12

Exotic and Endemic Species

A community can change dramatically after the introduction of an exotic species. An exotic species is a nonnative species, a species that evolved in one community, then dispersed from its home and became established elsewhere. By contrast, an endemic species is one that evolved in a particular community and exists nowhere else. The rate of exotic species introductions has accelerated along with the pace of global travel. More than 4,500 exotic species have become established in the United States. An estimated 25 percent of Florida's plant and animal species are exotics. In Hawaii, 45 percent are exotic. Some of these species were brought in as food crops, as ornamentals for gardens, or as a source of textiles. Other species arrived as stowaways along with cargo from distant regions. Many exotic species have little impact on their adoptive community, but some of them are invasive. An invasive species is an exotic species that harms members of its new community. In some cases, the arrival of an exotic species can lead to the extinction of endemic species. Invasive species often have a far greater impact in their new community than they did in the region in which they originated. When a species leaves its community of origin behind, it also leaves behind the competitors, predators, and parasites with which it coevolved and which helped to keep its population in check. If the invasive species is a parasite, predator, or herbivore, it also leaves behind hosts or prey that had coevolved with it and had defenses against it. Its new hosts are often defenseless against it. As a result, an invasive species often reaches a higher population density in its new home than it achieved in its old one. The water mold that is currently killing off oaks in the Pacific Northwest (Section 20.4) is an invasive species, as are the fungi that currently threaten North America's bats (Section 22.3) and frogs (Chapter 22 Application). Sea lampreys are an invasive species in the Great Lakes (Section 24.2). As another example, consider the woody perennial vine called kudzu (Pueraria lobata). Native to Asia, kudzu was introduced to the American Southeast as a food for grazers and to control erosion, but it quickly became an invasive weed. It is now present in 31 states, and continues to spread. Kudzu decreases the diversity of natural communities by overgrowing native trees and shrubs, and outcompeting them for sunlight (Figure 41.17A). Kudzu also poses a threat to agriculture; it serves as the main winter host for soybean rust, a fungal pathogen that can drastically reduce soybean yields. Gypsy moths (Lymantria dispar) are an invasive species too. They entered the northeastern United States in the mid-1700s and now range into the Southeast, Midwest, and Canada. Gypsy moth caterpillars (Figure 41.17B) preferentially feed on oaks. Loss of leaves to gypsy moths weakens the trees, making them less efficient competitors and more susceptible to pathogens. Large semiaquatic rodents called nutrias (Myocastor coypus) were imported from South America for their fur. Today, descendants of animals that either escaped or were intentionally released thrive in freshwater marshes and along rivers in twenty states (Figure 41.17C). Their appetite for plants threatens native vegetation and crop plants, and their burrowing contributes to marsh erosion and damages levees, increasing the risk of flooding.

Factors that Shape Communities

A biological community includes all the populations of all species that live in a particular place at the same time. The scale of a community is defined by a human observer, so one community can contain smaller communities or be a component of a larger one. Consider the community of microbial organisms that live inside the digestive tract of a termite. That termite is part of a larger community of organisms that live in and on a fallen log. The many log-dwellers are part of a still larger forest community. Communities can differ in species diversity even if they are of similar size. There are two components to diversity. The first, species richness, refers to the number of different species in a community. The second is species evenness, or the relative abundance of each species. For example, a pond that has five fish species in nearly equal numbers has a higher species diversity than a pond with one abundant fish species and four rare ones. Community structure is dynamic, which means that in any community, the array of species and their relative abundances tend to change over time. Communities change over a long time span as they form and then age. They also change over the short term as a result of disturbances. Each species can only live in a specific habitat, a place with resources it needs and conditions it can tolerate. Thus, geography and climate affect community structure. Factors such as soil quality, sunlight intensity, rainfall, and temperature vary with latitude and elevation. Tropical (low latitude) regions receive the most sunlight energy and have the most even temperature. For most plant and animal groups, the number of species is greatest near the equator, and declines as you move toward the poles. Tropical forest communities have more types of trees than temperate ones. Similarly, tropical reef communities are more diverse than marine communities farther from the equator. Species interactions also influence community structure. In some cases, the effect is indirect. For example, when songbirds eat caterpillars, the birds indirectly benefit the trees that the caterpillars feed on, while directly reducing the abundance of caterpillars. Biologists categorize direct species interactions by their effects on both participating species (Table 41.1). For example, commensalism helps one species and has no effect on the other. Commensal orchids live attached to the trunk or branches of a tree (Figure 41.1). Having a perch in the light benefits the orchid, and the tree is unaffected. Relationships are considered commensal only when one species benefits and the other neither benefits nor is harmed by the relationship. If evidence of helpful or harmful effects is discovered, the relationship is reclassified. Species interactions may be fleeting or longer term. Symbiosis, which means "living together," refers to a relationship in which two species have a prolonged close association. Mutualism, parasitism, and commensalism can be symbioses. Two species that interact closely for generations often coevolve. As Section 17.10 explained, coevolution is an evolutionary process in which each species acts as a selective agent that shifts the range of variation in the other.

Ecological Pyramids

A food web diagram is one way of depicting the trophic relationships of species in a particular ecosystem. Ecological pyramid diagrams are another. In such diagrams, primary producers collectively form a base for successive tiers of consumers above them. Figure 42.5 shows ecological pyramids for one freshwater spring ecosystem in Florida. A biomass pyramid shows the dry mass (weight) of organic material in the bodies of organisms at each trophic level at a specific time. Most commonly, producers account for the bulk of the biomass in a pyramid, and top carnivores contribute relatively little. The Florida ecosystem has lots of aquatic plants but very few gars (a top predator in this ecosystem). Similarly, if you walk through a prairie, you will see many more grams of grass than of coyote. An energy pyramid shows the amount of energy that flows through each trophic level in a given interval. An energy pyramid is always broadest at the bottom. Sunlight energy is captured at the base (by primary producers) and declines with successive levels to the pyramid's tip (top carnivores). People sometimes promote a vegetarian diet by touting the ecological benefits of "eating lower on the food chain." They are referring to energy that is lost in transfers between plants, livestock, and humans. When people eat plants, they get a larger proportion of the energy that the plant captured than they would if the plant was used to feed livestock. When plants are used to feed livestock, only a very small percentage of the energy that was stored in the plant body ends up in the meat a person can eat.

Keystone Species

A keystone species is a species that has a disproportionately large effect on a community relative to its size and abundance. It is named after a central, wedge-shaped stone that holds others stones of a archway in place. Robert Paine coined the term "keystone species" to describe the results of his experiments on the rocky shores of Washington state. Species in the rocky intertidal zone withstand pounding surf by clinging to rocks. A rock to cling to is a limiting factor. Paine set up control plots with the sea star Pisaster ochraceus and its main prey—chitons, limpets, barnacles, and mussels. Then he removed all sea stars from his experimental plots. Sea stars prey mainly on mussels. With sea stars missing from experimental plots, mussels took over, crowding out seven other species of invertebrates (Figure 41.16). Paine concluded that sea stars normally prevent competitive exclusion by mussels in the intertidal zone, so they keep the number of species here high. Keystone species need not be predators. For example, beavers are keystone species in some communities. These large, herbivorous rodents cut down trees by gnawing through their trunks. The beaver then uses the felled trees to build a dam, thus creating a deep pool where a shallow stream would otherwise exist. By altering the physical conditions in a section of the stream, the beaver changes which species of fish and aquatic invertebrates can live there.

An Experimental Study

A long-term study by the evolutionary biologists John Endler and David Reznick illustrates the effect of predation on life history traits. Endler and Reznick studied populations of guppies, small fishes that are native to shallow freshwater streams in the mountains of Trinidad (Figure 40.9). The scientists focused their attention on a region where many small waterfalls prevent guppies in one part of a stream from moving easily to another. As a result of these natural barriers, each stream holds several populations of guppies that have very little gene flow between them (Section 17.6). The waterfalls also keep guppy predators from moving from one part of the stream to another. The main guppy predators, killifishes and cichlids, differ in their size and prey preferences. The relatively small killifish preys mostly on immature guppies, and ignores the larger adults. The cichlids are larger fish. They tend to pursue large, mature guppies and ignore small ones. In some regions of the stream, guppies are exposed to one type of predator but not the other. Reznick and Endler looked at the life history traits of guppies in different regions to see if they varied. They discovered that guppies in regions where cichlids are the only predators grow faster and are smaller at maturity than guppies in regions where killifish are the only threat. Guppies in populations preyed on by cichlids alone also reproduce earlier, produce more offspring each time they breed, and breed more frequently. The researchers were not sure whether the observed differences in life history traits were genetic, or a result of some environmental variation between regions. To find out, they collected guppies from both cichlid- and killifish-dominated streams. They reared the groups in separate aquariums under identical predator-free conditions. Two generations later, the guppy groups continued to have the differences observed in natural populations. The researchers concluded that the predator-associated life history differences they had observed between guppies have a genetic basis. Reznick and Endler hypothesized that the predators act as selective agents on guppy life history patterns. They made a prediction: If life history traits evolve in response to predation, then these traits will change in a population after a new predator that favors different prey traits is introduced. To test their prediction, they carried out an experiment. They located a stream region that was above a waterfall and had killifish but no guppies or cichlids. To this region, they introduced guppies from a site below the waterfall, where there were cichlids but no killifish. Thus, at the experimental site, guppies that had previously lived only with cichlids (which eat large guppies) were now exposed to killifish (which eat smaller ones). The control site was the downstream region below the waterfall, where relatives of the transplanted guppies still coexisted with cichlids. Reznick and Endler revisited the stream over the course of eleven years and thirty-six generations of guppies. They monitored traits of guppies above and below the waterfall. The recorded data showed that guppies at the upstream experimental site were evolving. Exposure to a previously unfamiliar predator caused changes in the guppies' rate of growth, age at first reproduction, and other life history traits. By contrast, guppies at the control site showed no such changes. Reznick and Endler concluded that life history traits in guppies can evolve rapidly in response to the selective pressure exerted by predation.

Size, Density, and Distribution

A population is a group of organisms that interbreed with one another more often than they interbreed with other members of their species. Demographics are statistical characteristics that can be used to describe a population. They include population size, density, and distribution, which we discuss here, as well as age structure and fertility rates, which we consider in Section 40.6. Population size is the total number of individuals in a population. Population density is the number of individuals per unit area or volume. Examples of population density include the number of dandelions per square meter of lawn or the number of amoebas per milliliter of pond water. Population distribution describes the location of individuals relative to one another. Members of a population may be clumped together, be an equal distance apart, or be distributed randomly. The most common population distribution pattern is a clumped one, in which members of the population are closer to one another than would be predicted by chance alone. A patchy distribution of resources encourages clumping. Hippopotamuses clump in muddy river shallows, for example (Figure 40.1). Moisture-loving ferns may cover a damp, north-facing slope and be absent from an adjacent drier and sunnier south-facing slope. A limited ability to disperse increases the likelihood of a clumped distribution too: The nut really does not fall far from the tree. Asexual reproduction is another source of clusters. It results in colonies of coral and vast stands of poplar trees. Finally, some animals have a clumped distribution because they benefit by living in groups. Intense competition for limited resources can produce a near-uniform distribution, with individuals more evenly spaced than would be expected by chance. Creosote bushes in deserts of the American Southwest grow in this pattern. Competition for limited water among the root systems keeps the plants from growing in close proximity. Similarly, seabirds in breeding colonies often show a near-uniform distribution. Each nesting bird aggressively repels others that get within reach of its beak (Figure 40.1B). A random distribution occurs when resources are distributed uniformly through the environment, and proximity to others neither benefits nor harms individuals. For example, when the wind-dispersed seeds of dandelions land on the uniform environment of a suburban lawn, dandelion plants grow in a random pattern (Figure 40.1C). The scale of the area sampled and timing of a study can influence observed demographics. For example, seabirds are spaced almost uniformly at a nesting site, but the nesting sites are clumped along a shoreline. The birds crowd together during the breeding season, but disperse when breeding is over.

Life History Patterns

A population's growth rate is affected by its members' life history—the manner in which individuals allocate resources to growth, survival, and reproduction over the course of their lifetimes. Survival-related traits include the probability of surviving to a given age, and of dying at specific ages. Traits related to reproduction include the age at which reproduction begins, the frequency of reproduction, the number of offspring produced by each reproductive event, and the extent of parental investment in each offspring.

Producers and Consumers

All ecosystems run on energy captured from the environment by autotrophic species that are referred to as producers (Figure 42.1). Producers obtain energy directly from the environment and use an inorganic form of carbon to build sugars (Section 6.1). In most ecosystems, photoautotrophs such as plants, algae, and photosynthetic protists and bacteria are the main producers. In some dark environments such as deep-sea hydrothermal vent ecosystems, chemoautotrophs fill this role. As Section 19.3 explained, chemoautotrophs are bacteria or archaea that fuel the assembly of their food by oxidizing (removing electrons from) inorganic substances such as hydrogen sulfide. Primary production is the rate at which an ecosystem's producers capture and store energy. Day length, temperature, and the availability of nutrients, including nitrogen and phosphorus, affect the rate of photosynthesis and so influence primary production. As a result, primary production can vary seasonally within an ecosystem, and it also differs among ecosystems. Per unit area, primary production on land tends to be higher than that in the oceans. However, because oceans cover about 70 percent of Earth's surface, marine producers—photosynthetic bacteria and protists—contribute nearly half of Earth's overall primary production. As Section 1.2 explained, consumers obtain energy and carbon by feeding on tissues and remains of producers and one another. We describe consumers by their diets. Herbivores eat plants. Carnivores eat the flesh of animals. Parasites live inside or on a living host and feed on its tissues. Omnivores eat both animals and plants. Detritivores such as earthworms and some insects eat detritus: small bits of decaying organic matter. Decomposers break this material down into inorganic building blocks. Bacteria, archaea, and fungi serve as decomposers.

The Nature of Ecosystems

An ecosystem is a community of organisms together with the nonliving components of their environment. It is an open system, because it requires ongoing inputs of energy to persist. Most ecosystems also gain nutrients from and lose nutrients to other ecosystems. Features of ecosystems vary widely. In climate, soil type, array of species, and other features, prairies differ from forests, which differ from tundra and deserts. Reefs differ from the open ocean, which differs from streams and lakes. Yet, despite these differences, ecologists have found that all systems are alike in many aspects.

Parasitoids

An estimated 15 percent of insects are parasitoids, meaning their larvae develop in or on a host insect. Typically, the larvae hatch from the eggs laid in or on the host. As the larvae develop, they feed on the host's tissues. Parasitoids reduce the size of a host population in two ways. First, as the parasitoid larvae grow inside their host, they withdraw nutrients and prevent the host from reproducing. Second, the presence of parasitoid larvae often leads to the death of the host.

Food Webs

An organism that participates in one food chain usually has a role in many others as well. All of the food chains of an ecosystem cross-connect as a food web. Figure 42.3 shows some participants in an arctic food web. Most food webs include two types of food chains. The first step in a grazing food chain is consumption of a living producer by a grazer (an herbivore). In a detrital food chain, the first step is consumption of organic waste or remains by a detritivore. In most land ecosystems, the bulk of the energy that gets stored in producer tissues moves through detrital food chains. For example, in an arctic ecosystem, voles, lemmings, and hares eat some living plant parts. However, far more plant material ends up as detritus. Bits of dead plant material sustain detritivores such as nematodes and soil-dwelling insects. Understanding food webs helps ecologists predict how ecosystems will respond to change. Neo Martinez and his colleagues constructed the food web diagram shown in Figure 42.4. By comparing different food webs, Martinez realized that trophic interactions connect species more closely than people thought. On average, each species in a food web was two links away from all other species. Ninety-five percent of species were within three links of one another, even in large communities with many species. As Martinez concluded in his paper about these findings, "Everything is linked to everything else." He cautioned that the extinction of any species in a food web has a potential impact on many other species.

Reducing Human Impacts

An understanding that all life on Earth draws upon the same limited resources and that the health of the environment affects human well-being has given rise to call for more sustainable practices. In this context, sustainability means using resources in a way that meets the needs of the current human population without degrading the environment so that future generations will be unable to meet their own needs. Living sustainably begins with recognizing the environmental consequences of one's own lifestyle. People in industrial nations use enormous quantities of resources, and the extraction, delivery, and use of these resources has negative effects on biodiversity. In the United States, the size of the average family has declined since the 1950s, while the size of the average home has doubled. All of the materials used to build and furnish those larger homes come from the environment. For example, an average new home contains about 500 pounds of copper in its wiring and plumbing. Where does that copper come from? Like most other nonrenewable mineral elements used in manufacturing, most copper is mined (Figure 44.13A). Surface mining strips an area of vegetation and soil, creating an ecological dead zone. Like other types of mining, surface mining can put particulate matter into the atmosphere, creates mountains of rocky waste, and can contaminate nearby waterways. Minerals are mined worldwide, and global trade makes it difficult to know the source of the raw materials in products you buy. Keep in mind that resource extraction in developing countries is often carried out under regulations that are less strict or less stringently enforced than those in the United States. As a result, the environmental impact of mining is even greater in these countries. Nonrenewable mineral resources are used in electronic devices such as phones, computers, and televisions. Constantly trading up to the newest device may be good for the ego and for the economy, but it is bad for the environment. Reducing consumption by fixing existing products is a sustainable resource use, as is recycling. Reuse and recycling reduce the need for extracting nonrenewable resources, and they also help keep material out of landfills. In 2013, the United States recycled 87 million tons of material that would otherwise have ended up in landfills. Even so, there is plenty of room for improvement. That 87 million tons was only one-third of the municipal solid waste produced. Minimizing energy use is also part of living sustainably. Fossil fuels such as petroleum, natural gas, and coal supply most of the energy used by developed countries. You already know that burning these nonrenewable fuels contributes to global warming and acid rain. Extracting and transporting fossil fuels add environmental costs. For example, oil that escapes from ocean-drilling operations harms aquatic species (Figure 44.13B). Natural gas that leaks from wells and pipes contributes to global warming. For example, between October 2015 and February 2016, a natural gas well in Southern California leaked more than 2 million tons of methane into the air. Methane is one of the greenhouse gases. Nuclear energy produces no carbon dioxide, but occasional accidents allow dangerous radioactive material to escape into the environment. Renewable energy sources have their own drawbacks. For example, dams in rivers can generate renewable hydroelectric power, but they can also alter the array of species in the river by slowing its flow. Wind turbines kill birds and bats. This occurs most often if turbines are installed without regard for migration routes and/or the structure supporting the turbine provides a place to perch. Industrial-scale solar farms can pose a threat to birds too. Birds sometimes mistake the array of solar panels or mirrors at such facilities for a lake. As a result, the birds collide with the panels or are burned by intense sunlight concentrated by the mirrors. Placing a solar farm in an undisturbed area also displaces native species that lived in or used the area where the facility is situated. In short, all energy use has some negative environmental impacts, so the best way to minimize your impact is to use less energy. If you want to make more of a difference, learn about the threats to ecosystems in your own area. Support efforts to document, preserve, and restore local biodiversity. Many ecological restoration projects are supervised by trained biologists but carried out primarily by volunteers (Figure 44.14).

The Phosphorus Cycle

Atoms of phosphorus are highly reactive, so phosphorus does not occur naturally in its elemental form. Most of Earth's phosphorus is bonded to oxygen as phosphate, which occurs in rocks and sediments. In the phosphorus cycle, phosphorus passes quickly through food webs as it moves from land to ocean sediments, then slowly back to land (Figure 42.12). Little phosphorus exists in a gaseous form. Because the atmosphere plays little role in the phosphorus cycle and the major reservoir for phosphorus is sedimentary rock, the phosphorus cycle is called a sedimentary cycle. In the geochemical portion of the phosphorus cycle, weathering and erosion move phosphates from rocks into soil, lakes, and rivers . Leaching and runoff carry dissolved phosphate to the ocean . Here, most dissolved phosphorus comes out of solution and settles as rocky deposits along continental margins . Tectonic movements of Earth's crust can uplift these deposits onto land , where weathering releases phosphates from rocks once again. All organisms use phosphorus to build nucleic acids and phospholipids. The biological portion of the phosphorus cycle begins when producers take up phosphate. Land plants take up dissolved phosphate from soil water . Animals on land get phosphates by eating the plants or one another. Phosphorus returns to the soil in the wastes and remains of organisms . In the seas, phosphorus enters food webs when producers take up phosphate dissolved in seawater . As on land, wastes and remains continually replenish the phosphorus supply . Like nitrogen, phosphorus is often a limiting factor for plant growth, so most fertilizers contain phosphorus as well as nitrogen. Phosphate-rich rocks are mined for use in the industrial production of fertilizer. Guano, which is phosphate-rich droppings from seabird or bat colonies, is also mined and used as fertilizer. Phosphates from fertilizers often run off from the site where they are applied and enter aquatic habitats. Other sources of aquatic phosphate pollution include animal waste from farms, sewage released from cities, and phosphate-rich detergents used to wash laundry and dishes. An influx of phosphorus can encourage the growth of aquatic producers, resulting in an algal bloom.

The Value of Biodiversity

Biodiversity (biological diversity) refers to the variety and variability of life forms. A region's biodiversity is measured at three levels: the genetic diversity within species, species diversity, and ecosystem diversity. Biodiversity is currently declining at all three levels, in all regions. Conservation biology addresses these declines. The goals of this relatively new field of biology are to survey the range of biodiversity, and to find ways to maintain and use biodiversity to benefit human populations by encouraging people to value their region's natural resources and use those resources in nondestructive ways. Why should we protect biodiversity? From a selfish standpoint, doing so is an investment in our future. Healthy ecosystems are essential to the survival of our species. Other organisms produce the oxygen we breathe and the food we eat. They remove waste carbon dioxide from the air and decompose and detoxify wastes. Plants take up rain and hold soil in place, preventing erosion and reducing the risk of flooding. Wild species produce medically valuable compounds that we are still discovering; wild relatives of crop plants are reservoirs of genetic diversity that plant breeders draw on to protect and improve crops. There are ethical reasons to preserve biodiversity too. All living species are the result of an ongoing evolutionary process that stretches back billions of years. Each species has a unique combination of traits, and extinction removes that collection of traits from the world forever. The more diverse a biological system is, the greater its capacity to recover from damage. This applies at all levels of life. Thus, reducing the biodiversity of ecosystems makes them less resilient to disturbances.

Biological Pest Control

Biological pest control is the practice of using a pest's natural enemies to reduce its numbers. Commercially raised parasites and parasitoids are often used as agents of biological pest control (Figure 41.10). The method of pest control has some advantages over pesticides. Most chemical insecticides kill a wide variety of insects, including helpful species. Insecticides also have negative effects on human health. By contrast, the parasites and parasitoids usually used as biological control agents target only a limited number of species. For a species to be an effective biological control agent, it must target only one specific host species and be able to survive in that host's habitat. The ideal biological control agent excels at finding the target host species, has a population growth rate comparable to the host's, and has offspring that disperse widely. Introducing a species into a community as a biological control agent always entails some risks. The introduced parasites sometimes attack nontargeted species in addition to, or instead of, those that they were expected to control. Consider the parasitoid wasps that were introduced to Hawaii to control stinkbugs that feed on some Hawaiian crops. Instead, the parasitoids decimated the population of koa bugs, Hawaii's largest native bug. Introduced parasitoids have also been implicated in ongoing declines of many native Hawaiian butterfly and moth populations.

Managing Canada Geese

Canada geese were hunted to near extinction in the late 1800s. In the early 1900s, federal laws and international treaties were put in place to protect them and other migratory birds. In recent decades, the number of geese in the United States has soared. For example, in 1970, Michigan had about 9,000 geese; today, it has 300,000. These plant-eating birds often congregate at golf courses and parks (Figure 40.14), where they are considered pests. Large numbers of Canada geese also pose a serious problem for air traffic. They are one of the species most commonly involved in collisions with aircraft. Controlling the number of Canada geese poses a challenge because multiple different populations spend time in the United States. In the past, nearly all Canada geese seen in the United States were migratory. The geese nested in northern Canada, flew to the United States to spend the winter, then returned to Canada. Most Canada geese still migrate, but some populations have lost this trait. During the winter, migratory birds often mingle with nonmigratory ones. Life is more difficult for migratory geese than for nonmigratory ones. Compared to a migratory bird, one that stays put can devote more energy to producing young. If the nonmigrant lives in a suburban or urban area, it also benefits from an unnatural abundance of food (grass) and an equally unnatural lack of predators. Not surprisingly, the biggest increases in Canada geese have been among nonmigratory birds that live where humans are plentiful. We have increased the carrying capacity for these birds. The U.S. Fish and Wildlife Service now encourages wildlife managers to reduce the size of nonmigratory Canada goose populations, without harming migratory ones. To do so, these biologists need to know about the traits that characterize different goose populations, as well as how these populations interact with one another, and with other species.

Competitive Exclusion

Competition between species will be most intense when the supply of a shared resource is an important limiting factor for both species. In the 1930s, G. Gause conducted experiments with two species of ciliated protozoans (Paramecium) that compete for the same prey: bacteria. Gause cultured the Paramecium species separately and together (Figure 41.5). Within weeks, population growth of one species outpaced the other, which went extinct. This and other experiments are the basis for the concept of competitive exclusion: Whenever two species require the exact same limited resource to survive or reproduce, the better competitor will drive the less competitive species to extinction in that habitat. When resource needs of competitors are not exactly the same, competing species can coexist, but the presence of each reduces the carrying capacity (Section 40.3) of the habitat for the other. For example, the reproductive success of a flowering plant is decreased by competition from other species that flower at the same time and share the same pollinators. As another example, consider again those fly-eating sundew plants. A sundew grown in the presence of wolf spiders makes fewer flowers than one grown in a spider-free area. Competition for insect prey reduces the availability of resources that a sundew could otherwise use to make flowers.

A Demographic Transition

Demographic factors vary among countries, with the most highly developed countries having the lowest birth rates and infant mortality, and the highest life expectancy. The demographic transition model describes how changes in population growth often unfold in four stages of economic development (Figure 40.13). Living conditions are harshest during the preindustrial stage, before technological and medical advances become widespread. Birth and death rates are both high, so the growth rate is low . For most of human history, all regions were in this stage, but today no country is. In the transitional stage, food production and health care improve as industrialization begins. The death rate drops fast, but the birth rate declines more slowly . As a result, the rate of population growth increases. The world's least developed countries are in this stage. Examples include Afghanistan, Haiti, Cambodia, and Ethiopia. During the industrial stage, when industrialization is in full swing, the birth rate declines. The population distribution shifts. As people move from rural areas to towns and cities, couples tend to want smaller families and birth control is more easily available. The birth rate moves closer to the death rate, and the population grows less rapidly . The United States is in this stage. In the postindustrial stage, a population's growth rate becomes negative. The birth rate falls below the death rate, and population size slowly decreases . Germany and some countries in eastern Europe would be in this stage if they did not take in immigrants from elsewhere. The demographic transition model was developed based on analysis of what happened when western Europe and North America industrialized in the late 1800s. Whether it can accurately predict changes in modern developing countries remains to be seen. Global movement of people and resources is far greater today than it was during the period on which the model is based.

Desertification

Deserts naturally expand and contract over geological time as climate conditions vary. However, human activities sometimes result in the rapid conversion of a grassland or woodland to desert, a process called desertification. As human populations increase, greater numbers of people are forced to farm in areas that are ill suited to agriculture. In other places, people allow livestock to overgraze in grasslands. In both cases, the result can be habitat degradation through desertification. A well-documented instance of desertification occurred in the United States during the mid-1930s, when large portions of prairie on the southern Great Plains were plowed under to plant crops. Plowing exposed deep prairie topsoil to the force of the region's constant winds. Then came a drought, and the result was an economic and ecological disaster. Winds carried more than a billion tons of topsoil aloft as sky-darkening dust clouds (Figure 44.2A), turning the region into what came to be known as the Dust Bowl. Millions of tons of soil was displaced, and some fell to earth as far away as New York City and Washington, D.C. Today, Africa's Sahara desert is expanding south into the Sahel region. Overgrazing in this region strips grasslands of their vegetation, exposing the soil to erosion by wind. Wind carries billions of tons of soil aloft and westward (Figure 44.2B). Soil particles land as far away as the southern United States and the Caribbean. In China's northwestern regions, overplowing and overgrazing have expanded the Gobi desert so that dust clouds periodically darken skies above Beijing. Winds carry some of this soil across the Pacific to the west coast of the United States. Drought encourages desertification, which results in more drought (a positive feedback cycle). Plants cannot thrive in a region where the topsoil has blown away. Fewer plants means less transpiration (Section 26.3), so less water enters the atmosphere and local rainfall decreases. The best way to prevent desertification is to avoid farming in areas subject to high winds and periodic drought. If these areas must be used, methods that do not repeatedly disturb the soil can minimize risk of desertification.

Trophic Structure

Ecologists often look at energy transfers in terms of who eats whom. The hierarchy of feeding relationships in an ecosystem is the ecosystem's trophic structure. Troph refers to feeding, as in autotroph, which means self-feeding. All organisms at the same trophic level are the same number of transfers away from the ecosystem's source of energy. Producers are at the first trophic level. The primary consumers that eat them are at the second trophic level. The second-level consumers eat primary consumers, and so on. A food chain is a sequence of steps by which some energy captured by primary producers is transferred to higher trophic levels. For example, in one tallgrass prairie food chain, energy flows from grasses to grasshoppers, to sparrows, and finally to bird-eating hawks (Figure 42.2). At the first trophic level in this food chain, grasses and other plants are the producers. At the second trophic level, grasshoppers are primary consumers. At the third trophic level, sparrows that eat grasshoppers are second-level consumers. At the fourth trophic level, hawks that eat sparrows are third-level consumers. Energy captured by producers usually passes through no more than four or five trophic levels: Even in ecosystems with many species, the number of participants in each food chain is limited. The inefficiency of energy transfers constrains the length of food chains. Only 5 to 30 percent of the energy in tissues of an organism at one trophic level ends up in tissues of an organism at the next trophic level. Several factors limit the efficiency of transfers. All organisms lose energy as metabolic heat, and this energy is not available to organisms at the next trophic level. Also, some energy gets stored in molecules that most consumers cannot break down. For example, most carnivores cannot access energy in molecules that make up bones, scales, hair, feathers, or fur. Many herbivores cannot digest cellulose and lignin that reinforce the tissues of plants.

Characteristics of a Population

Ecology is the study of interactions between organisms and their environments. Ecology is not the same as environmentalism, which is advocacy for protection of the environment. However, environmentalists sometimes cite the results of ecological studies when drawing attention to their concerns. Population ecology investigates the factors that influence the size, distribution, and other properties of natural populations. Data from population ecology studies can be used to make decisions about how to manage a species. For example, information about a specific population can help wildlife managers determine the number of individuals that can be hunted or fished without driving the population into decline. Population ecology studies are also used to assess the effects of other human activities on wild species.

Energy Flows, Nutrients Cycle

Energy captured by producers is converted to bond energy in organic molecules. This energy is released by metabolic reactions that give off heat. Energy flow through living organisms is a one-way process. Producers cannot capture and store the energy of heat in chemical bonds, so heat energy escapes from the ecosystem (Section 5.1). In contrast, nutrients cycle within an ecosystem. Producers take up hydrogen, oxygen, and carbon from inorganic sources in (for example) air and water. They also take up dissolved nitrogen, phosphorus, and other necessary minerals. Nutrients that producers use to build their bodies are used in turn to build the bodies of the consumers who eat them. When producers or consumers die, decomposition returns nutrients to the environment, from which producers take them up again.

The Unknown Losses

Estimates of the number of currently living species range as high as one trillion, but only about 2 million species have been described and named. Simply put, we do not yet know what we have to lose. Scientists racing to document Earth's diversity are attempting to identify the species most in need of protection before it is too late. The International Union for Conservation of Nature and Natural Resources (IUCN) monitors threats to species worldwide. However, its species listings have historically focused on vertebrates. Threats to invertebrates and plants have only recently been considered. Our impact on protists and fungi is largely unknown, and the IUCN does not address threats to bacteria or archaea. These microorganisms are essential to nutrient cycles that humans rely upon, yet we have almost no idea whether or how our activities affect their numbers and distribution.

Threatened and Endangered Species

Extinction, like speciation, is a natural process; species arise and become extinct on an ongoing basis. The overwhelming majority of all species that have ever lived are now extinct. The rate of extinction increases dramatically during a mass extinction, when many kinds of organisms in many different habitats become extinct in a relatively short period. Such an event is happening now. Scientists estimate that the current extinction rate is about 1,000 times that of the rate when a mass extinction is not occurring. Unlike most previous mass extinctions, this one is not the inevitable result of a physical catastrophe such as a volcanic eruption or asteroid impact. Humans are driving the current rise in extinctions, and our actions will determine the extent of the losses. For purposes of conservation, a species is considered extinct if repeated, extensive surveys of its known range fail to turn up signs of any individuals. It is "extinct in the wild" if the only known members of the species are in captivity. Some species are difficult to find, so occasionally a population of species previously thought to be extinct or extinct in the wild turns up, but this does not happen very often. An endangered species faces extinction in all or part of its range. A threatened species is one that is likely to become endangered in the near future. Keep in mind that not all rare species are threatened or endangered. Some species have always been uncommon. In the United States, species are listed as endangered or threatened by the United States Fish and Wildlife Service (USFWS).

Expansions and Innovations

For most of its history, the human population grew very slowly (Figure 40.11). The growth rate began to pick up about 10,000 years ago, and it soared during the past two centuries. Three trends promoted the large increases. First, humans expanded into new habitats. Second, they developed technologies that increased the carrying capacity of their habitats. Third, they sidestepped limiting factors that typically restrain population growth. Modern humans evolved in Africa by about 200,000 years ago, and by 43,000 years ago, their descendants were established in much of the world (Section 24.12). The invention of agriculture about 11,000 years ago provided a more dependable food supply than traditional hunting and gathering. In the middle of the eighteenth century, people learned to harness energy in fossil fuels to operate machinery. This innovation opened the way to high-yielding mechanized agriculture and improved food distribution systems. Food production was further enhanced in the early 1900s, when the invention of synthetic nitrogen fertilizers increased crop yields. The invention of synthetic pesticides in the mid-1900s also contributed to increased food production. Disease has historically dampened human population growth. During the mid-1300s, one-third of Europe's population was lost in a pandemic known as the Black Death. Beginning in the mid-1800s, an increased understanding of the link between microorganisms and illness led to improvements in food safety, sanitation, and medicine. People began to pasteurize foods and drinks, heating them to kill harmful bacteria. They also began to protect their drinking water. Advances in sanitation also lowered the death rate associated with medical treatment. In the mid-1800s, Ignaz Semmelweis, a physician in Vienna, began urging doctors to wash their hands between patients. His advice was largely ignored until after his death, when Louis Pasteur popularized the idea that unseen organisms cause disease. Acceptance of this idea also revolutionized surgery, which had been carried out without regard for cleanliness. A worldwide decline in death rates without an equivalent drop in birth rates is responsible for the ongoing explosion in human population size. The population is now more than 7 billion and is expected to reach 9 billion by 2050.

Fertility and Future Growth

Human birth rates have begun to slow as a result of contraception use. The total fertility rate of a population is the number of offspring a woman would be expected to have during her reproductive years given the current age-specific birth rate. In 1950, the total fertility rate for humans averaged 6.5 worldwide (women had 6.5 children, on average, during their lifetime). By 2014, the fertility rate had declined to 2.5. The replacement fertility rate, which is the number of children a woman must bear to replace herself with one daughter of reproductive age, varies among regions. At present, it is about 2.1 for developed countries. It is higher in developing countries because more daughters die before reaching the age of reproduction. As long as the total fertility rate of our species as a whole exceeds our total replacement rate, our population will continue to grow. The rate at which a population grows depends in part on its age structure—the distribution of its members among various age groups. Individuals are often categorized as pre-reproductive, reproductive, or post-reproductive. Members of the pre-reproductive category have a capacity to produce offspring when mature. Along with reproductive individuals, they constitute a population's reproductive base. Figure 40.12 shows age structure diagrams for the world's three most populous countries. China and India already have more than one billion people apiece; together, they hold 38 percent of the world population. Next in line is the United States, with more than 320 million. Notice the size of the reproductive base in each diagram. The broader the base of an age structure diagram, the greater the proportion of young people, and the greater the expected growth. Government policies that favor couples who have only one child have helped China to narrow its pre-reproductive base. Even if every couple now living decides to have no more than two children, two factors will keep the world population increasing for many years. First, longevity continues to increase. Second, the population has a broad prereproductive base. Worldwide, about 1.9 billion people are about to enter their reproductive years.

Zero to Exponential Growth

If we set aside the effects of immigration and emigration, we can define zero population growth as an interval during which the number of births is balanced by an equal number of deaths. As a result, population size remains unchanged, with no net increase or decrease in the number of individuals. We can measure births and deaths in terms of rates per individual, or per capita. Capita means head, as in a head count. Subtract a population's per capita death rate (d) from its per capita birth rate (b) and you have the per capita growth rate, or r: Imagine 2,000 mice living in the same field. If 1,000 mice are born each month, then the birth rate is 0.5 births per mouse per month (1,000 births/2,000 mice). If 200 mice die one way or another each month, then the death rate is 200/2,000 or 0.1 deaths per mouse per month. Thus, r is 0.5 - 0.1, or 0.4 per mouse per month. As long as r remains constant and greater than zero, exponential growth will occur, which means that the population's size will increase by the same proportion of its total in every successive time interval. We can calculate population growth (G) for each interval based on the number of individuals (N) and the per capita growth rate: Let's use this equation to study growth of our hypothetical population of field mice. In one month, the population will expand from 2,000 to 2,800 individuals (Figure 40.2A). A net increase of 800 mice has increased the number of breeders. If all of the mice reproduce, the population size will expand by 1,120 individuals (2,800 × 0.4) in the next month. The total population size is now 3,920. At this growth rate, the number of mice would rise from 2,000 to more than 1 million in under two years! Graphing the increases against time results in a J-shaped curve, which is characteristic of exponential population growth (Figure 40.2B). With exponential growth, the number of new individuals increases with each generation, even though the per capita growth rate stays the same. Exponential population growth is analogous to compound interest in a bank account. The annual interest rate stays fixed, yet every year the amount of interest paid increases. The annual interest paid into the account adds to the size of the balance, so the next interest payment will be based on the increased balance. Similarly, with exponential growth, the number of new individuals increases in each generation. Exponential growth will occur in any population in which the birth rate exceeds the death rate—in other words, as long as r is greater than zero. Imagine a single bacterium in a culture flask. After thirty minutes, the cell divides in two. Those two cells divide, and so on every thirty minutes. If no cells die between divisions, then the population size will double in every interval—from 1 to 2, then 4, 8, 16, 32, and so on. After 9-1/2 hours, there have been nineteen doublings, so the population now consists of more than 500,000 cells. Ten hours (twenty doublings) later, there are more than a million. Curve 1 in Figure 40.3 is a plot of this increase. Now suppose that 25 percent of the descendant cells die every thirty minutes in our hypothetical population of bacteria. In this scenario, it takes seventeen hours, not ten, for that population to reach 1 million. Thus, deaths slow the rate of increase but do not stop exponential growth (curve 2 in Figure 40.3).

Biogeochemical Cycles

In a biogeochemical cycle, a chemical element or compound moves from one or more environmental reservoirs, into, through, and out of a food web, and then returns to the reservoirs (Figure 42.6). The environmental reservoirs vary depending on the substance, but can include Earth's rocks and sediments, waters, and atmosphere. The biological component of the ecosystem includes the bodies of living organisms, as well as their wastes and remains. Geologic and chemical processes move elements to, from, and among environmental reservoirs. For example, elements locked in rocks can become part of the atmosphere as a result of volcanic activity. Movement of Earth's tectonic plates (Section 16.5) can uplift rocks, so an area that was once seafloor becomes part of a landmass. On land, erosion (Section 26.1) breaks down rocks. The slow breakdown of rocks releases their component chemical elements. These elements enter rivers, and eventually reach the seas. Compared to the movement of elements among organisms of an ecosystem, the movement of elements among nonbiological reservoirs is far slower. Processes such as erosion and uplifting operate over thousands or millions of years.

Immigration and Emigration

In nature, populations continually change in size. Individuals are added to a population by births and immigration, the arrival of new residents that previously belonged to another population. Individuals are removed from it by deaths and emigration, the departure of individuals who take up permanent residence elsewhere. In many animal species, young of one or both sexes leave the area where they were born to breed elsewhere. For example, young freshwater turtles typically emigrate from their parental population and become immigrants at another pond some distance away. By contrast, seabirds typically breed where they were born. However, some individuals may emigrate and end up at breeding sites more than a thousand kilometers away. In most species, the tendency of individuals to emigrate to a new breeding site is related to resource availability and crowding. As resources decline and crowding increases, the likelihood of emigration rises.

Carbon Reservoirs and Flow

In the carbon cycle, carbon moves among Earth's atmosphere, oceans, soils, and into and out of food webs (Figure 42.8). It is an atmospheric cycle, a biogeochemical cycle in which a gaseous form of the element plays a significant role. On land, plants take up carbon dioxide from the atmosphere and incorporate it into their tissues when they carry out photosynthesis . Plants and most other land organisms release carbon dioxide into the atmosphere as they carry out aerobic respiration . The greatest flow of carbon between nonbiological reservoirs takes place between the atmosphere and the oceans. The atmosphere, which is less than 1 percent , holds about 750 gigatons of carbon. Seawater holds about 40,000 gigatons of dissolved carbon, primarily in bicarbonate and carbonate ions. Bicarbonate ions form when atmospheric carbon dioxide dissolves in water . Aquatic producers take up bicarbonate and convert it to for use in photosynthesis. They, like organisms on land, carry out aerobic respiration and release carbon dioxide . Earth's rocks and sediments are its single greatest reservoir of carbon, with more than 65 million gigatons. Limestone is a sedimentary rock that forms over millions of years when sediments containing calcium carbonate shells of marine organisms such as foraminifera (Section 20.4) are compacted . Limestone and other carbon-containing rocks derived from marine sediments can be uplifted onto land by movements of tectonic plates. However, producers take up carbon from the air rather than from rocks, so carbon in these reservoirs has little effect on ecosystems. Soil contains about 1,600 gigatons of carbon, more than twice as much as the atmosphere. The carbon in soil is part of molecules making up humus and living soil organisms. As bacteria and fungi in the soil decompose humus, they release carbon into the air (as ). The speed of decomposition increases with temperature. In a warm tropical forest, decomposition and nutrient uptake proceed rapidly, so most of the forest's carbon is in living plants, rather than in soil. By contrast, in cooler temperate zone forests and grasslands, soil holds more carbon than the plants do. Soils that hold the most carbon occur in the arctic, where consistent low temperature slows decomposition, and in peatbogs (Section 21.2), where acidic, anaerobic conditions do the same. Fossil fuels such as coal, oil, and natural gas hold an estimated 5,000 gigatons of carbon. These substances formed over hundreds of millions of years from carbon-rich remains of ancient organisms. High pressure and temperature transformed the remains of land plants to coal (Section 21.4). A similar process transformed the remains of plankton to oil and natural gas. Until the industrial revolution, the carbon in fossil fuels, like the carbon in rocks, had little impact on ecosystems. Today, burning this fuel adds billions of tons of carbon dioxide to the atmosphere every year .

Alterations to the Cycle

In the early 1900s, scientists invented a method of fixing atmospheric nitrogen and producing ammonium on an industrial scale. This process allowed production of synthetic nitrogen fertilizers that have boosted crop yields (Figure 42.11). The use of these fertilizers has helped feed a rapidly increasing human population, but it has also added large amounts of nitrogen-containing compounds to our air and water. Here we consider two of these compounds and their effects. Nitrate from synthetic fertilizers sometimes runs off from agricultural fields and contaminates aquatic ecosystems, where it can encourage algal blooms (Chapter 20 Application). When nitrate contaminates drinking water, it can pose a threat to human health. Among other effects, ingested nitrate inhibits iodine uptake by the thyroid gland and may increase the risk of thyroid cancer. The U.S. Environmental Protection Agency (EPA) has set a maximum standard for nitrate in public drinking water. Nitrous oxide gas is another nitrogen-containing pollutant. Burning of fossil fuels, use of synthetic nitrogen fertilizers, and industrial livestock production all add nitrous oxide to the atmosphere. Burning fossil fuel releases nitrous oxide directly into the air. Chemical fertilizers and manure from livestock increase atmospheric nitrous oxide by encouraging growth of bacteria that release this gas. An increase in atmospheric nitrous oxide is a matter of concern for two reasons. First, nitrous oxide is a greenhouse gas, and a highly persistent and effective one. It can remain in the atmosphere for more than 100 years, and it traps 300 times as much heat as an equivalent amount of . Second, nitrous oxide contributes to destruction of the ozone layer. As Section 18.3 explains, ozone high in the atmosphere protects life at Earth's surface from damaging effects of ultraviolet radiation. We discuss ozone destruction in Section 44.4.

Depletion of the Ozone Layer

In the upper layers of the atmosphere, between 17 and 27 kilometers (10.5 and 17 miles) above sea level, the ozone concentration is so great that scientists refer to this region as the ozone layer. The ozone layer benefits living organisms by absorbing most ultraviolet (UV) radiation from incoming sunlight. UV radiation, remember, damages DNA and causes mutations (Section 8.5). In the mid-1970s, scientists noticed that Earth's ozone layer was thinning. Its thickness had always varied a bit with the season, but now the average level was declining steadily from year to year. By the mid-1980s, the spring thinning of the ozone layer over Antarctica had become so pronounced that people began referring to an "ozone hole" over this region (Figure 44.7A). Declining ozone quickly became an international concern. With a thinner ozone layer, people would be exposed to more UV radiation, the main cause of skin cancers. Higher UV levels also harm animals, which do not have the option of using sunscreeens or avoiding sunlight. In addition, exposure to higher-than-normal UV levels damages plants and other producers, slowing their rate of photosynthesis and the release of oxygen into the atmosphere. In the 1980s, compounds called chlorofluorocarbons, or CFCs, were the main ozone destroyers. These odorless gases were widely used as propellants in aerosol cans, as coolants, and in solvents and plastic foam. In response to the potential threat posed by the thinning ozone layer, countries worldwide agreed in 1987 to phase out the production of CFCs and other ozone-destroying chemicals. As a result of that agreement, known as the Montreal Protocol, the concentrations of CFCs in the atmosphere are no longer rising dramatically (Figure 44.7B). However, CFCs break down quite slowly, so they are expected to remain at a level that significantly thins the ozone layer for several decades. Today, the main ozone-depleting pollutant entering the atmosphere is nitrous oxide . This gas is released when people burn fossil fuels and when bacteria feed on fertilizers and animal waste (Section 42.6).

Types of Competition

Interspecific competition is competition among members of different species. It is not usually as intense as intraspecific competition (competition within a species). The requirements of two species might be similar, but they are never as close as they are for members of the same species. Each species has a unique set of ecological requirements and roles that we refer to as its ecological niche. Both physical and biological factors define the niche. Aspects of an animal's niche include the temperature range it can tolerate, the species it eats, and the places it can breed. A description of a flowering plant's niche would include its soil, water, light, and pollinator requirements. The more similar the niches of two species are, the more intensely those species will compete. Competition generally takes one of two forms. With interference competition, one species actively prevents the other from accessing some limited resource. As an example, one species of scavenger will often chase another away from a carcass (Figure 41.4A). Plants engage in interference competition too, although it is less obvious. For example, some plants use chemicals to fend off potential competitors. Aromatic compounds that ooze from tissues of sagebrush plants, black walnut trees, and eucalyptus trees seep into the soil and prevent most plants from germinating or growing nearby. In exploitative competition, species do not interact directly, but by using the same resource, each reduces the amount of that resource available to the other. For example, wolf spiders and carnivorous plants called sundews both feed on insects in Florida swamps (Figure 41.4B). These two very different organisms do not fight over food, but they do compete. By catching insects, each reduces the number of insects available to the other.

Island Biogeography

Islands are natural laboratories for population studies. They have also been laboratories for community studies. Consider Surtsey, an island that appeared in the mid-1960s when an undersea volcano erupted 33 kilometers (21 miles) from the coast of Iceland (Figure 41.18). Bacteria and fungi were early colonists. Vascular plants followed in 1965, and mosses appeared two years later. The first lichens were found five years after that. The rate at which vascular plants were introduced to the island increased dramatically after a seagull colony became established in 1986, but the number of species on Surtsey can not continue increasing forever. How many species will there be when the number levels off? According to the equilibrium model of island biogeography, the eventual number of species living on any island reflects a balance (or equilibrium) between immigration of new species and extinction of established ones. Two factors determine the equilibrium number of species (Figure 41.19). First, there is a distance effect: Islands far from a source of colonists receive fewer immigrants than those closer to a source. Most species cannot disperse very far, so they will not turn up far from a mainland. Second, there is an area effect: An island's size affects both immigration rates and extinction rates. More colonists will happen upon a larger island simply by virtue of its size. Also, big islands are more likely to offer a variety of habitats, such as high and low elevations. These options make it more likely that a new arrival will find a suitable habitat. Finally, big islands can support larger populations of species than small islands. The larger a population, the less likely it is to become locally extinct as the result of some random event. For example, a fire large enough to wipe out the population of a small island might leave survivors on a larger island. Robert H. MacArthur and Edward O. Wilson developed the equilibrium model of island biogeography in the late 1960s. Since then, the model has been modified and its use has been expanded to help scientists think about habitat islands, which are natural settings surrounded by a "sea" of habitat that has been disturbed by humans. Many parks and wildlife preserves fit this description. Island-based models help ecologists estimate the size of an area needed to ensure survival of a species in a region.

Sampling a Population

It is often impractical to count all members of a population, so biologists frequently use sampling techniques to estimate population size. Plot sampling is a method of estimating the total number of individuals in an area based on data from direct counts in some portion of the area. For example, ecologists might estimate the number of grass plants in a grassland, or the number of clams in a mudflat, by measuring the number of individuals in each of several 1-meter by 1-meter square plots. To estimate total population size, scientists first determine the average number of individuals per sample plot. They then multiply that average by the number of plots that would fit in the population's range. Estimates derived from plot sampling are most accurate when species are not very mobile and conditions across their habitat are uniform. Mark-recapture sampling is used to estimate the population size of mobile animals. With this technique, animals are captured, marked with a unique identifier of some sort, then released. Some time later, scientists capture another group of individuals from the same population. The proportion of marked animals in the second sample is taken to be representative of the proportion marked in the population as a whole. Suppose 100 deer are captured, marked, and released. Later, 50 of these deer are recaptured along with 50 unmarked deer. Marked deer constitute half the recaptured group, so the group previously caught and marked (100 deer) must have been half of the population. Thus the total population is estimated at 200. Information about the traits of individuals in a sample plot or capture group can be used to infer properties of the whole population. For example, if half the recaptured deer are of reproductive age, half of the population is assumed to share this trait. Giving each captured animal a unique mark (such as the bands on the sandpiper in the photo that opens the chapter) allows individuals to be followed over time.

Fighting Foreign Fire Ants - Application

Like most ants, red imported fire ants (Solenopsis invicta) nest in the ground (Figure 41.20). Accidentally step on one of their nests, and you will quickly realize your mistake. S. invicta defend their nest by stinging, and their venom causes a burning sensation that gives the ants their common name. S. invicta is native to South America. The species first arrived in the southeast United States in the 1930s when some of the ants traveled as stowaways on a cargo ship. Since then, S. invicta has gradually expanded its range across the south, and has been accidentally introduced to California and New Mexico. More recently, it became established in the Caribbean, Australia, New Zealand, and several countries in Asia. Genetic comparisons among S. invicta populations revealed that the ants involved in these recent introductions originated in the southeastern United States, rather than South America. Increased dispersal of pest species is an unanticipated side effect of increased global trade and improvements in shipping. Speedier ships make quicker trips, increasing the likelihood that pests hidden away in cargo holds will survive a journey. The spread of S. invicta concerns ecologists because this species has a negative impact on communities where it is introduced. Competition from S. invicta typically causes a region's native ant populations to decline, and the resulting change in species composition can harm ant-eating animals. For example, the Texas horned lizard feeds mainly on native harvester ants, and cannot eat the red imported fire ants that have now largely replaced its natural prey. Red imported fire ants also harm native species by feeding on their eggs, and feeding on or stinging their young. Ground-nesting animals such as quail are especially vulnerable to fire ant predation. The arrival of red imported fire ants can even affect native plants. The ants interfere with pollination by displacing or preying on native pollinators such as ground-nesting bees. They also impede dispersal of native plants whose seeds would normally be spread by native ants. Given the problems that the imported ants are causing in the United States, you might wonder what things are like in their native South America. The ants are not considered much of a concern there, in part because they are far less common. Coevolved parasites, predators, and diseases keep the ants' numbers in check. Invicta means "invincible" in Latin, and S. invicta lives up to its name. Pesticides have not halted its spread, so scientists are now turning to some of the ants' coevolved South American enemies. Phorid flies are one such natural enemy. In South America, some species of these parasitoids target red imported fire ants. A female phorid fly pierces the cuticle of an adult ant and lays an egg in the ant's soft tissues. The egg hatches into a larva, which grows and eats its way through the tissues to the ant's head. When the larva is ready to undergo metamorphosis, it secretes an enzyme that makes the ant's head fall off. The fly larva then develops into an adult within the shelter of the detached ant head. Several South American phorid fly species have been introduced to southern states. The flies are surviving, reproducing, and, in some cases, increasing their range. The imported flies are not expected to kill off all S. invicta in affected areas. Rather, the hope is that these flies will reduce the density of invading colonies.

Logistic Growth

Logistic growth occurs when density-dependent factors affect population size over time, so that a plot of numbers versus time yields an S-shaped curve (Figure 40.5). When the population is small, density-dependent limiting factors have little effect and the population grows exponentially . Then, as population size and the degree of crowding rise, these factors begin to limit growth . Eventually, the population size levels off at the carrying capacity . Carrying capacity (K) is the maximum number of individuals of a particular species that a population's environment can support indefinitely. As with the exponential growth equation, G is growth per unit time, r is the per capita growth rate, and N is current population size. K is carrying capacity. The (K-N)/K part of the equation represents the proportion of carrying capacity not yet used. As a population grows, this proportion decreases, so G becomes smaller and smaller. At carrying capacity, the equation become , which means the size of the population cannot increase. Carrying capacity is species-specific, environment-specific, and can change over time . For example, the carrying capacity for a plant species decreases when nutrients in the soil become depleted. Human activities can affect carrying capacity. For example, human harvesting of horseshoe crabs has decreased the carrying capacity for red knot sandpipers (shown in the chapter opening photo). Horseshoe crab eggs are the birds' main food during their long-distance migration.

Predation Effects on Life History

Many predators prefer prey of a specific size, and individuals of most prey species change in size over their lifetime. Thus, predation can affect life history traits of prey. When predators prefer large prey, prey individuals who reproduce when still small and young are at a selective advantage. When predators focus on small prey, fast-growing individuals have the selective advantage.

Ecological Succession

The species composition of a community will change over time. Often, some species alter the habitat in ways that allow others to come in and replace them. This type of change, which takes place over a long interval, is called ecological succession. (Succession refers to a series of things, one following after the other.) Ecological succession begins with the arrival of pioneer species, which are species whose traits allow them to colonize new or newly vacated habitats. Pioneer species have an opportunistic life history (Section 40.4): They grow and mature quickly, and they produce many offspring capable of dispersing. Later, other species replace the pioneers. Then the replacements are replaced, and so on. There are two types of succession: primary succession and secondary succession. Primary succession takes place in a barren habitat that lacks soil, such as land exposed by the retreat of a glacier, a newly formed volcanic island, or a region where volcanic material has buried existing soil. The earliest pioneers to colonize such environments are often mosses and lichens (Sections 21.2 and 22.3), which are small, have a brief life cycle, and can tolerate intense sunlight, extreme temperature changes, and little or no soil. Some hardy annual flowering plants with wind-dispersed seeds are also frequent pioneers. Pioneer species help build and improve the soil. In doing so, they often set the stage for their own replacement. Many pioneer species partner with nitrogen-fixing bacteria, so they can grow in nitrogen-poor habitats. Seeds of later species can take root inside mats of the pioneers. Organic wastes and remains accumulate and, by adding volume and nutrients to soil, this material helps other species take hold. Later successional species often shade and eventually displace earlier ones. In secondary succession, a disturbed area within a community recovers. It commonly occurs in abandoned agricultural fields and burned forests. Because improved soil is present from the start, secondary succession usually occurs faster than primary succession. When the concept of ecological succession was first developed in the late 1800s, it was thought to be a predictable and directional process. Which species are present at each stage in succession was thought to be determined primarily by physical factors such as climate, altitude, and soil type. In this view, succession culminates in a "climax community," an array of species that persists over time and will be reconstituted in the event of a disturbance. Ecologists now know that the species composition of a community changes in unpredictable ways. Communities do not journey along a well-worn path to a predetermined climax state. Random events determine the order in which species arrive in a habitat, and thus affect the course of succession. Ecologists had an opportunity to investigate how random factors influence succession after the 1980 eruption of Mount Saint Helens leveled about 600 square kilometers (235 square miles) of forest in Washington State (Figure 41.12). They recorded the natural pattern of colonization, and carried out experiments in plots inside the blast zone. The results of these and other studies showed that the types of pioneer species that arrive first were most important. Which species arrived first in a particular area was a random event that had a major influence on which species followed.

Mutualism

Mutualism is an interspecific interaction that benefits both participants. Flowering plants and their pollinators are a familiar example. In the most extreme cases, two species coevolve and become mutually dependent. For example, each species of yucca plant is pollinated by one species of yucca moth, whose larvae develop only on that plant (Figure 41.2). Mutualistic relationships are typically less exclusive: Most flowering plants have more than one pollinator, for example, and most pollinators service more than one species of plant. Photosynthetic organisms often supply food for nonphotosynthetic partners, as when plants lure pollinators with sugary nectar. In addition, many plants make fruits that attract seed-dispersing animals. Plants also provide sugars to mycorrhizal fungi and nitrogen-fixing bacteria. The plants' fungal or bacterial symbionts return the favor by supplying their host with other essential nutrients. Similarly, photosynthetic dinoflagellates provide sugars to reef-building corals, and photosynthetic bacteria or algae in a lichen feed their fungal partner. All are examples of mutualisms. Many animals have mutualistic microorganisms living in their digestive tract. For example, Escherichia coli bacteria in your colon provide you with vitamin K, and you provide them with a steady food supply and a warm habitat. Other mutualisms involve protection. For example, an anemonefish and a sea anemone fend off one another's predators (Figure 41.3). Ants protect bull acacia trees from leaf-eating insects, and in return the tree houses the ants in special hollow thorns and provides them with sugar-rich food. The oxpecker bird shown in the chapter's opening photo protects its partner from ticks, which it eats. From an evolutionary standpoint, mutualism is best described as reciprocal exploitation. Each individual increases its fitness by extracting a resource, such as protection or food, from its partner. If taking part in the mutualism has a cost, then minimizing that cost is evolutionarily advantageous. Most flowering plants produce the minimum amount of nectar necessary to attract pollinators. Producing more nectar than necessary would waste resources that could otherwise be used in growth and reproduction.

Timing of Births and Deaths

One way to investigate life history traits is to focus on a cohort—a group of individuals born during the same interval—from their time of birth until the last one dies. Ecologists often divide a natural population into age classes and record the age-specific birth rates and mortality. The resulting data is summarized in a life table (Table 40.1). Information about age-specific death rates can also be illustrated by a survivorship curve, a plot that shows how many members of a cohort remain alive over time. Ecologists describe three types of curves. A type I curve is convex, indicating that the death rate remains low until relatively late in life (Figure 40.7A). Humans and other large mammals that produce and care for one or two offspring at a time have this pattern. A diagonal, type II curve indicates that the death rate of the population does not vary much with age (Figure 40.7B). In lizards, small mammals, and large birds, old individuals are about as likely to die of disease or predation as young ones. A type III curve is concave, indicating that the death rate for a population peaks early in life (Figure 40.7C). Marine animals that release eggs into water have this type of curve, as do plants that release huge numbers of tiny seeds.

Density-Dependent Factors

No population can grow exponentially forever. As the degree of crowding increases, density-dependent limiting factors cause birth rates to slow and death rates to rise, so the rate of population growth decreases. These factors include predation, parasitism and disease, and competition for a limited resource. Any natural area has limited resources. Thus, an increase in the number of individuals in an area leads to increased intraspecific competition—competition for a resource among members of the same species. As a result of increased competition, some individuals may fail to secure what they need to survive and reproduce. Competition has a detrimental effect even on winners, because energy they use in competition for resources is not available for reproduction. Essential resources for which animals might compete include food, water, hiding places, and nesting sites (Figure 40.4). Plants compete for nutrients, water, and access to sunlight. The negative effects of parasites and contagious disease increase with crowding because the closer individuals are to one another, the more easily parasites and pathogens can spread. Predation increases with density too, because predators often concentrate their efforts on the most abundant prey.

Global Climate Change

Ongoing climate change affects ecosystems worldwide. Average temperatures are increasing (Figure 44.8), with the more pronounced rise at temperate and polar latitudes. This rising temperature is elevating sea level by two mechanisms. Water expands as it is heated, and heating also melts sea ice and glaciers (Figure 44.9), adding meltwater to the sea. In the past century, sea level has risen about 20 centimeters (8 inches). As a result, some coastal wetlands and low-lying islands are disappearing underwater. Temperature changes are important cues for many temperate zone species. Abnormally warm spring temperatures are causing deciduous trees to put leaves out earlier, and spring-blooming plants to flower earlier. Animal migration times and breeding seasons are also shifting. Species arrays in biological communities are changing as warmer temperatures allow some species to expand their range to higher latitudes or elevations that were previously too cold for them. Not all species can move or disperse quickly to a new location, and warmer temperatures are expected to drive some of these species to extinction. In tropical seas, warming water is stressing reef-building corals and increasing the frequency of coral bleaching events (Section 43.10). In the arctic, seasonal sea ice now forms later and melts earlier, stressing polar bears. During late spring and early fall, regions where these bears previously would have walked across sea ice to gain access to their seal prey are now open water. Global warming is just one aspect of global climate change. Temperature affects evaporation, winds, and currents, so many weather patterns are expected to change as the world continues to warm. For example, warmer temperatures are correlated with extremes in rainfall patterns: periods of drought interrupted by unusually heavy rains. In addition, warmer seas tend to increase the intensity of hurricanes. As Section 42.5 explained, the main cause of global climate change is a rise in the atmospheric concentrations of greenhouse gases such as carbon dioxide. Fossil fuel combustion is the single biggest source of greenhouse gas emissions, and the use of these fuels continues to rise as China and India become increasingly industrialized. Reducing greenhouse gas emissions will be a challenge, but international efforts are under way to increase the efficiency of processes that require fossil fuels, to shift to alternative energy sources such as solar and wind power, and to develop innovative ways to store carbon dioxide.

Causes of Species Decline

Overharvesting In some cases, people directly reduce a species' number. Consider that when European settlers first arrived in North America, they found between 3 and 5 billion passenger pigeons. In the 1800s, commercial hunting of these birds for food caused a steep decline in their number. The last time anyone saw a wild passenger pigeon was 1900, and he shot it. The last captive member of the species died in 1914. We continue to overharvest species. The overfishing of the Atlantic codfish population, described in Section 40.5, is one recent example. Consider also the fate of the white abalone, a gastropod mollusk native to kelp forests off the coast of California (Figure 44.1A). Heavy harvesting of this species during the 1970s reduced the population to about 1 percent of its original size. In 2001, it became the first invertebrate to be listed as endangered by the USFWS. Although some white abalone remain in the wild, their population density remains too low for effective reproduction. The species' only hope for survival is a program of captive breeding. If this program succeeds, individuals will be reintroduced to the wild. Species are overharvested not only as food, but also for use in traditional medicine, for the pet trade, and for ornamentation. Some orchids prized by collectors have become nearly extinct in the wild. Most of the orphan elephants shown in the chapter opening photo lost their mother to poachers who kill to obtain ivory tusks. The majority of elephant tusks harvested this way end up in China, in the form of decorative carved objects. Habitat Degradation and Fragmentation Humans also harm species indirectly by altering the species' habitat. Each species requires a specific type of habitat, and any degradation, fragmentation, or destruction of that habitat reduces population numbers. An endemic species occurs only in the area in which it evolved. Endemic species are more likely to go extinct as a result of habitat degradation than species that have spread to many areas. Consider Pyne's ground plum (Figure 44.1B), a flowering plant that lives only in cedar forest near a rapidly growing city in Tennessee. The plant is threatened by conversion of its habitat to homes and industrial use. Texas blind salamanders (Figure 44.1C) are among the species endemic to Edwards Aquifer, a series of water-filled, underground limestone formations. Excessive withdrawals of water from the aquifer, in combination with water pollution, threaten the salamander and other species in the aquifer. Deliberate or accidental species introductions can be another type of habitat degradation (Section 41.7). Rats that reached islands by stowing away on ships endanger many ground-nesting birds that evolved in the absence of egg-eating ground predators. Exotic species can outcompete native ones. For example, California's endemic golden trout declined after European brown trout and eastern brook trout were introduced into California's mountain streams for sport fishing. The introduced trout outcompete the native trout for food. Habitat fragmentation creates islands of suitable habitat scattered across what was once a continuous range. Buildings, roads, and fences can fragment a habitat, thus breaking a large population into smaller populations in which inbreeding is more likely. Inbreeding can have deleterious genetic effects (Section 17.6). Fragmentation also prevents individuals from accessing essential resources. Roads, fences, and houses prevent spotted turtles endemic to the eastern United States from moving among wetlands to feed and hibernate. The wetlands still exist, but turtles cannot move between them.

Near-Ground Ozone Pollution

Ozone does not naturally occur in the lower atmosphere, but it can form when compounds released by burning or evaporating fossil fuels are exposed to sunlight. Warm temperature speeds this reaction, so ground-level ozone varies daily (it is higher in the daytime) and seasonally (it is higher during the summer). The ozone that forms in the lower atmosphere never reaches the ozone layer where it would be useful. Instead, ozone that forms in the lower atmosphere acts as a harmful pollutant. It irritates the eyes and respiratory tracts of animals and interferes with plant growth. To help reduce ozone pollution, avoid actions that put fossil fuels or their combustion products into the air at times that favor ozone production. During hot, sunny, still weather, postpone filling your gas tank or using gasoline-powered appliances until the evening, when there is less sunlight to power the conversion of pollutants to ozone.

Effects of Pollution

Pollutants are natural or synthetic substances released into soil, air, or water in greater than natural amounts. The presence of a pollutant disrupts the physiological processes of organisms that evolved in its absence, or that are adapted to lower levels of it. Some pollutants come from a few easily identifiable sites, or point sources. A factory that discharges pollutants into the air or a landfill that releases pollutants into water is a point source. Pollutants that come from point sources are easiest to control: Identify the few specific sources, and you can take action there. Eradicating pollution from nonpoint sources is more challenging. Pollution from nonpoint sources stems from widespread release of a pollutant. For example, leaked motor oil that pollutes waterways comes from vehicles in many roads and driveways.

Atmospheric Deposition

Pollutants that enter the atmosphere can harm organisms when they fall to Earth as dust or in precipitation.Acid Deposition Common air pollutants include sulfur dioxides (released by coal-burning power plants) and nitrogen oxides (released by combustion of gasoline and oil). In dry weather, airborne sulfur and nitrogen oxides coat dust particles that fall to the ground as dry acid deposition. Wet acid deposition, or acid rain, occurs when pollutants combine with water and fall as acidic precipitation. The pH of acid rain can be as low as 2; normal rainwater has a pH above 5 (Section 2.5). In the United States, federal regulations limiting sulfur dioxide emissions from coal-burning power plants have made precipitation less acidic (Figure 44.6A). The world's main sulfur dioxide emitters are now China and India, where industrialization and coal use continue to increase. Acid rain that falls on or drains into waterways, ponds, and lakes can harm aquatic organisms. Acidic water can kill adult fish and prevent fish eggs from developing. Acid rain that falls on a forest burns tree leaves and alters the nutrient content of forest soil. As acidic water drains through the soil, positively charged hydrogen ions displace positively charged nutrient ions such as calcium, which leach from the soil in runoff. The acidity also causes soil particles to release metals such as aluminum that can harm plants. A combination of fewer nutrients and greater exposure to toxic metals weakens trees, making them more susceptible to insects and pathogens, and thus more likely to die. Trees at high elevations, where exposure to clouds of acidic droplets occurs more frequently, are most at risk of harm from acid rain (Figure 44.6B). Ammonium Deposition Sulfur dioxides and nitrogen oxides that enter the air can also combine with gaseous ammonia released by fertilizers and animal wastes. When the resulting ammonium salts fall to Earth, they act like fertilizer. Addition of these salts to soil allows plants with high nitrogen needs to thrive where they would otherwise be excluded, thus altering the array of species in an ecosystem. Mercury Deposition Emissions from coal-burning power plants and factories are the main source of mercury in the atmosphere. When mercury falls to Earth, microorganisms combine it with carbon to form methylmercury, which is toxic to animals. As explained in the Chapter 2 and 42 Applications, methylmercury commonly enters aquatic food chains and undergoes biological magnification—its concentration in organisms increases as it passes from one trophic level to the next. Thus upper-level consumers of aquatic organisms are most affected. Mercury does not harm plants; in fact, plants are sometimes used to bioremediate mercury pollution.

Predator-Prey Arms Races

Predator and prey exert selective pressure on one another. Suppose a mutation arises that gives a prey species a more effective defense. Over generations, this mutation will tend to spread through the prey population as a result of natural selection. As this occurs, traits that make individual predators better at thwarting the improved prey defense become adaptive in predator populations. Thus, predators exert selective pressure that favors improved prey defense, which in turn exerts selective pressure on predators, and so it goes over many generations. You have already learned about some defensive adaptations. Many prey species have hard or sharp parts that make them difficult to eat. Think of a snail's shell or a sea urchin's spines. Other prey contain chemicals that taste bad or sicken predators. Most such defensive compounds in animals come from the plants they eat. For example, a monarch butterfly caterpillar takes up chemicals from the milkweed that it feeds on. A bird that eats a monarch butterfly will be sickened by these chemicals and avoid similar butterflies later. Prey animals use a variety of mechanisms to fend off a predator. Section 1.6 described how eyespots and a hissing sound protect some butterflies. A lizard's tail may detach from the body and wiggle a bit as a distraction. Many animals, including skunks, exude or squirt a foul-smelling, irritating repellent when frightened. Bees and wasps sting. Well-defended prey often have warning coloration, a conspicuous color pattern that predators learn to avoid. For example, many species of stinging wasps and bees have a pattern of black and yellow stripes (Figure 41.7A). The similar appearance of bees and wasps is an example of one type of mimicry, an evolutionary pattern in which one species comes to resemble another. Bees and wasps benefit from their similar appearance. The more often a predator is stung by a black-and-yellow-striped insect, the less likely it is to attack any similar-looking insect. In another type of mimicry, an undefended prey species masquerades as a well-defended species. For example, some flies that cannot sting resemble bees or wasps that can (Figure 41.7B). The fly benefits when predators avoid it after an encounter with the better-defended look-alike species. Camouflage is a body form and coloration pattern that allows an animal to blend into its surroundings, and thus avoid detection. For example, snowshoe hares such as the one in Figure 41.6 turn white in winter, making it harder for predators to spot them in a snowy landscape. Predators benefit from camouflage that hides them from their prey (Figure 41.8). Other predator adaptations include sharp teeth and claws that can pierce protective hard parts. Speedy prey select for faster predators. For example, the cheetah, the fastest land animal, can run 114 kilometers per hour (70 mph). Its preferred prey, Thomson's gazelles, run 80 kilometers per hour (50 mph).

Setting Priorities

Protecting biological diversity is often a tricky proposition. Even in developed countries, people often oppose environmental protections because they fear adverse economic consequences. However, taking care of the environment can make good economic sense. With a bit of planning, people can both preserve and profit from their biological wealth. The resources available for conserving areas are limited, so conservation biologists must often make difficult choices about which areas should be targeted for protection first. These biologists identify biodiversity hot spots, places that are home to species found nowhere else and are under great threat of destruction. Once identified, hot spots can take priority in worldwide conservation efforts. On a broader scale, conservation biologists define ecoregions, which are land or aquatic regions characterized by climate, geography, and the species found within them. The most widely used system of ecoregion definitions was developed by scientists of the World Wildlife Fund. This system defines 867 distinctive land ecoregions, each with a distinctive arrays of species. Figure 44.10 shows the locations and conservation status of major ecoregions considered the top priority for conservation efforts. The Klamath-Siskiyou forest in southwestern Oregon and northwestern California is one of North America's endangered ecoregions (Figure 44.11). It is home to many rare conifers. Two endangered birds, the northern spotted owl and the marbled murrelet, nest in old-growth parts of the forest, and endangered coho salmon breed in streams that run through the forest. Logging threatens all of these species. By focusing on protecting biodiversity hot spots and critical ecoregions rather than on individual endangered species, scientists hope to maintain the ecosystem interactions that naturally sustain biological diversity.

Resource Partitioning

Resource partitioning is an evolutionary process by which different species become adapted to share a limiting resource in a way that minimizes competition. Consider three species of annual plants that commonly coexist in abandoned fields. All require water and nutrients, but the roots of each species extend to and take resources up from soil at a different depth. This variation in root form allows the plants to coexist. Resource partitioning arises as a result of directional selection that occurs when species with similar requirements share a habitat and compete for a limiting resource. In each species, those individuals whose traits minimize their need to compete with the other species for the resource will be at a selective advantage. Thus, competition between species can lead to character displacement: The range of variation for one or more characters shifts in a direction that lessens the intensity of competition for a limiting resource. In short, the resource needs of the competing species become less similar. Results from a long-term study of Galápagos finches demonstrate how character displacement can occur. In 1982, an established population of medium ground finches on one of the Galápagos islands was joined by a new competitor, the large ground finch. Both types of ground finches eat seeds, but the large ground finch (which has a bigger beak) is better at opening big seeds. From 2003 to 2005 a drought decreased the availability of all seeds, competition for food increased, and populations of both finch species declined. By the end of this period, the distribution of beak size among the medium ground finches had shifted; smaller beaks had become more common. Smaller-beaked individuals had survived the drought-induced seed shortage better than larger-beaked members of their species because they concentrated their efforts on smaller seeds. Thus, they faced less competition for food from the large ground finches.

Talking Trash

Seven billion people use and discard a lot of stuff. Where does all the waste go? Historically, much of it was buried in the ground or dumped out at sea. Trash was out of sight, and also out of mind. We now know that these practices have negative impacts on both biodiversity and human health. Piling up or burying garbage on land can contaminate groundwater, as when lead from discarded batteries seeps into aquifers. Modern landfills in developed countries are lined with clay and/or plastic to prevent toxins in the trash from reaching the groundwater. However, many landfills in less-developed countries have no such barrier. Toxins leached from trash enter the groundwater and can turn up in waterways and in wells that supply drinking water. Discarding solid waste in oceans threatens marine life (Figure 44.5). In the United States, solid waste can no longer legally be dumped at sea. Nevertheless, plastic constantly enters coastal waters. Plastic shopping bags, plastic water bottles, foam cups and containers from fast-food outlets, and other litter often enters storm drains. From there it is carried to streams and rivers that can convey it to the sea. A recent study estimated that the world's oceans currently contain 268,940 tons of floating plastic. Ocean currents can carry bits of plastic for thousands of miles. These plastic bits accumulate in some areas of the ocean. Consider the Great Pacific Garbage Patch, a region of the north central Pacific that the media often describe as an "island of trash." In fact, the plastic is not easily visible. Rather, the garbage patch is a region where a high concentration of confetti-like plastic particles swirl slowly around an area as large as the state of Texas. The small bits of plastic absorb and concentrate toxic compounds such as pesticides and industrial chemicals from the seawater around them, making the plastic all the more harmful to marine organisms that mistakenly eat it.

Density-Independent Factors

Sometimes, natural disasters or weather-related events affect population size. A volcanic eruption, hurricane, or flood can decrease population size. So can human-caused events such as oil spills. These events are called density-independent limiting factors, because crowding does not influence the likelihood of their occurrence or the magnitude of their effect. In nature, density-dependent and density-independent factors often interact to determine a population's size. Consider what happened after the 1944 introduction of 29 reindeer to St. Matthew Island, an uninhabited island off the coast of Alaska. When biologist David Klein visited the island in 1957, he found 1,350 well-fed reindeer (Figure 40.6). Klein returned in 1963 and counted 6,000 reindeer. The population had soared far above the island's carrying capacity. A population can temporarily overshoot an environment's carrying capacity, but the high density cannot be sustained. Klein observed that some effects of density-dependent limiting factors were already apparent. For example, the average body size of the reindeer had decreased. When Klein returned in 1966, only 42 reindeer survived, and only one was male. There were no fawns. Thousands of reindeer had starved to death during the winter of 1963-1964. That winter was unusually harsh, with low temperatures, high winds, and 140 inches of snow. Most reindeer, already in poor condition as a result of increased competition, starved when deep snow covered their food. A decline in the number of reindeer in this population had been expected—populations that overshoot their environment's carrying capacity will necessarily shrink—but bad weather magnified the extent of the crash. By the 1980s, there were no reindeer left on the island.

Deforestation

The amount of forested land is currently stable or increasing in North America, Europe, and China, but tropical forests continue to disappear at an alarming rate. In Brazil, increases in the export of soybeans and free-range beef have helped make the country the world's seventh-largest economy. However, this economic expansion has come at the expense of the country's woodlands and forests (Figure 44.3). Deforestation has detrimental effects beyond the immediate destruction of forest organisms. For example, deforestation encourages flooding, because water that no longer is taken up by tree roots runs off instead. Deforestation in hilly areas raises the risk of landslides. Tree roots tend to stabilize the soil. When they are removed, waterlogged soil becomes more likely to slide. Soils of deforested areas become nutrient-poor because of the increased loss of nutrient ions in runoff. Figure 44.4 shows results of an experiment in which scientists deforested a region in New Hampshire and monitored the nutrient content of runoff. Deforestation caused a spike in loss of essential soil nutrients such as calcium. Like desertification, deforestation affects local climate. The loss of plants means reduced transpiration, so the amount of local rainfall declines. In shady forests, transpiration also results in evaporative cooling. When a forest is cut down, shade disappears and the evaporative cooling ceases. Thus, the temperature in a deforested area is typically higher than in an adjacent forested area. Once a tropical forest has been logged, the resulting nutrient losses and drier, hotter conditions can make it impossible for tree seeds to germinate or for seedlings to survive. Thus, deforestation can be difficult to reverse. Forests take up and store huge amounts of carbon dioxide (Chapter 25 Application), so forest losses also contribute to global climate change. With fewer trees, less carbon dioxide is taken up from the atmosphere and stored in wood.

Collapse of a Fishery

The evolution of life history traits in response to predation is not merely of theoretical interest. It has economic importance. Just as guppies evolved in response to predators, a population of Atlantic codfish (Gadus morhua) evolved in response to human fishing pressure. From the mid-1980s to early 1990s, the number of fishing boats targeting the North Atlantic population of codfish increased. As the yearly catch rose, the age at which codfish become sexually mature shifted; fishes that reproduce while young and small increased in frequency in the population. These early-reproducing individuals were at an advantage because both commercial fisherman and sports fishermen preferentially caught and kept larger fish (Figure 40.10). Fishing pressure continued to rise until 1992, when declining cod numbers caused the Canadian government to ban cod fishing in some areas. That ban, and later restrictions, came too late to stop the Atlantic cod population from crashing. In some areas, the population declined by 97 percent and still shows no signs of recovery. Looking back, it is clear that life history changes were an early sign that the North Atlantic cod population was in trouble. Had biologists recognized what was happening, they might have been able to save the fishery and protect the livelihood of more than 35,000 fishers and associated workers. Ongoing monitoring of the life history data for other economically important fishes may help prevent similar disastrous crashes in the future.

Biotic Potential

The growth rate for a population under ideal conditions is its biotic potential. This is a theoretical rate at which the population would grow if shelter, food, and other essential resources were unlimited and there were no predators or pathogens. Microbes such as bacteria have some of the highest biotic potentials, whereas large-bodied mammals have some of the lowest. Factors that affect biotic potential include the age at which reproduction typically begins, how long individuals remain reproductive, and the number of offspring that are produced each time an individual reproduces. Section 40.4 considers how natural selection influences these factors. Regardless of the species, populations seldom reach their biotic potential because of the effects of limiting factors, a topic we discuss in detail in the next section.

Single-Species Effects

The loss or addition of one species sometimes alters the abundance of many other species in a community.

Effects of Disturbance

The magnitude and frequency of disturbances affect communities. A variety of field studies support the intermediate disturbance hypothesis, which states that species richness is greatest when physical and biological disturbances are moderate in their intensity or frequency (Figure 41.13). When disturbance is infrequent and of low intensity (does not remove many individuals), the most competitive species will exclude others, so diversity is low. By contrast, when disturbance occurs often or is of high intensity, most species present will be colonizers. With a moderate level of disturbance, the community will contain a mix of early and late successional species, and so will be most diverse. In communities that repeatedly experience a particular type of disturbance, individuals that withstand or benefit from that disturbance have a selective advantage. For example, some plants in areas subject to periodic fires produce seeds that germinate only after a fire. Seedlings of these plants benefit from the lack of competition for resources in newly burned areas. Other plants have an ability to resprout fast after a fire (Figure 41.14). Because different species respond differently to fire, the frequency of this disturbance affects competitive interactions. Human suppression of naturally occurring fires can alter the composition of a biological community. In the absence of fires, species whose numbers would otherwise be suppressed by fire can become dominant. Some species are especially intolerant of physical disturbance of their environment. These indicator species are the first to decline or disappear when conditions change, so they can provide an early warning of environmental degradation. For example, a decline in a trout population can be an early sign of problems in a stream, because trout are highly sensitive to pollutants and they cannot tolerate low oxygen levels. Some lichens that are intolerant of air pollution serve as indicators of air quality (Figure 41.15).

Nitrogen Reservoirs and Flow

The nitrogen cycle is an atmospheric cycle in which nitrogen moves among the atmosphere, soil, and water, and into and out of food webs (Figure 42.10). The main nitrogen reservoir is the atmosphere, which is about 80 percent nitrogen gas. Nitrogen gas consists of two atoms of nitrogen held together by a triple covalent bond as , or N≡N. Remember from Section 2.3 that a triple bond holds atoms together more strongly than a single or double bond. All organisms use nitrogen to build ATP, nucleic acids, and proteins. Photosynthetic organisms also use it to build chlorophyll. Despite the universal need for nitrogen and the abundance of atmospheric , no eukaryote can make use of nitrogen gas. Eukaryotes do not have an enzyme that can break the strong bond between the two nitrogen atoms. Some bacteria and archaea can break the triple bond in nitrogen gas. They carry out nitrogen fixation, combining gaseous nitrogen with hydrogen to produce ammonium , the dissolved (ionic) form of ammonia . Biological nitrogen fixation has a high activation energy (Section 5.2): It requires an input of 16 molecules of ATP to convert one molecule of nitrogen to ammonia. You have already learned about two major groups of nitrogen-fixers. Nitrogen-fixing cyanobacteria live in aquatic habitats, soil, and as components of lichens (Sections 19.4 and 22.3). Some other nitrogen-fixing bacteria are free-living in soil. Still others live inside plant parts such as nodules on roots of peas and other legumes (Section 26.2). Some deepsea archaea also fix nitrogen. In addition to biological nitrogen fixation, a small amount of ammonium forms as a result of lightning-fueled reactions in the atmosphere. The energy of the lightning causes nitrogen gas to react with atmospheric water vapor. Once ammonium has formed, plants can take it up from soil water and use it in metabolic reactions. Animals meet their nitrogen needs by eating plants or one another. Nitrogen then cycles back to soil when bacterial and fungal decomposers release ammonium from organic remains, a process called ammonification . Ammonium can be converted to nitrate by a two-step process called nitrification . In the first step, ammonia-oxidizing bacteria or archaea convert ammonium to nitrite . In the second step, nitrite-oxidizing bacteria convert nitrites to nitrates. Like ammonium, nitrates can be taken up and used by plants . Nitrification is essential to ecosystems because it prevents toxic ammonium from accumulating to high concentrations. Sewage treatment plants often make use of bacteria that carry out nitrification. Sewage contains large amounts of ammonium formed from the urea excreted in urine (Section 37.1). Some nitrates are converted to nitrogen gas and returned to the atmosphere by denitrification, a process carried out by anaerobic bacteria . In ecosystems, denitrification can have a negative effect on primary production because it results in a decline in the amount of soluble nitrogen available to producers. In sewage treatment plants, denitrifying bacteria are used to remove nitrates from wastewater before the water is released into the environment.

The Greenhouse Effect

The ongoing increase in atmospheric carbon dioxide is a matter of concern. Carbon dioxide is a greenhouse gas, an atmospheric gas whose ability to absorb and reradiate heat energy helps keep Earth warm enough to sustain life. The mechanism by which this occurs is called the greenhouse effect (Figure 42.9). Remember from Section 6.2 that electromagnetic energy is emitted by sun. Earth's atmosphere reflects some of that energy back into space , but more energy penetrates the atmosphere and is absorbed by Earth's surface . Earth's surface is warmed by absorbing energy, and it emits heat. Greenhouse gases absorb some of this radiated heat, then emit a portion of it back toward Earth . The emitted heat further warms Earth's surface and lower atmosphere. Without greenhouse gases, heat emitted by Earth's surface would escape into space, leaving the planet cold and lifeless. Given the greenhouse effect, increases in the atmospheric concentration of carbon dioxide and other greenhouse gases would be expected to raise the temperature of Earth's surface. Since the mid-1800s, the concentration of atmospheric has risen from 280 parts per million (ppm) to more than 400 ppm. The result is global climate change, a trend toward rising temperature and shifts in climate patterns worldwide. Earth's climate has always varied over time. During ice ages, much of the planet was covered by glaciers. Other periods were warmer than the present, and tropical plants and coral reefs thrived at now-cool latitudes. Scientists can correlate past large-scale temperature changes with shifts in Earth's orbit (which deviates a bit in a 100,000-year cycle) and tilt (which varies in a 40,000-year cycle). Changes in solar output and volcanic eruptions also affect Earth's temperature. However, the overwhelming majority of scientists agree that these factors are not sufficient to cause the climate change occurring now. There is broad scientific consensus that the current climate change is due to the rise in greenhouse gases, and also that it poses a threat to ecosystems worldwide. We discuss the consequences of global climate change in more detail in Section 44.5, when we consider human effects on the biosphere.

The Water Cycle

The water cycle moves water from the ocean to the atmosphere, onto land, and back to the oceans (Figure 42.7). Sunlight energy causes water to evaporate (change from a liquid to a vapor). Water vapor that enters the cool upper layers of the atmosphere condenses into droplets, forming clouds. When droplets get large and heavy enough, they fall as precipitation: rain, snow, or hail. Oceans cover about 70 percent of Earth's surface, so most rainfall returns water directly to the oceans. Most precipitation that falls on land seeps into the ground. Some of this water remains between soil particles as soil water, which can be taken up by plants. Most water that enters plants is returned to the atmosphere by transpiration (evaporation from plant parts). Water that drains through soil layers often collects in aquifers, which are layers of water-saturated porous rock. Groundwater is water in soil and aquifers. Water that falls on impermeable rock or on saturated soil becomes runoff, meaning it flows over the ground into streams. The flow of groundwater and surface water returns water to oceans. Movement of water results in movement of other nutrients. Carbon, nitrogen, and phosphorus all have soluble forms that can be carried from place to place in flowing water. As water trickles through soil, it brings soluble nutrient ions from topsoil into deeper soil layers. As a stream flows over limestone, water slowly dissolves the rock and carries carbonates back to the seas where the limestone formed. The vast majority of Earth's water (97 percent) is in oceans, and most fresh water is frozen as ice (Table 42.1). Thus, the fresh water available to meet human needs and sustain land ecosystems is limited. Water overdrafts are now common—humans remove water from aquifers, lakes, or rivers faster than natural processes replenish it. Aquifers supply about half of the drinking water in the United States. Overdrawing water from an aquifer can lower the water table, which is the uppermost layer of soil that is saturated with water. When the water table falls, wells that tap an aquifer can run dry. Consider what has happened to the largest aquifer in the United States, the Ogallala aquifer. This aquifer stretches from South Dakota to Texas and supplies irrigation water for 27 percent of the nation's crops. For the past thirty years, withdrawals have exceeded replenishment by a factor of ten. As a result, the water table has dropped as much as 50 meters (150 feet) in some regions. Water in many rivers is currently over-allocated, meaning the amount of water promised to various stakeholders such as cities and farmers exceeds the amount that currently flows through the river. Diversion of water from these rivers for human use results in lowered or nonexistent flow farther downstream. Lack of water can have a devastating effect on biological communities that depend on the river. Rivers convey sediment and nutrients as well as water, so decreased flow alters ecosystems by slowing delivery of these materials to the river's delta, the region where the river approaches the sea.

r-Selection and K-Selection

To produce offspring, an individual must invest resources that it could otherwise use to grow and maintain itself. Species differ in the ways which they distribute parental investment among offspring. Life history patterns vary continuously among species, but ecologists have described two theoretical extremes at either end of this continuum. Which pattern evolves will depend on what particular allocation of resources to growth, survivorship, and reproduction will maximize the number of offspring that survive to adulthood. When a species lives where conditions vary in an unpredictable manner, its populations seldom reach the carrying capacity of their environment. As a result, there is little competition for resources, and deaths occur mainly as a result of density-independent factors. These conditions favor an opportunistic life history, in which individuals produce as many offspring as possible, as quickly as possible. Opportunistic species are said to be subject to r-selection, because they maximize r, the per capita growth rate. They tend to have a short generation time and small body size. Opportunistic species usually have a type III survivorship curve, with mortality heaviest early in life. For example, weedy plants such as dandelions have an opportunistic life history. They mature within weeks, produce many tiny seeds, then die. Flies are opportunistic animals. A female fly can lay hundreds of eggs in a temporary food source such as a rotting tomato (Figure 40.8A). When a species lives in a more stable environment, its populations often approach carrying capacity. Under these circumstances, the ability to successfully compete for resources has a major influence on reproductive success. Thus, an equilibrial life history, in which parents produce a few, high-quality offspring, is adaptive. Equilibrial species are shaped by K-selection, in which adaptive traits provide a competitive advantage when population size is near carrying capacity (K). Such species tend to have a large body and a long generation time. This type of life history is typical of large mammals that take years to reach adulthood and begin reproducing. For example, a female blue whale reaches maturity at the age of 6 to 10 years. She then produces only one large calf at a time, and continues to invest in the calf by nursing it after its birth (Figure 40.8B). Similarly, a coconut palm grows for years before beginning to produce a few coconuts at a time. In both whales and coconut palms, a mature individual produces young for many years. Many equilibrial species have a type I survivorship curve, with mortality heaviest later in life. Some species have combinations of traits that cannot be explained by r-selection or K-selection alone. For example, Atlantic eels and Pacific salmon are among the few vertebrates that have a one-shot reproductive strategy. They breed once, then die. This strategy can evolve when opportunities for reproduction are unlikely to be repeated. In the eels and salmon, physiological changes related to migration between fresh water and saltwater make a repeat journey impossible.

Development and Consumption

What is Earth's carrying capacity for humans? There is no simple answer to this question. For one thing, we cannot predict what new technologies may arise or the effects they will have. For another, different types of societies require different amounts of resources to sustain them. On a per capita (per-indvidual) basis, people in highly developed countries use far more resources than those in less developed countries, and they also generate more waste and pollution. Ecological footprint analysis is one widely used method of measuring and comparing resource use. An ecological footprint is the amount of Earth's surface required to support a particular level of development and consumption in a sustainable fashion. It includes the amount of area required to grow crops, graze animals, produce forest products, catch fish, hold buildings, and take up any carbon emitted by burning fossil fuels. In 2010, the per capita global footprint for the human population was 2.7 hectares, or about 6.5 acres. The world's two most populous countries, China and India, were below that average; the per capita footprint of the United States was more than three times the average (Table 40.2). In other words, the lifestyle of an average person in the United States requires three times as much of Earth's sustainable resources as the lifestyle of an average world citizen. It requires more than eight times the resources of a person in India. Consider that the United States accounts for about 5 percent of the world's population, yet it uses about 25 percent of the world's minerals and energy supply. The United States is unlikely to lower its resource consumption to match that of India. In fact, billions of people in India, China, and other less developed nations dream that one day they or their offspring will enjoy the same type of lifestyle as the average American. Ecological footprint analysis tells us that, with current technology, Earth may not have enough resources to make those dreams come true. The Global Footprint Network estimates that Earth does not have enough resources for everyone now alive to live like an average person in the United States. Ecological footprint analysis suggests that the human population is currently living well beyond its ecological means, and is in the process of racking up a deficit that will take its toll on future generations.

Brood Parasitism

With brood parasitism, one egg-laying species benefits by having another raise its offspring. The host species suffers by squandering care on unrelated individuals. The European cuckoos described in Section 39.2 are brood parasites, as are North American cowbirds. Not having to invest in parental care allows a female cowbird to produce a large number of eggs—as many as thirty in a single reproductive season. The presence of brood parasites decreases the reproductive rate of the host species and favors an ability to detect and eject foreign young. Some avian brood parasites counter this host defense by producing eggs that closely resemble those of their host species. In cuckoos, different subpopulations have different host preferences and egg coloration. Females of each subpopulation lay eggs that closely resemble those of their preferred host. Some butterflies are brood parasites that outsource care of their larvae to ants. Caterpillars (larvae) of the Alcon blue butterfly smell like an ant worker and mimic the sounds made by an ant queen. Worker ants, fooled by these false cues, carry the caterpillars into their nest, where they care for them as if they were members of the colony, feeding them and protecting them from predators (Figure 41.11). The large blue butterfly described in Section 17.10 is a relative of the Alcon blue butterfly, and it too is a brood parasite of ants.

Plant-Herbivore Arms Race

With herbivory, an animal eats a plant or plant parts. The number and type of plants in a community can influence the abundance and type of herbivores present. Two types of defenses have evolved in response to herbivory. Some plants can withstand and recover quickly from the loss of some parts. For example, prairie grasses store enough resources in their roots to grow back lost shoots, so they are seldom killed by grazers such as bison. Other plants have traits that deter herbivory: spines or thorns, leaves that are hard to chew or digest, or chemicals that taste bad or sicken herbivores. Ricin, the toxin made by castor bean plants, is an example. Plant defenses select for traits that allow herbivores to overcome those defenses. For example, eucalyptus leaves contain toxins that make them poisonous to most mammals, but not to koalas. Koalas have specialized liver enzymes that break down these toxins.

Parasitism

With parasitism, one species (the parasite) benefits by feeding on another (the host), without immediately killing it. Endoparasites such as parasitic roundworms live and feed in their host (Figure 41.9A). An ectoparasite such as a tick feeds while attached to a host's external surface (Figure 41.9B). A parasitic way of life has evolved in a diverse variety of groups. Bacterial, fungal, protistan, and invertebrate parasites feed on vertebrates. Lampreys (Section 24.2) attach to other fish and feed on them. There are even a few parasitic plants that withdraw nutrients from other plants (Figure 41.9C). In terms of evolutionary fitness, killing a host too fast is bad for the parasite. The longer the host survives, the more parasite offspring can be produced. Thus, parasites with less-than-fatal effects on hosts are at a selective advantage. Although parasites typically do not kill their hosts, many still have an important impact on a host population. Many parasites are pathogens that cause disease in their hosts. Even when a parasite does not cause obvious symptoms, its presence can weaken a host, making it more vulnerable to predation or less attractive to potential mates. Some parasites cause their host to become sterile or limit its number of offspring. Adaptations to a parasitic lifestyle include traits that allow the parasite to locate hosts and to feed undetected. Ticks that feed on mammals or birds move toward a source of heat and carbon dioxide, which may be a potential host. A chemical in tick saliva acts as a local anesthetic; it prevents an animal from feeling a tick that is feeding on it. Endoparasites often have traits that help them evade a host's immune defenses. For example, some parasitic worms that live in the human digestive tract turn down our inflammatory response. By one hypothesis, the increasing rate of autoimmune disease in highly developed countries is an unexpected consequence of a reduction in inflammation-reducing worm infections. Among hosts, an ability to fend off parasites or reduce the toll that a parasite takes on fitness is adaptive. The crested auklet (a type of seabird) secretes a citrus-scented compound that repels lice and ticks. In many other birds and mammals, preening or grooming behavior removes such ectoparasites. Chimpanzees and other primates sometimes fold up tough, indigestible leaves and swallow them whole, a practice that helps rid their gut of parasitic worms.

Predator and Prey Abundance

With predation, one species (the predator) captures, kills, and eats another species (the prey). Predation removes a prey individual from the population immediately. In any community, predator abundance and prey abundance are interconnected. Generally, an increase in the size of a predator population results in a decrease in the abundance of its prey. Understanding how a predator population responds to changes in prey density is important because it helps ecologists predict long-term effects of predation. For passive predators, such as web-spinning spiders, the proportion of prey killed is constant, so the number killed in any given interval depends solely on prey density. The number of flies caught in spider webs is proportional to the total number of flies: The more flies there are, the more get caught in webs. More typically, the rate at which prey are killed depends in part on the time it takes predators to process prey. For example, a wolf that just killed a caribou will not hunt another until it has eaten and digested the first one. As prey density increases, the rate of kills rises steeply at first because there are more prey to catch. The rate slows when predators are exposed to more prey than they can handle at one time. Predator and prey populations sometimes rise and fall in a cyclical fashion. Figure 41.6 shows historical data for the numbers of lynx and their main prey, the snowshoe hare. Both populations rise and fall over an approximately ten-year cycle, with predator abundance lagging behind prey abundance. Field studies indicate that lynx numbers fluctuate mainly in response to hare numbers. However, the size of the hare population is affected by the abundance of the hare's food as well as the number of lynx. Hare populations continue to rise and fall even when predators are experimentally excluded from areas.

Preservation and Restoration

Worldwide, many biodiverse regions have been protected in ways that benefit local people. The Monteverde Cloud Forest in Costa Rica is one example. In the 1970s, George Powell was studying birds in this forest, which was rapidly being cleared. Powell decided to buy part of the forest as a nature sanctuary. His efforts inspired individuals and conservation groups to donate funds, and much of the forest is now a nature reserve. The reserve's plants and animals include more than 100 mammal species, 400 bird species, and 120 species of amphibians and reptiles. It is one of the few habitats left for jaguars and ocelots. A tourism industry centered on the reserve provides economic benefits to local people. Sometimes an ecosystem is so damaged, or so little of it is left, that conservation alone is not enough to sustain biodiversity. Ecological restoration is the process of renewing a natural ecosystem that has been degraded or destroyed. Consider the ecological restoration occurring in Louisiana's coastal wetlands. More than 40 percent of the coastal wetlands in the United States are in Louisiana. These marshes are an ecological and economic treasure, but they are in trouble. Dams and levees built upstream of the marshes hold back sediments that should replace sediment the marshes lose to the sea. Channels cut through the marshes for oil exploration and extraction have encouraged erosion, and the rising sea level threatens to drown existing plants. Restoration efforts now under way aim to reverse some of those losses by restoring marsh grass to regions that have become open water (Figure 44.12).