, followed by seeds of gymnosperms and/or angiosperms, are carried into the area by wind, water, or animals from the surrounding regions (Figs. 31.4 and 31.5).
The first species to appear in an area undergoing either primary or secondary succession are called opportunistic pioneer species. These species are small in stature, short-lived, quick to mature, and they produce numerous offspring per reproductive event. The first pioneer species to arrive are photosynthetic organisms such as lichens and mosses. The fungal partners of lichens play a critical role by breaking down rock or lava into usable mineral nutrition, not only for their algal partners but also for pioneer plants. The mycorrhizal fungal partners of plants (see Fig. 18.26) pass minerals directly to plants so that they can grow successfully in poor soil. Pioneer plant species that become established in an area are accompanied by pioneer herbivore species (e.g., insects) and then carnivore species (e.g., small mammals). As the community continues to change, equilibrium species become established in the area. Equilibrium species, such as deer, wolves, and bears, are larger in size, long-lived, and slow to mature, and they produce few offspring per reproductive event.
Interactions in Communities
Species interactions, especially competition for resources, fashion a community into a dynamic system of interspecies relationships. In Table 31.1, the plus and minus signs tell how the relationship affects the abundance of the two interacting species. Competition between two species for limited resources has a negative effect on the abundance of both species. In predation, one animal, the predator, feeds on another, the prey; in parasitism, one species obtains nutrients from another species, called the host, but does not kill the host. Commensalism is a relationship in which one species benefits while the second species is not harmed. Commensalism often occurs when one species provides a home or transportation for another. In mutualism, two species interact in a way that benefits both of them.
Ecological Niche
Each species occupies a particular position in the community, both in a spatial and a functional sense. Spatially, species live in a particular area of the community, or habitat, such as underground, in the trees, or in shallow water. Functionally, each species plays a role, such as whether it is a photosynthesizer, predator, prey, or parasite. The ecological niche of a species incorporates the role the species plays in its community, its habitat, and its interactions with other species. The niche includes the living and nonliving resources that individuals in the population need to meet their energy, nutrient, and survival demands. The habitat of an insect, called a backswimmer, is a pond or lake, where it eats other insects (Fig. 31.6). The pond or lake must contain vegetation where the backswimmer can hide from its predators, including fish and birds. The pond water must be clear enough for the backswimmer to see its prey and warm enough for it to maintain a good metabolic rate. Since it is difficult to describe and measure the total niche of a species in a community, ecologists often focus on a certain aspect of a species’ niche, as with the birds featured in Figure 31.7.
Competition
Competition for resources such as light, space, or nutrients contributes to the niche of each species and helps structure the community. Laboratory experiments helped ecologists formulate the competitive exclusion principle, which states that no two species can occupy the same niche at the same time. In the 1930s, G. F. Gause grew two species of Paramecium in one test tube containing a fixed amount of bacterial food. Although populations of each species survived when grown in separate test tubes, only one species, Paramecium aurelia, survived when the two species were grown together (Fig. 31.8). P. aurelia acquired more of the food resource and had a higher population growth rate than did P. caudatum. Eventually, as the P. aurelia population grew and obtained an increasingly greater proportion of the food resource, the number of P. caudatum individuals decreased, and the population died out.
Niche SpecializationCompetition for resources does not always lead to localized extinction of a species. Multiple species coexist in communities by partitioning, or sharing, resources. In another laboratory experiment using other species of Paramecium, Gause found that the two species could survive in the same test tube if one species consumed bacteria at the bottom of the tube and the other ate bacteria suspended in solution. This resource partitioning decreased competition between the two species, leading to increased niche specialization. What could have been one niche became two more specialized niches due to species differences in feeding behavior.
When three species of ground finches of the Galápagos Islands live on the same islands, their beak sizes differ, and each feeds on a different-sized seed (Fig. 31.9). When the finches live on separate islands, their beaks tend to be the same intermediate size, enabling each to feed on a wider range of seeds. Such so-called character displacement often is viewed as evidence that competition and resource partitioning have taken place.
The niche specialization that permits coexistence of multiple species can be very subtle. Species of warblers that occur in North American forests are all about the same size, and all feed on budworms, a type of caterpillar found on spruce trees. Robert MacArthur recorded the length of time each warbler species spent in different regions of spruce canopies to determine where each species did most of its feeding. He discovered that each species primarily used different parts of the tree canopy and, in that way, had a more specialized niche. As another example, consider that three types of birds—swallows, swifts, and martins—all eat flying insects and parachuting spiders. These birds even frequently fly in mixed flocks. But each type of bird has a different nesting site and migrates at a slightly different time of year.
Mutualism
Mutualism, a symbiotic relationship in which both members benefit, is now recognized to be at least as important as competition in shaping community structure. The relationship between plants and their pollinators mentioned previously is a good example of mutualism. Perhaps the relationship began when herbivores feasted on pollen. The provision of nectar by the plant may have spared the pollen, and at the same time allowed the animal to become an agent of pollination. By now, pollinator mouthparts are adapted to gathering the nectar of a particular plant species, and this species is dependent on the pollinator for dispersing pollen. As also mentioned previously, lichens can grow on rocks because the fungal member leaches minerals that are provided to the algal partner. The algal partner, in turn, photosynthesizes and provides organic food for both members of the relationship. 
In tropical America, ants form mutualistic relationships with certain plants. The bullhorn acacia tree is adapted to provide a home for ants of the species Pseudomyrmex ferruginea (Fig. 31.10). Unlike other acacias, this species has swollen thorns with a hollow interior where ant larvae can grow and develop. In addition to housing the ants, acacias provide them with food. The ants feed from nectaries at the base of the leaves and eat fat- and protein-containing nodules called Beltian bodies, which are found at the tips of the leaves. For 24 hours a day, ants constantly protect the tree from herbivores that would like to feed on it. The ants are so critical to the trees’ survival that when the ants on experimental trees were poisoned, the trees died. 
The outcome of mutualism is an intricate web of species interpendencies critical to the community. For example, in areas of the western United States, the branches and cones of whitebark pine are turned upward, meaning that the seeds do not fall to the ground when the cones open. Birds called Clark’s nutcrackers eat the seeds of whitebark pine trees and store them in the ground (Fig. 31.11). Therefore, Clark’s nutcrackers are critical seed dispersers for the trees. Also, grizzly bears find the stored seeds and consume them. Whitebark pine seeds do not germinate unless their seed coats are exposed to fire. When natural forest fires in the area are suppressed, whitebark pine trees decline in number, and so do Clark’s nutcrackers and grizzly bears. When lightning-ignited fires are allowed to burn, or prescribed burning is used in the area, the whitebark pine populations increase, as do the populations of Clark’s nutcrackers and grizzly bears.
Community Stability
As witnessed by our discussion of succession, community stability is fragile. However, some communities have one species that stabilizes the community, helps maintain its characteristics, and essentially helps hold its web of interactions together. Such a species is known as a keystone species, or species on which the existence of a large number of other species in the ecosystem depends. The term “keystone” comes from the name for the center stone of an arch that holds the other stones in place so that the arch can keep its shape. 
Keystone species are not always the most numerous in the community. Howev-er, the extinction of a keystone species can lead to other species extinctions and a loss of diversity. For example, bats are designated as keystone species in tropical forests. Bats are pollinators and also disperse the seeds of certain tropical trees. When bats are killed off or their roosts destroyed, these trees fail to reproduce. The grizzly bear is a keystone species in the northwestern United States and Canada. Grizzly bears disperse as many as 7,000 berry seeds in one dung pile. Grizzlies also kill the young of many herbivorous mammals, such as deer, and thereby keep their populations under control.
The sea otter is a keystone species of a kelp forest ecosystem. Kelp forests, created by large brown seaweeds, provide a home for a vast assortment of organisms. The kelp forests occur just off the coast and protect coastline ecosystems from damaging wave action. Among other species, sea otters eat sea urchins, keeping their population size in check. Otherwise, sea urchins feed on the kelp, causing the kelp forest and its associated species to severely decline. Fishermen don’t like sea otters because they also prey on abalone, a mollusc prized for its commercial value. They do not realize that without the otters, abalone and many other species would not be around because their natural habitat, a kelp forest, would no longer exist. 
Native Versus Exotic Species
Native species are species indigenous to an area. They colonize an area without intentional or accidental human assistance. For example, you naturally find maple trees in Vermont and many other states of the eastern United States. The introduction of exotic species, or nonnative species, into a community greatly disrupts normal interactions, and therefore a community’s web of species. Populations of exotic species tend to grow exponentially because they are better competitors, or because their numbers are not controlled by predators or disease. The unique assemblage of native species on an island often cannot compete well against an exotic species. For example, myrtle trees, introduced into the Hawaiian Islands from the Canary Islands, are mutualistic with a type of bacterium capable of fixing atmospheric nitrogen. The bacterium provides the plant with a form of nitrogen it can utilize. This feature allows myrtle trees to become established on nitrogen-poor volcanic soil, a distinct advantage in Hawaii. Once established, myrtle trees halt the normal succession of native plants on volcanic soil.
Exotic species disrupt communities in continental areas as well. The red fox was deliberately imported into Australia to prey on the previously introduced European rabbit, but instead, the red fox has now reduced the populations of native small mammals (Fig. 31.12a). The brown tree snake was introduced onto a number of islands in the Pacific Ocean (Fig. 31.12b). The brown tree snake eats eggs, nestlings, and adult birds. On Guam, it has reduced ten native bird species to the point of extinction. On the Galápagos Islands, black rats accidentally carried to the islands by ships have reduced populations of the giant tortoise. Goats and feral pigs have changed the vegetation on the islands from highland forest to pampas-like grasslands and destroyed stands of cactus. In the United States, gypsy moths, zebra mussels, the Chestnut blight fungus, fire ants, and African bees are well-known exotic species that have killed native species. At least two species, fire ants and African bees, have attacked humans with serious consequences. 
31.2		Ecology of Ecosystems 
An ecosystem is more inclusive than a community because when studying a community, we only consider how species interact with one another. When studying an ecosystem, we also consider interactions with the physical environment. For example, one important aspect of ecological niche is how an organism acquires food. It is obvious that autotrophs interact with the physical environment but so do heterotrophs.
Autotrophs
Autotrophs take in only inorganic nutrients (e.g., CO2 and minerals) and an outside energy source to produce organic nutrients for their own use and for all the other members of a community. They are called producers because they produce food. Photoautotrophs, often called photosynthetic organisms, produce most of the organic nutrients for the biosphere (Fig. 31.13). Algae of all types possess chlorophyll and carry on photosynthesis in freshwater and marine habitats. Algae make up the phytoplankton, which are photosynthesizing -organisms suspended in water. Green plants are the dominant photosynthesizers on land. All photosynthesizing organisms release O2 to the atmosphere.
Some bacteria are chemoautotrophs. They obtain energy by oxidizing inorganic compounds such as ammonia, nitrites, and sulfides, and they use this energy to synthesize organic compounds. Chemo-autotrophs have been found to support communities in some caves and also at hydrothermal vents along deep-sea oceanic ridges.
Heterotrophs
Heterotrophs need a source of preformed organic nutrients, and they release CO2 to the atmosphere. They are called consumers because they consume food. Herbivores are animals that graze directly on -algae or plants (Fig. 31.14a). In aquatic habitats, zooplankton, such as protozoans, are herbivores; in terrestrial habitats, insects, including caterpillars, play that role. Among larger herbivores, giraffes browse on trees. Carnivores eat other animals; for example, a praying mantis catches and eats caterpillars, and an osprey preys on and eats fish (Fig. 31.14b). These examples illustrate  that there are primary consumers (e.g., giraffes), secondary consumers (e.g., praying mantis), and tertiary consumers (e.g., osprey). Sometimes tertiary consumers are called top predators. Omnivores are animals that eat both plants and animals. As you likely know, most humans are omnivores.
The decomposers are heterotrophic bacteria and fungi, such as molds and mushrooms, that break down dead -organic matter, including animal wastes (Fig. 31.15). They perform a very valuable service because they release inorganic nutrients (CO2 and minerals) that are then taken up by plants once more. Otherwise, plants would have to wait for minerals to be released from rocks. Detritus is composed of the remains of dead organisms plus the bacteria and fungi of decay. Fanworms feed on detritus floating in marine waters, while clams take detritus from the sea bottom. Earthworms and some beetles, termites, and maggots are soil detritus feeders.
Energy Flow and Chemical Cycling
The living components of ecosystems process energy and chemicals. Energy flow through an ecosystem begins when producers absorb solar energy, and chemical cycling begins when producers take in inorganic nutrients from the physical environment (Fig. 31.16). Thereafter, via photosynthesis, producers convert the solar energy and inorganic nutrients into chemical energy in the form of organic nutrients, such as carbohydrates. Producers synthesize organic nutrients directly for themselves and indirectly for the heterotrophic components of the ecosystem. Energy flows through an ecosystem because as organic nutrients pass from one component of the ecosystem to another, as when a herbivore eats a plant or a carnivore eats a herbivore, a portion is used as an energy source. Eventually, the energy dissipates into the environment as heat. Therefore, the vast majority of ecosystems cannot exist without a continual supply of solar energy. 
Only a portion of the organic nutrients made by producers is passed on to consumers because plants use organic molecules to fuel their own cellular respiration. Similarly, only a small percentage of nutrients consumed by lower-level consumers, such as herbivores, is available to higher-level consumers, or carnivores. As Figure 31.17 demonstrates, a certain amount of the food eaten by a herbivore is never digested and is eliminated as feces. Metabolic wastes are excreted as urine. Of the assimilated energy, a large portion is utilized during cellular respiration for the production of ATP and thereafter becomes heat. Only the remaining energy, which is converted into increased body weight or additional offspring, becomes available to carnivores.
The elimination of feces and urine by a heterotroph, and indeed the death of all organisms, does not mean that organic nutrients are lost to an ecosystem. Instead, they represent the organic nutrients made available to decomposers. Decomposers convert the organic nutrients back into inorganic chemicals and release them to the soil or atmosphere. Chemicals complete their cycle within an ecosystem when inorganic chemicals are absorbed by the producers from the atmosphere or from soil.
Energy Flow
Applying the principles discussed so far to a temperate deciduous forest, ecologists can draw a food web to represent the interconnecting paths of energy flow between the component species of the ecosystem. In Figure 31.18, the green arrows are part of a grazing food web because the web begins with plants, such as the oak trees depicted. A detrital food web (brown arrows) begins with bacteria and fungi. In the grazing food web, caterpillars and other herbivorous insects feed on the leaves of the trees, while other herbivores, including mice, rabbits, and deer, feed on leaves at or near the ground. Birds, chipmunks, and mice feed on fruits and nuts of the trees, but they are in fact omnivores because they also feed on caterpillars and other insects. These herbivores and omnivores all provide food for a number of different carnivores. In the detrital food web, detritus, which includes smaller decomposers such as bacteria and fungi, is food for larger organisms. Because some of these organisms, such as shrews and salamanders, become food for aboveground animals, the detrital and the grazing food webs are joined.
We tend to think that the aboveground parts of trees are the largest storage form of organic matter and energy, but this is not necessarily the case. In temperate deciduous forests, the organic matter lying on the forest floor and mixed into the soil, along with the belowground roots of the trees, contains over twice the energy of the leaves, branches, and trunks of living trees combined. Therefore, more energy and matter in a forest may be stored in or funneled through the detrital food web than the grazing food web.
Trophic Levels and Ecological PyramidsThe arrangement of component species in Figure 31.19 suggests that organisms are linked to one another in a straight line according to feeding relationships, or who eats whom. Diagrams that show a single path of energy flow in an ecosystem are called food chains (Fig. 31.19). A trophic level is a level of nourishment within a food web or chain. In the grazing food web (see Fig. 31.18), going from left to right: The trees are producers (the first trophic level), the first series of animals are herbivores (the second trophic level), and many of the animals in the next series are carnivores (the third and possibly fourth trophic levels). Food chains are short because energy is lost between trophic levels. In general, only about 10% of the energy of one trophic level is available to the next trophic level. Therefore, if an herbivore population consumes 1,000 kg of plant material, only about 100 kg is converted to herbivore tissue, -10 kg to first-level carnivores, and 1 kg to second-level carnivores. The so-called 10% rule of thumb explains why few carnivores can be supported in a food web. The flow of energy with large losses between successive trophic levels is sometimes depicted as an ecological pyramid (Fig. 31.20).
A pyramid based on the number of organisms can run into problems because, for example, one tree can support many herbivores. Pyramids of biomass, which is the number of organisms multiplied by their weight, eliminate size as a factor. Even then, apparent inconsistencies can arise. In aquatic ecosystems, such as lakes and open seas where algae are the only producers, the herbivores at some point in time may have a greater biomass than the producers. Why? Because even though the algae reproduce rapidly, they are also consumed at a high rate.
Chemical Cycling
The pathways by which chemicals cycle within ecosystems involve both living (producers, consumers, decomposers) and nonliving (rock, inorganic nutrients, atmosphere) components, and therefore are known as biogeochemical cycles. Biogeochemical cycles can be sedimentary or gaseous. In a sedimentary cycle, such as the phosphorus cycle, the chemical is absorbed from the sediment by plant roots, passed to heterotrophs, and eventually returned to the soil by decomposers, usually in the same general area. In a gaseous cycle, such as the nitrogen and carbon cycles, the element returns to and is withdrawn from the atmosphere as a gas.
Chemical cycling of an element may involve reservoirs and exchange pools as well as the biotic community (Fig. 31.21). A reservoir is a source normally unavailable to organisms. For example, much carbon is found in calcium carbonate shells in ocean bottom sediments. An exchange pool is a source from which organisms generally take elements. For example, photosynthesizers can utilize carbon dioxide in the atmosphere for their carbon needs. The biotic community consists of the autotrophic and heterotrophic species of an ecosystem that feed on one another. Human activities, such as mining or burning fossil fuels, increase the amounts of chemical elements removed from reservoirs and cycling within ecosystems. As a result, the physical environment of the ecosystem contains excess chemicals that, in turn, can alter the species composition and diversity of the biotic community.
Phosphorus Cycle
On land, the very slow weathering of rocks fostered by fungi adds phosphates (PO432 and HPO422) to the soil, some of which become available to terrestrial plants for uptake (Fig. 31.22). Phosphates made available by weathering also run off into aquatic ecosystems, where algae absorb phosphates from the water before they become trapped in sediments. Phosphates in sediments become available again only when a geological upheaval exposes sedimentary rocks to weathering once more.
Producers use phosphates in a variety of molecules, including phospholipids, ATP, and the nucleotides that become a part of DNA and RNA. Animals consume producers and incorporate some of the phosphates into teeth, bones, and shells that take many years to decompose. Decomposition of dead plant and animal material and animal wastes does, however, make phosphates available to producers at a faster rate than weathering. Because much of the available phosphates are utilized within food chains, phosphate is usually a limiting inorganic nutrient for ecosystems. In other words, the limited supply of phosphates limit plant growth, and therefore primary productivity. 
The importance of phosphates and calcium to population growth is demonstrated by considering the fate of lemmings every four years (Fig. 31.23). You may have heard that lemmings dash mindlessly over cliffs into the sea; ecologists tell us that actually these lemmings are migrating to find food. What happened? Every four years or so, grasses and sedges of the tundra (see Fig. 31.28) become rich in minerals, and the lemming population starts to explode. Once the lemmings number in the millions, the grasses and sedges of the tundra suffer a decline caused by a lack of minerals. Now the lemming population suffers a crash, but it takes about four years before the animals decompose in this cold region and minerals return to the producers. Then the cycle begins again.
Human ActivitiesA transfer rate is defined as the amount of a nutrient that moves from one component of the environment to another within a specified period of time. Human activities alter the dynamics of a community by changing transfer rates. For example, humans mine phosphate ores and use them to make fertilizers, animal feed supplements, and detergents. Phosphate ores are slightly radioactive, and therefore mining phosphate poses a health threat to all organisms, including the miners. Animal wastes from livestock feedlots, fertilizers from lawns and cropland, and untreated and treated sewage discharged from cities all add excess phosphates to nearby waters. The result is eutrophication, or overenrichment, of a body of water, which causes an algal overpopulation called an algal bloom. When the algae die and decay, oxygen is consumed, causing fish kills. In the mid-1970s, Lake Erie was dying because of eutrophication. Control of nutrient phosphates, particularly in sewage effluent and household detergents, reversed the situation.
Nitrogen Cycle
Nitrogen, in the form of nitrogen gas (N2), comprises about 78% of the atmosphere by volume. But plants cannot make use of nitrogen gas. Instead, plants rely on various types of bacteria to make nitrogen available to them. Therefore, nitrogen, like phosphorus, is also a limiting inorganic nutrient of producers in ecosystems.
Plants can take up both ammonium (NH41) and nitrate (NO32) from the soil and incorporate the nitrogen into amino acids and nucleic acids. Two processes, nitrogen fixation and nitrification, convert nitrogen gas, N2, into NH41 and NO32, respectively (Fig. 31.24). Nitrogen fixation occurs when nitrogen gas is converted to ammonium. Some cyanobacteria in aquatic ecosystems and some free-living, nitrogen-fixing bacteria in soil are able to fix nitrogen in this way. Other nitrogen-fixing bacteria live in nodules on the roots of legumes, plants such as peas, beans, and alfalfa. They make organic compounds containing nitrogen available to the host plant. 
Nitrification is the production of nitrates. Ammonium in the soil is converted to nitrate by certain nitrifying soil bacteria in a two-step process. (First, nitrite-producing bacteria convert ammonium to nitrite (NO22), and then nitrate-
producing bacteria convert nitrite to nitrate.) In Figure 31.24, notice that the biotic community subcycle in the nitrogen cycle does not depend on the presence of nitrogen gas.
Denitrification is the conversion of nitrate to nitrogen gas, which enters the atmosphere. Denitrifying bacteria are chemoautotrophs living in the anaerobic mud of lakes, bogs, and estuaries that carry out this process as a part of their own metabolism. In the nitrogen cycle, denitrification counterbalanced nitrogen fixation until humans started making fertilizer. 
Human ActivitiesHuman activities significantly alter the transfer rates in the nitrogen cycle by producing fertilizers from N2—in fact, humans nearly double the fixation rate. Unfortunately, industrial nitrogen fixation requires tremendous heat and pressure, usually produced by burning great quantities of fossil fuels, with accompanying air pollution. The nitrate in fertilizers, just like phosphate, can leach out of agricultural soils into surface waters, leading to eutrophication. Deforestation by humans also causes a loss of nitrogen to groundwater and makes regrowth of the forest difficult. The underground water supplies in farming areas today are apt to contain excess nitrate. Infants below the age of six months who drink water containing excessive amounts of nitrate can become seriously ill and, if untreated, may die. 
To cut back on fertilizer use, it might be possible to genetically engineer soil bacteria with increased nitrogen fixation rates. Also, farmers could grow legumes that increase the nitrogen content of soil (Fig. 31.25). In one study, the rotation of legumes and winter wheat produced a better yield than fertilizers after several years. 
Carbon Cycle
In the carbon cycle, organisms in both terrestrial and aquatic ecosystems exchange carbon dioxide with the atmosphere (Fig. 31.26). On land, plants take up carbon dioxide from the air, and through photosynthesis, they incorporate carbon into organic nutrients that are used by autotrophs and heterotrophs alike. When aerobic organisms respire, a portion of this carbon is returned to the atmosphere as carbon dioxide, a waste product of cellular respiration.
In aquatic ecosystems, the exchange of carbon dioxide with the atmosphere is indirect. Carbon dioxide from the air combines with water to produce bicarbonate ion (HCO32), a source of carbon for algae that produce food for themselves and for heterotrophs. Similarly, when aquatic organisms respire, the carbon dioxide they give off becomes bicarbonate ion. The amount of bicarbonate in the water is in equilibrium with the amount of carbon dioxide in the air.
Living and dead organisms contain organic carbon and serve as one of the reservoirs for the carbon cycle. The world’s biotic components, particularly trees, contain billions of tons of organic carbon, and additional tons are estimated to be held in the remains of plants and animals in the soil. If dead plant and animal remains fail to decompose, they are subjected to extremely slow physical processes that transform them into coal, oil, and natural gas, the fossil fuels. Most of the fossil fuels were formed during the Carboniferous period, 286–360 mya, when an exceptionally large amount of organic matter was buried before decomposing. Another reservoir is the  calcium carbonate (CaCO3) that accumulates in limestone and in shells. Many marine organisms have calcium carbonate shells that remain in bottom sediments long after the organisms have died. Geological forces change these sediments into limestone.
Human ActivitiesThe transfer rates of carbon dioxide due to photosynthesis and cellular respiration are just about even. However, more carbon dioxide is being deposited in the atmosphere than is being removed. This increase is largely due to the burning of fossil fuels and the destruction of forests to make way for farmland and pasture. When we do away with forests, excess carbon dioxide enters the atmosphere. But only about half of this excess CO2 remains in the atmosphere; it is believed that the rest is dissolved in the ocean.
The increased amount of carbon dioxide (and other gases) in the atmosphere is causing a rise in temperature called global warming. These gases allow the sun’s rays to pass through, but they absorb and reradiate heat back to the Earth, a phenomenon called the greenhouse effect. Scientists predict that if the ice at the poles melts and sea levels rise as a result, many of the world’s most populous cities will be flooded. Furthermore, weather pattern changes might cause the American Midwest to become a dust bowl. 
31.3		Ecology of Major Ecosystems
The biosphere, which encompasses all the ecosystems on planet Earth, is the final level of biological organization. Aquatic ecosystems are divided into those composed of fresh water and those composed of salt water (marine ecosystems) (Fig. 31.27). The ocean is a marine ecosystem that covers 70% of the Earth’s surface. Two types of freshwater ecosystems are those with standing water, such as lakes and ponds, and those with running water, such as rivers and streams. The richest marine ecosystems lie near the coasts. Coral reefs are located offshore, while marshes occur where rivers meet the sea.
Scientists recognize several distinctive major types of terrestrial ecosystems, also called biomes (Fig. 31.28). Temperature and rainfall define the biomes, which contain communities adapted to the regional climate. The tropical rain forests, which occur at the equator, have a high average temperature and the greatest amount of rainfall of all the biomes. They are dominated by large evergreen, broad-leaved trees. The savanna is a tropical grassland with alternating wet and dry seasons. Temperate grasslands receive less rainfall than temperate forests (in which trees lose their leaves during the winter) and more water than deserts, which lack trees. The taiga is a very cold northern coniferous forest, and the tundra, which borders the North Pole, is also very cold, with long winters and a short growing season. A permafrost persists even during the summer in the tundra and prevents large plants from becoming established.
Primary Productivity
One way to compare ecosystems is based on primary productivity, the rate at which producers capture and store energy as organic nutrients over a certain length of time. Temperature and moisture, and secondarily the nature of the soil, influence the primary productivity and, as already discussed, the assemblage of species in an ecosystem. In terrestrial ecosystems, primary productivity is generally lowest in high-latitude tundras and deserts, and highest at the equator where tropical forests occur (Fig. 31.29). The high productivity of tropical rain forests provides varied niches and much food for consumers. The number and diversity of species in tropical rain forests are the highest of all the terrestrial ecosystems. Therefore, conservation biologists are interested in preserving as much of this biome as possible.
The primary productivity of aquatic communities is largely dependent on the availability of inorganic nutrients. Estuaries, swamps, and marshes are rich in organic nutrients and in decomposers that convert those organic nutrients into their inorganic chemical components. Estuaries, swamps, and marshes also contain a large number of varied species, particularly in the early stages of their development before they venture forth into the sea. Therefore, all of these coastal regions are in great need of preservation. The open ocean has a productivity somewhere between that of a desert and the tundra because it lacks a concentrated supply of inorganic nutrients. Coral reefs exist near the coasts in warm tropical waters where currents and waves bring nutrients and where sunlight penetrates to the ocean floor. Coral reefs are areas of remarkable biological abundance, equivalent to that of tropical rain forests.
The Chapter in Review
Summary
31.1		Ecology of Communities
Knowledge of community and ecosystem ecology is important for understanding the impacts of human alterations to the environment.
CommunityA community is an assemblage of the populations of different species interacting with each other in a given area. 
EcosystemAn ecosystem consists of species interacting with one another and with the physical environment. 
Ecological Succession 
bare rocklichens/mossesgrassesshrubstrees
The two types of ecological succession are primary succession (begins on bare rock) and secondary succession (following a disturbance; begins where soil is present). Ecological succession leads to a stable climax community.
Interactions in Communities
Species in communities interact with one another in the following ways: 
	•	CompetitionSpecies vie with one another for resources, such as light, space, and nutrients. Aspects of competition are the competitive exclusion principle, resource partitioning, and character displacement.
	•	PredationOne species (predator) eats another species (prey). 
	•	ParasitismOne species (parasite) obtains nutrients from another species (host) but does not kill the host species. 
	•	CommensalismOne species benefits from the relationship, while the other species is not harmed.
	•	MutualismTwo species interact in a way that benefits both.
Ecological NicheThe ecological niche of a species is defined by the role it plays in its community, the habitat, and its interactions with other species.
Keystone SpeciesThe interactions of a keystone species in the community hold the community and its species together. Removal of a keystone species can lead to species extinctions and loss of diversity. An example of a keystone species is the grizzly bear.
Native Versus Exotic SpeciesNative species are indigenous to a given area and thrive without assistance. Exotic species are introduced into an area, and greatly disrupt the balance and interactions between native species in that area’s community. 
31.2		Ecology of Ecosystems
In the food chain of an ecosystem, some populations are autotrophs and some are heterotrophs.
Autotrophs are the producers. They require only inorganic nutrients (e.g., CO2 and minerals) and an outside energy source to produce organic nutrients for their own use and for the use of other members of the community. Examples of autotrophs are algae, cyanobacteria, and plants.
Heterotrophs are the consumers. They require a preformed source of organic nutrients and give off CO2. Examples of consumers are herbivores (feed on plants), carnivores (feed on other animals), and omnivores (feed on both plants and animals). Other heterotrophs are the decomposers (the bacteria and fungi of decay).
Energy Flow and Chemical Cycling
Energy flows through an ecosystem, while chemicals cycle within an ecosystem.
Food Webs and Food ChainsEnergy flows in an ecosystem through food chains and detrital and grazing food webs.
Trophic LevelA trophic level is a level of nourishment in a food web or chain. 
Ecological PyramidAn ecological pyramid illustrates the energy losses that occur between trophic levels. 
	•	Only about 10% of the energy of one trophic level is available to the next trophic level.
	•	Top carnivores occupy the last and smallest trophic level.
Biogeochemical CycleChemicals cycle within an ecosystem through various biogeochemical cycles, such as the phosphorus cycle, the nitrogen cycle, and the carbon cycle. Human activities significantly alter the transfer rates in these cycles.
31.3		Ecology of Major Ecosystems
The biosphere encompasses all the major ecosystems of the Earth.
Aquatic Ecosystems
Aquatic ecosystems are classified as freshwater ecosystems (rivers, streams, lakes, ponds) and saltwater, or marine, ecosystems (oceans, coral reefs, saltwater marshes). 
Terrestrial Ecosystems
The terrestrial ecosystems are called biomes. The major biomes are:
	•	Tundra
	•	Taiga
	•	Temperate forests
	•	Tropical grasslands (savanna)
	•	Temperate grasslands (prairie)
	•	Deserts
	•	Tropical rain forests
Primary Productivity 
Primary productivity is the rate at which producers capture solar energy and convert it to chemical energy over a specified length of time. The number of species in an ecosystem is positively related to its primary productivity.
Thinking Scientifically
	1.	One of the most striking examples of coevolution is between insects and flowers. The earliest angiosperms produced wind-pollinated flowers, which released large quantities of pollen. The ovules exuded tiny droplets of sugary sap to catch passing pollen. Outline the course of events that probably took place between insects and flowers to result in the highly specialized interactions we see today.
	2.	Over 200 wildlife species have been observed around prairie dog colonies in the Great Plains of the United States. The prairie dog, which burrows, forages, and feeds in the area, acts as a keystone species. If the prairie dogs are destroyed, many other species will die as well. How do you think the activities of the prairie dogs influence the survival of so many other species? 
Testing Yourself
Choose the best answer for each question.
	1.	As diversity increases,
	a.	species richness increases, and distribution of species becomes more even.
	b.	species richness decreases, and distribution of species becomes more even.
	c.	species richness increases, and distribution of species becomes less even.
	d.	species richness decreases, and distribution of species becomes less even.
	2.	Which is not a feature of an opportunistic pioneer species?
	a. long life span
	b. short time to maturity
	c. small size
	d. high reproductive output
For statements 3-–7, indicate the type of interaction described in each scenario.
Key:
	a. competition
	b. predation
	c. parasitism
	d. commensalism
	e. mutualism
	3.	An alfalfa plant gains fixed nitrogen from the bacterial species Rhizobium in its root system, while Rhizobium gains carbohydrates from the plant. 
	4.	Both foxes and coyotes in an area feed primarily on a limited supply of rabbits. 
	5.	Roundworms establish a colony inside a cat’s digestive tract. 
	6.	A fungus captures nematodes as a food source. 
	7.	An orchid plant lives in the treetops, gaining access to sun and pollinators, but not harming the trees.
	8.	The abundance of both species is expected to increase as a result of which type of interaction?
	a. predation	d. competition
	b. commensalism	e. parasitism 
	c. mutualism
	9.	According to the competitive exclusion principle,
	a.	one species is always more competitive than another for a particular food source.
	b.	competition excludes multiple species from using the same food source.
	c.	no two species can occupy the same niche at the same time.
	d.	competition limits the reproductive capacity of species.
	10.	Fungi are examples of
	a. autotrophs.	d. omnivores.
	b. herbivores.	e. decomposers.
	c. carnivores.
	11.	In the following diagram, fill in the components of chemical cycling and nutrient flow. 
	12.	An ecological pyramid depicts the amount of _________ in various trophic levels.
	a. food	c. energy
	b. organisms	d. nutrients 
	13.	Which of the following would be a primary consumer in a vegetable garden?
	a. aphid sucking sap from cucumber leaves
	b. lady beetle eating aphids
	c. songbird eating lady beetles
	d. fox eating songbirds
	e. All of these are correct.
	14.	Detritus always contains
	a. bacteria and fungi.	c. decaying logs.
	b. leaf litter.	d. animal carcasses.
	15.	Producers are 
	a. autotrophs.	d. carnivores.
	b. herbivores.	e. Both a and b are correct.
	c. omnivores.
	16.	The first trophic level in a food web is occupied by the 
	a. producers.
	b. primary consumers.
	c. secondary consumers.
	d. tertiary consumers.
	17.	Which of the following represents a grazing food chain?
	a. leaves  detritus feeders  deer  owls
	b. birds  mice  snakes
	c. nuts  leaf-eating insects  chipmunks  hawks
	d. leaves  leaf-eating insects  mice  snakes
	18.	In a grazing food web, carnivores that eat herbivores are
	a. producers.
	b. primary consumers.
	c. secondary consumers.
	d. tertiary consumers.
	19.	Identify the components of the ecological pyramid in the following diagram. 
	20.	Minerals in rocks are considered members of this component of an ecosystem.
	a. exchange pool 
	b. community
	c. reservoir
	d. More than one of these are correct.
	21.	Which of the following is a sedimentary biogeochemical cycle?
	a. carbon
	b. nitrogen
	c. phosphorus
	22.	Underground oil is an example of a carbon
	a. cycle.
	b. pathway.
	c. reservoir.
	d. exchange pool.
For questions 23–25, match the description to the process in the key.
Key:
	a. nitrogen fixation
	b. nitrification
	c. denitrification
	23.	Nitrate to nitrogen gas.
	24.	Nitrogen gas to nitrate.
	25.	Nitrogen gas to ammonium.
	26.	Which of the following is not a component of the nitrogen cycle?
	a. proteins
	b. ammonium
	c. decomposers
	d. photosynthesis
	e. bacteria in root nodules
	27.	Which biome is characterized by a coniferous forest with low average temperature and moderate rainfall?
	a. taiga
	b. savanna
	c. tundra
	d. tropical rain forest
	e. temperate forest 
	28.	Which biome has the lowest primary productivity?
	a. tundra
	b. lake
	c. sandy beach
	d. prairie
	e. temperate forest
For questions 29–34, match the description to the biome in the key. 
Key:
	a. tundra
	b. taiga
	c. tropical rain forest
	d. temperate grassland
	e. tropical grassland
	f. desert
	29.	Very cold northern coniferous forest.
	30.	Prairie.
	31.	Northernmost terrestrial ecosystem; persisting permafrost.
	32.	Changeable temperatures with minimal rainfall; low primary productivity.
	33.	Occurs near the equator; high temperatures, large amount of rainfall; high primary productivity.
	34.	Savanna, alternating wet and dry seasons.
For questions 35–38, match the description to the type of aquatic ecosystem in the key. 
Key:
	a. ocean
	b. marshes
	c. rivers, lakes
	d. coral reefs
	35.	Lie near the coast; high primary productivity.
	36.	Lowest primary productivity.
	37.	Freshwater ecosystems.
	38.	Lie offshore; high primary productivity.
Go to www.mhhe.com/maderessentials for more quiz questions.
Bioethical Issue
Many exotic species, such as zebra mussels and sea lampreys, are so obviously troublesome that most people do not object to programs aimed at controlling their populations. However, some exotic species eradication programs meet with more resistance. For example, the mute swan, one of the world’s largest flying birds, is beautiful, graceful, and makes an impressive presence. However, it is very aggressive and territorial. The mute swan was introduced to the United States from Asia and Europe in the nineteenth century as an ornamental bird. The birds consume large amounts of aquatic vegetation and displace native birds from feeding and nesting areas. The U.S. Fish and Wildlife Service believes it will be necessary to kill 3,000 mute swans in Maryland in the next two years in order to protect native bird populations. Attempts to limit the size of the mute swan populations in Maryland and other states have been met with opposition by citizens who find the birds beautiful.
Do you feel that native populations need not be protected as long as the exotic species serves a suitable human purpose? Or, do you feel that native species should be protected regardless?
Understanding the Terms
aquatic ecosystem568
biogeochemical cycle564
biomass563
biosphere568
character displacement 557
climax community554
coevolution552
commensalism556
community552
competition556
competitive exclusion 
principle557
consumer560
decomposer560
detrital food web562
detritus560
diversity553
ecological niche556
ecological pyramid563
ecological succession554
ecosystem552
eutrophication565
exotic species559
food chain563
food web562
fossil fuel567
global warming567
grazing food web562
greenhouse effect567
habitat556
keystone species558
mutualism556
native species559
parasitism556
predation556
primary productivity570
primary succession554
producer560
resource partitioning557
secondary succession554
species richness553
terrestrial ecosystem568
transfer rate565
trophic level563
Match the terms to these definitions:
a.	_______________	Assemblage of populations of different species. 
b.	_______________	Relationship in which one species obtains nutrients from another species but does not kill it. 
c.	_______________	Combination of the role a species plays in its community, its habitat, and its interactions with other species. 
d.	_______________	Tendency for characteristics to be more divergent when populations of different species belong to the same community than when they are isolated. 
e.	_______________	One species in a community that stabilizes the community, helps maintain its characteristics, and helps hold the web of interactions together. 
f.	_______________	Remains of dead organisms plus the bacteria and fungi of decay. 
g.	_______________	All the organisms that feed at a particular link in a food chain. 
h.	_______________	Amount of a nutrient that moves from one component of the environment to another within a certain period of time. 
i.	_______________	All the ecosystems on planet Earth. 

Approximately 70% of the Earth is covered by water, but less than 1% of that water is drinkable.
Too much of a nutrient such as nitrogen can harm an ecosystem.
Figure 31.2Community species composition. 
Communities differ in their species composition, as exemplified by the predominant plants and animals in (a) a coniferous forest and (b) a tropical rain forest. Some mammals found in coniferous forests and in tropical rain forests are listed.
Figure 31.1Coevolution. 
Flowers and pollinators have evolved to be suited to one another. a. Hummingbird pollinated flowers are usually red, a color that these birds can see, and the petals are recurved to allow the stamens to dust the birds’ heads. b. The reward offered by the flower is not always food. This orchid looks and smells like the female of this wasp’s species. The male tries to copulate with flower after flower and in the process transfers pollen. c. Bats are nocturnal, and the flowers they pollinate are white or light-colored making them visible in moonlight. The flowers smell like bats and are large and sturdy enabling them to withstand insertion of the bat’s head as it uses its long, bristly tongue to lap up nectar and pollen.
Check Your Progress
	1.	Describe what is meant by coevolution.
	2.	
Contrast a community with an ecosystem.
	3.	Contrast species richness with diversity.
	4.	Contrast primary succession with secondary succession.
Answers:1. An evolutionary change in one species results in an evolutionary change in another species.2. A community is a group of populations, while an ecosystem is all the species in a community interacting with each other and with the environment.3. Species richness is the list of species in a community (species composition), while diversity encompasses both composition and the relative abundance of each species.4. Primary succession occurs where soil has not yet been formed, while secondary succession occurs where soil is present.
Figure 31.5Secondary succession.
Secondary successional changes in a western Pennsylvania field from (a) first year, (b) second year, (c) fifth year, and (d) after twenty years.
Figure 31.4Primary and secondary succession. 
Primary succession begins on areas of bare rock. Secondary succession begins in areas where soil remains following natural or human-caused disturbance.
Figure 31.8Competitive exclusion principle demonstrated by Paramecium. 
The competitive exclusion principle states that no two species occupy the exact same niche. When grown separately, Paramecium caudatum and Paramecium aurelia exhibit logistic growth. When grown together, P. aurelia excludes P. caudatum.
Data from G.F. Gause, The Struggle for Existance, 1934, Williams & Wilkins Company, Baltimore, MD. p. 557.
Figure 31.9Character displacement in finches on the Galápagos Islands.
When G. fuliginosa, G. fortis, and G. magnirostris coexist on the same island, their beak sizes are appropriate for eating small-, medium-, and large-sized seeds, respectively. When G. fortis and G. fuliginosa are on separate islands, their beaks have the same intermediate size, which allows them to eat seeds of various sizes. Character displacement is evidence that resource partitioning has occurred. 
Figure 31.6Niche of a backswimmer. 
Backswimmers require warm, clear pond water containing insects that they can eat and vegetation where they can hide from predators. 
Figure 31.7Feeding niches for wading birds. 
Flamingos feed in deeper water by filter feeding; dabbling ducks feed in shallower areas by upending; avocets feed by sifting. Oystercatchers and plovers have adaptations for feeding in shallows, such as shorter legs.
Check Your Progress
	1.	List the five major types of species interactions in a community.
	2.	
Distinguish between habitat and ecological niche.
	3.	Why is character displacement a form of resource partitioning?
	4.	
Describe the role of a keystone species.
Answers:1. Competition, predation, parasitism, commensalism, and mutualism.2. A habitat is the special location of a species. It is one component of an ecological niche, which also includes the role the species plays in the community and its interactions with other species.3. Due to character displacement, species are able to feed on different types of food. 
4. A keystone species stabilizes the community, helps maintain its characteristics, and helps hold the web of interactions together.
Figure 31.12Exotic species. 
Human introduction of exotic species, such as (a) the red fox and (b) the brown tree snake, have disrupted communities in Australia and Guam, respectively.
Figure 31.10Mutualism.
The bullhorn acacia tree is adapted to provide nourishment for a mutualistic ant species. a. The thorns are hollow, and the ants live inside. b. The bases of the leaves have nectaries (openings) where ants can feed. c. The tips of the leaves of the bullhorn acacia have Beltian bodies, which ants harvest for larval food. 
Figure 31.11Interpendence of species. 
Clark’s nutcrackers feed on the seeds of whitebark pines. But the storing of the seeds by the nutcrackers is the primary means of seed dispersal for whitebark pine. 
Figure 31.16Chemical cycling and energy flow.
Chemicals cycle within, but energy flows through, an ecosystem. As energy is repeatedly passed from one component to another, all the chemical energy derived from solar energy dissipates as heat.
Figure 31.17Energy balances.
Only about 10% of the nutrients and energy taken in by a herbivore is passed on to carnivores. A large portion goes to detritus feeders. Another large portion is used for cellular respiration.
Figure 31.13Producers.
Green plants and algae are photoautotrophs.
Figure 31.14Consumers.
a. Caterpillars and giraffes are herbivores. b. A praying mantis and an osprey are carnivores. 
Figure 31.15Decomposers.
Mushrooms and bacteria are decomposers.
Figure 31.20Ecological pyramid.
An ecological pyramid depicts the loss of nutrients and energy from one trophic level to the next. 
Figure 31.19Food chain.
A food chain diagrams a single path of energy flow in an ecosystem.
Figure 31.18Food webs.
The grazing and detrital food webs of ecosystems are linked.
Check Your Progress
	1.	Contrast the two types of autotrophs.
	2.	
Contrast herbivores with carnivores.
	3.	Contrast the first organisms in the grazing food web with those in the detrital food web.
	4.	
Contrast a food chain with an ecological pyramid.
	5.	Contrast a sedimentary biogeochemical cycle with a gaseous one.
Answers:1. Photoautotrophs use photosynthesis to produce organic nutrients, while chemoautotrophs use energy from inorganic compounds to produce organic nutrients.2. Herbivores are animals that feed on autotrophs, while carnivores feed on other animals.3. The first organisms in the grazing food web are herbivorous insects, while those in the detrital food web are bacteria and fungi.4. A food chain depicts a single path of energy flow in an ecosystem, while an ecological pyramid depicts the entire flow of energy between trophic levels.5. A sedimentary cycle involves the exchange of chemicals in the soil, while a gaseous cycle involves chemicals in the atmosphere.
Figure 31.22The phosphorus cycle.
The phosphorus cycle is a sedimentary biogeochemical cycle. Globally, phosphates flow into large bodies of water and become a part of sedimentary rocks. Thousands or millions of years later, the seafloor can rise; the phosphates are then exposed to weathering and become available. Locally, phosphates cycle within a community when plants on land and algae in the water take them up. Animals gain phosphates when they feed on plants or algae. Decomposers return phosphates to plants or algae, and the cycle within the community begins again. 
Figure 31.23Lemmings.
Lemmings, small furry rodents, feed on plant species in an Arctic community called the tundra. 
Figure 31.21Model for chemical cycling.
Chemical nutrients cycle between these components of ecosystems. Reservoirs, such as fossil fuels, minerals in rocks, and sediments in oceans, are normally relatively unavailable sources, but exchange pools, such as those in the atmosphere, soil, and water, are available sources of chemicals for the biotic community. When human activities (purple arrows) remove chemicals from reservoirs and make them available to the biotic community, pollution can result.
Check Your Progress
	1.	Describe how phosphate enters the phosphorus cycle.
	2.	
Describe the significance of nitrogen fixation and nitrification to the nitrogen cycle.
	3.	Describe the relationship between the greenhouse effect and global warming.
Answers:1. Most phosphate comes from decomposition of organisms, but some comes from weathering.2. Both processes allow nitrogen to enter the nitrogen cycle.3. The greenhouse effect (absorption of the sun’s rays) is believed to contribute to global warming (a rise in the temperature of the atmosphere).
Figure 31.26The carbon cycle.
The carbon cycle is a gaseous biogeochemical cycle. Producers take in carbon dioxide from the atmosphere and convert it to organic molecules that feed all organisms. Fossil fuels arise when organisms die but do not decompose. The burning of fossil fuels releases carbon dioxide and causes environmental pollution. 
Figure 31.24The nitrogen cycle.
The nitrogen cycle is a gaseous biogeochemical cycle normally maintained by the work of several populations of soil bacteria. 
Check Your Progress
	1.	Describe the two major types of ecosystems of the biosphere.
	2.	
List the terrestrial ecosystems of the world.
	3.	Explain why swamps have higher levels of primary productivity than open oceans.
Answers:1. The biosphere is divided into aquatic ecosystems and terrestrial ecosystems.2. Taiga, savanna, prairie, temperate forest, desert, tropical rain forest, and tundra.3. Swamps have a more concentrated supply of nutrients.
Figure 31.28The major terrestrial ecosystems.
The tundra is the northernmost terrestrial ecosystem and has the lowest average temperature of all the terrestrial ecosystems, with minimal to moderate rainfall. The taiga, a coniferous forest that encircles the globe, also has a low average temperature, but moderate rainfall. Temperate forests have moderate temperatures and occur where rainfall is moderate, yet sufficient to support trees. A tropical grassland (savanna) has high temperatures and moderate/seasonal rainfall. A temperate grassland (prairie) has low to high temperatures, with low annual rainfall. Deserts have changeable temperatures with minimal rainfall. Tropical rain forests, which generally occur near the equator, have a high average temperature and the greatest amount of rainfall of all the terrestrial ecosystems.
Figure 31.27The major aquatic ecosystems.
Aquatic ecosystems are divided into those that have salt water, such as the ocean (a) and those that have fresh water, such as a river (b). Saltwater, or marine, ecosystems also include coral reefs (c) and marshes (d).
Figure 31.29Primary productivity.
Ecologists can compare ecosystems based on primary productivity, the rate at which producers convert and store solar energy as chemical energy.

Table 31.1	Species Interactions	

Interaction	Expected Outcome	
Competition (22)	Abundance of both species decreases.	
Predation (12)	Abundance of predator increases, and abundance of prey decreases.	
Parasitism (12)	Abundance of parasite increases, and abundance of host decreases.	
Commensalism (10)	Abundance of one species increases, and the other is not affected.	
Mutualism (11)	Abundance of both species increases.	

a. During the first year, only the remains of corn plants are seen. 
b. During the second year, wild grasses have invaded the area.
c. By the fifth year, the grasses look more mature, and sedges have joined them. 
d. 
After twenty years, the juniper trees are mature, and there are also birch and maple trees in addition to the blackberry shrubs.
a. Herbivores
b. Carnivores
Figure 31.3Climax communities. 
Does succession in a particular area always lead to the same climax community? For example, temperate forests (a) only occur where there is adequate rainfall, and deserts (b) occur where rainfall is minimal. Even so, the exact same mix of plants and animals may not always arise because the assemblage of organisms depends on which organisms, by chance, migrate to the area.
Figure 31.25Root nodules.
Bacteria that live in nodules on the roots of plants in the legume family, such as pea plants, convert nitrogen in the air to a form that plants can use to make proteins.
Grassland (savanna)
Tundra
Tropical rain forest

DesertTemperate forest

Taiga
Temperate grassland (prairie)