Chapter 1
Extinction and Endangered Species

Earth is richly supplied with different types of living organisms, including animals, plants, fungi, and bacteria. Various living organisms co-exist in their environments, forming complex, interrelated communities. Living organisms depend on one another for nutrients, shelter, and other benefits. The extinction of any one species can set off a chain reaction that affects many other species, particularly if the loss occurs near the bottom of the food chain. For example, the extinction of a particular insect or plant might seem inconsequential. However, there may be fish or small animals that depend on that resource for foodstuff. The loss can threaten the survival of these creatures, and larger predators that prey upon them. Extinction can have a ripple effect that spreads throughout nature.

In addition to its biological consequences, extinction poses a moral dilemma for humans, the only species capable of saving the others. The presence of humans on the planet has affected all other life forms, particularly plants and animals. Human lifestyles have proven to be incompatible with the survival of some other species. Purposeful efforts have been made to eliminate animals that prey on people, livestock, crops, or pose any threat to human livelihoods. Some wild animals have been decimated by human desire for meat, hides, fur, or other body parts with commercial value. Likewise demand for land, water, timber, and other natural resources has left many wild plants and animals with little to no suitable habitat. Humans have also affected nature by introducing nonnative species to local areas and producing pollutants that have a negative impact on the environment. The combination of these anthropogenic (human-related) effects with natural obstacles that limit survival, such as disease or low birthrates, have proven to be too much for some species to overcome. They have no chance of survival without human help.

As a result, societies have difficult choices to make about the amount of effort and money they are willing to spend to keep imperiled species from becoming extinct. Will people accept limits on their property rights, recreational activities, and means of livelihood to save a plant or animal? Should saving such popular species as whales and dolphins take priority over saving obscure, annoying, or feared species? Is it the responsibility of humans to save every kind of life form from disappearing, or is extinction an inevitable part of nature, in which the strong survive and the weak perish? These are some of the difficult questions that people face as they ponder the fate of other species living on this planet.

DEFINING AND NAMING LIFE ON EARTH

Living organisms are named and categorized according to a taxonomy, a hierarchical system of order based on the natural relationships among all types of life. For example, Table 1.1 shows the taxonomic chart for blue whales, the largest creatures on Earth. Blue whales are described by eight taxonomic levels ending with "species." A species is a term assigned to a group of organisms that are considered capable of interbreeding with one another. There is another category called "subspecies," that ranks immediately below species. A subspecies (abbreviated ssp.) is a population of a particular geographical region that is genetically different from other populations of the same species, but can still interbreed with them.

Animals and plants are identified by their common names and by unique scientific names. Some organisms have more than one common name. The animal known as the mountain lion is also called a puma or a cougar. To avoid confusion, scientific bodies have established systems of nomenclature (naming) for animals and plants. These systems are based on the example set by Carolus Linnaeus (1707–78), a Swiss botanist who published classifications for thousands of plants and animals. Linnaeus popularized the use of a binary naming system in Which the first word names the genus of the organism. The genus, which identifies a group of closely related species, is followed by a specific epithet (a descriptive word or phrase) that differentiates one species from another. Linnaeus used Latin words in his nomenclature because Latin was the preferred language for scientific publications during the eighteenth century.

Since the time of Linnaeus many thousands of additional plants and animals have been discovered or intentionally bred. Modern convention dictates that the scientist who first describes an organism in a scholarly publication chooses the scientific name for that organism. The scientific name must be in Latin or contain words that have been "Latinized" (rendered to appear Latin). During the 1800s the American researcher Frank Higgins discovered a new species of mussel (clam) in the Mississippi River. He called it Higgins' Eye. The scientific name is Lampsilis higginsii. In this case the specific epithet reflects the common name of the organism, but it is not required to do; it can be any descriptive term.

Scientific names are either italicized or underlined in print to distinguish them from surrounding text. The genus is capitalized, while the specific epithet is not capitalized.

Subspecies are indicated in scientific nomenclature with an additional term. For example, the scientific name of the blue whale is Balaenoptera musculus. A subspecies, the pygmy blue whale, is called Balaenoptera musculus brevicauda. Genus reassignments are indicated with the "=" sign in a scientific name. When the royal snail was discovered in 1977 it was assigned to the genus Marstonia. Ten years later biologists decided the snail was more properly a member of the genus Pyrgulopsis. Thus, the scientific name of the royal snail is written as Pyrgulopsis (=Marstonia) ogmorhaphe. When the species is not known for an organism of known genus, the scientific name is written with sp. (indicating a single species) or spp. (indicating multiple species) as the specific epithet. The latter format is also used when referring to all species in a genus. For example, Pyrgulopsis spp., refers to all species within the genus Pyrgulopsis.

The rules governing scientific names for animals are overseen by the International Commission on Zoological Nomenclature, headquartered in London, England. The International Code of Botanical Nomenclature for plants is set by the International Botanical Congress (IBC), a meeting of botanists from around the world held every six years. The most recent IBC took place in Vienna, Austria, in 2005.

BIODIVERSITY

Biodiversity is short for biological diversity. It refers to the richness and variety of living organisms across the planet. Biodiversity is important at levels within the taxonomic table and at the genetic level. For example, all humans are members of one species—Homo sapiens—but humans can vary widely in their personal characteristics, such as race, hair color, and eye color. These differences are due to slight variations in genetic material from person to person. Genetic biodiversity results in different individual properties within a species. It also helps ensure that deformities or disorders in genetic material do not become concentrated in a population.

Inbreeding is mating between closely related individuals with extremely similar genetic material. It is almost certain that if one of these individuals has any kind of gene disorder, the other individual will also have it. This disorder might not cause any notable problems in the parents, but could become concentrated in the offspring and cause serious health problems for them. This explains why there is a certain lower limit to the population of some species, particularly those that are isolated in a specific location. If the population falls too low, the remaining individuals will be so closely related that any inherent gene problems can kill off the resulting offspring and ultimately wipe out the entire species.

WHAT ARE ENDANGERED SPECIES?

A species is described as extinct when no living members remain. Scientists know from the study of fossils that dinosaurs, mammoths, saber-toothed cats, and countless other animal and plant species that once lived on Earth no longer exist. These species have "died out", or become extinct. Once a species is extinct, there is no way to bring it back.

The U.S. government defines endangered species as those that are at risk of extinction through all or a significant portion of their natural habitats. Threatened species are defined as those likely to become endangered in the foreseeable future. The management at the federal level of endangered and threatened species is handled by two agencies, the U.S. Fish and Wildlife Service (FWS) and the National Marine Fisheries Service (NMFS). The FWS is an agency of the Department of the Interior and oversees the terrestrial (land-based) and freshwater species. The NMFS is an agency of the National Oceanic and Atmospheric Administration (NOAA) under the U.S. Department of Commerce. The NMFS is responsible for marine (ocean-dwelling) species and those that are anadromous (migrate between the ocean and freshwater).

The U.S. Fish and Wildlife Service maintains a list of species that are endangered or threatened in the United States and abroad. Both endangered and threatened species are protected by laws intended to save them from extinction. In many cases, recovery plans for endangered species have also been developed and implemented. These include measures designed to protect endangered and threatened species and to help their populations grow.

MASS EXTINCTION

In the billions of years since life began on Earth, species have formed, existed, and then become extinct. Scientists call the natural extinction of a few species per million years a background, or normal, rate. When the extinction rate doubles for many different groups of plants and animals at the same time, this is described as a mass extinction. Mass extinctions have occurred infrequently in Earth's history and, in general, have been attributed to major cataclysmic geological or astronomical events. Five mass extinctions have occurred in the last 600 million years. These episodes, known as the Big Five, occurred at the end of five geologic periods:

• Ordovician (505–440 million years ago)
• Devonian (410–360 million years ago)
• Permian (286–245 million years ago)
• Triassic (245–208 million years ago)
• Cretaceous (146–65 million years ago)

After each mass extinction the floral (plant) and faunal (animal) composition of the Earth changed drastically. The largest mass extinction on record occurred at the end of the Permian, when an estimated 90% to 95% of all species became extinct. The Cretaceous extinction is perhaps the most familiar—it was at the end of the Cretaceous that many species of dinosaurs became extinct. The Cretaceous extinction is hypothesized to have resulted from the collision of an asteroid with the Earth.

The Sixth Mass Extinction?

Scientists estimate that hundreds, or even thousands, of species are being lost around the world each year. This suggests that we are currently in the midst of another mass extinction. Unlike previous mass extinctions, however, the current extinction does not appear to be associated with a cataclysmic physical event. Rather, the heightened extinction rate has coincided with the success and spread of human beings. Researchers predict that as humans continue to alter natural ecosystems through destruction of natural habitats, pollution, introduction of nonnative species, and global climate change, the extinction rate may eventually approach several hundred species per day. This would be a rate millions of times higher than normal background levels. The United Nations, in Global Biodiversity Outlook 2 (March 2006), concluded that without immediate intervention, more species of flora and fauna may disappear than were lost in the mass extinction that wiped out the dinosaurs sixty-five million years ago.

In 1948 an international conference on conservation resulted in formation of the International Union for the Protection of Nature. In 1956 the name was changed to the International Union for the Conservation of Nature and Natural Resources (IUCN; now the World Conservation Union). The IUCN is based in Gland, Switzerland, and is the world's largest conservation organization. As of 2006 more than eighty nations and 800 nongovernmental organizations are members. The IUCN reports that in the last 500 years, at least 816 species are known to have become extinct as a result of human activity. The actual number is probably much higher.

U.S. HISTORY—SOME EXTINCTIONS AND SOME CLOSE CALLS

Colonization of the New World by European settlers severely depleted the ranks of some native wild species. The introduction of livestock brought new animal diseases that devastated some native animals. Widespread hunting and trapping led to the demise of other species. During the early 1800s the United States was home to millions, perhaps billions, of passenger pigeons. These migratory birds traveled in enormous flocks and were extremely popular with hunters. By the beginning of the twentieth century the species was virtually exterminated. The last known passenger pigeon died in the Cincinnati Zoo in 1914. The heath hen, a small wild fowl native to the United States and once very abundant, was wiped out of existence by 1932. Stocks of other animals—beaver, elk, and bison (American buffalo)—were driven to the brink of extinction, but saved by conservation efforts.

Bison Comeback

The bison is the largest terrestrial animal in North America. It has short, pointed horns and a hump over the front shoulders. The head, neck, and front parts of the body are covered by a thick, dark coat of long, curly hair; the rear has shorter, lighter hair. Adult males weigh as much as 1,800 to 2,400 pounds; females are smaller. Adult males also have black "beards" about a foot long. Bison are social animals and travel in herds. Considering their size and weight, bison are remarkably light on their hooves—unlike cattle, they love to run and are surprisingly fast. Bison were central to the existence of Plains Native Americans, who used them for food and made clothing from their hides and tools from their bones. The dried dung, called buffalo chips, was used for fuel.

Sixty million bison—or buffalo—once roamed the grasslands of America. (See Figure 1.1.) Historical accounts describe herds stretching as far as the eye could see. Although Native Americans hunted bison, it was not until European settlers came with firearms that their numbers fell drastically. Many people shot the animals for fun, while others sold the hides. Bison numbers were eventually reduced to fewer than one thousand.

Bison first received protection from the U.S. government in 1872, with the establishment of Yellowstone National Park in Wyoming and Montana. However, the welfare of the small herd of bison in the park was largely ignored until 1901, when it was discovered that only twenty-five individuals remained. The herd was restored to 1,000 by 1930 with bison imported from the Great Plains. As the Yellowstone herd multiplied, the park service shot animals to keep the population under control. This practice was unnecessary, however, because harsh winters caused the herd to dwindle naturally. The park service stopped shooting bison in the 1960s, and by 1994 the population of the Yellowstone herd had reached a peak of 4,200 animals. According to the National Park Service there are more than 150,000 bison on public and private lands across the United States, including approximately 4,000 at Yellowstone National Park.

Today some populations of bison are managed as livestock because they have become a food source for humans. Bison are a source of high-protein, low-fat, low-cholesterol meat. According to the National Bison Association, more than 35,000 bison were slaughtered for food during 2005.

HOW MANY SPECIES ARE ENDANGERED?

Since 1960 the International Union for the Conservation of Nature and Natural Resources has compiled the IUCN Red List of Threatened Species, which aims to examine the status of biological species across the globe. The so-called "Red List" categorizes species based on the level of risk of their extinction in the wild, as follows:

• Critically endangered—extremely high risk
• Endangered—very high risk
• Vulnerable—high risk
• Near threatened—likely to qualify for a risk category in the near future

The IUCN refers to species in all of these categories as "threatened" species.

Determining how many species of plants and animals are threatened or endangered is difficult. In fact, only a small fraction of the species in existence have even been identified and named, let alone studied in detail. Various estimates of the total number of threatened/endangered species on Earth range from five million to 100 million with most estimates figuring around ten million species worldwide. Of these, about 1.5 million species have been named and described. Mammals, which are probably the best-studied group—and the one that includes humans—make up less than 1% of all known organisms. Insects are a particularly rich biological group—over 900,000 insect species have been identified, with countless more to be described.

Worldwide, in the IUCN's 2004 Red List, a total of 15,503 species were listed as threatened. Just over 38,000 species were examined out of the 1.5 million species that the IUCN considers "described species." Thus, only 2.5% of all known species were evaluated by the IUCN.

The listed species were as follows:

• 1,101 of 4,853 mammal species evaluated (23%)
• 1,213 of 9,917 bird species evaluated (12%)
• 304 of 499 reptile species evaluated (61%)
• 1,770 of 5,743 amphibian species evaluated (31%)
• 800 of 1,721 fish species evaluated (46%)
• 559 of 771 insect species evaluated (73%)
• 974 of 2,163 mollusk species evaluated (45%)
• 429 of 498 crustacean species evaluated (86%)
• 30 of 55 other species evaluated (55%)

In addition, the IUCN listed 8,321 plant species as threatened in 2004 out of 11,824 species evaluated. More than 280,000 plant species are known to the IUCN.

According to the IUCN nearly all described bird, amphibian, and mammal species were evaluated during 2004. The other species have not yet been thoroughly assessed. Further study will likely result in many more species being added to the Red List.

Table 1.2 lists the number of species identified as threatened or endangered under the Endangered Species Act (ESA) as of February 1, 2006. Of the 1,090 animal species listed, 527 are found in the United States. Among these, 398 are endangered and 129 are threatened. Among animals, the greatest numbers of listed species occur among fish, birds, and mammals. Of the 748 plant species listed, 745 are found in the United States. Among these, 599 are endangered, and 146 are threatened. Nearly all of the endangered plants are flowering plants.

Figure 1.2 shows the number of listed U.S. species per calendar year from 1980 through 2005. Table 1.3 shows the breakdown by species group and year. The number of listed species increased dramatically between 1980 and the mid-1990s. The plateau in listings during the late 1990s and early 2000s reflects, in part, budgetary constraints on listing activity at the U.S. Fish and Wildlife Service.

Within the United States, endangered and threatened species are not evenly distributed but are clustered in specific geographical areas. Figure 1.3 shows the number of federally listed endangered and threatened species in each state on February 1, 2006. Regions where the number of listed species is particularly high include southern Appalachia, Florida, the Southwest, California, and Hawaii. Hawaii harbors more threatened and endangered species than any other state, despite its small size. This is due largely to the fact that a significant proportion of Hawaiian plant and animal life is endemic—that is, found nowhere else on Earth. Endemism is very dangerous for imperiled species for a variety of reasons. A single calamitous event, such as a hurricane, earthquake, or disease epidemic, could wipe out the entire population at one time. The likelihood of interbreeding and resulting genetic problems is also higher for a species that is so geographically limited

SPECIES LOSS—CRISIS OR FALSE ALARM?

Environmental issues, which have a tendency to pit conservation against business or economic development, are often hotly debated. With respect to current threats to biodiversity, some critics argue that the scale of loss is not as great as we imagine. They point to uncertainty regarding the total number of species, as well as the geographic distributions of species. Other challengers claim that loss of habitat and disruption by human activity are not powerful enough to cause the massive extinction being documented. Still other challengers contend that extinction is inevitable, and that the Earth has experienced, and recovered from, mass extinctions before. They conclude that the current biodiversity loss, while huge, is not disastrous.

In addition, opponents of conservation frequently argue that "green" policies such as the U.S. Endangered Species Act place the needs of wildlife before those of humans. This was the central issue in one of the bitterest battles over an endangered species, that concerning protection of northern spotted owl habitat. (See Figure 1.4.) In 1990 declining populations resulted in the listing of the northern spotted owl as a threatened species. In 1992 the Fish and Wildlife Service set aside seven million acres of forestland in the Pacific Northwest—both private and public—as critical habitat for the species. Logging was banned on federal lands within these areas. Loggers protested this ban, arguing that jobs would be lost. Supporters of the ban, on the other hand, claimed that the logging industry in the area was already in decline and that continued logging would preserve existing jobs only for a short time. Eventually, a compromise was reached in which logging was limited to trees under a certain size, leaving the mature growth for owl habitat. By early 1993 almost all old-growth logging on federal lands had been stopped by court action.

In 1994 a group of federal agencies adopted the Northwest Forest Plan for management of old-growth forests in the Pacific Northwest. The plan has three goals:

• Manage federal forests so that sustainable timber production and biological diversity are achieved
• Coordinate actions by various federal agencies involved in forest management and ensure that they receive input from nonfederal parties
• Provide economic assistance and job retraining for displaced timber workers and other parties adversely affected by reduced timber harvesting

The compromise worked out in the Plan did not fully please either side in the controversy. In 2002 organizations representing the timber industry sued the U.S. Fish and Wildlife Service claiming that northern spotted owl populations had recovered enough to remove the bird from the list of threatened species. The FWS conducted a status review and concluded in 2004 that the threatened listing should remain in place. The agency noted that the Northwest Forest Plan has successfully minimized habitat loss on federal lands. However, populations of northern spotted owls in Washington, Oregon, and California have continued to decline due to a combination of threats, including forest fires, bird and tree diseases, and competition for habitat from barred owls.

WHY SAVE ENDANGERED SPECIES?

Proponents of conservation believe that saving species from extinction is important for many reasons. Species have both aesthetic and recreational value, as the tremendous popularity of zoos, wildlife safaris, recreational hiking, and wildlife watching (bird watching, whale watching, etc.) indicate. Wildlife also has educational and scientific value. In addition, because all species depend on other species for resources, the impact of a single lost species is difficult to predict and could potentially be immense. Scientists have shown that habitats with greater biodiversity are more stable—that is, they are better able to adjust to and recover from disturbances. This is because different species may perform overlapping functions in a biologically diverse ecosystem. Habitats with less diversity are more vulnerable, because a disturbance affecting one species may cause the entire network of interactions to collapse. Furthermore, many species have great economic value to human beings. Plants provide the genetic diversity used to breed new strains of agricultural crops, and many have been used to develop pharmaceutical products. Aside from the economic or utilitarian reasons for preserving species, however, many people think that humankind has a moral responsibility to maintain the Earth's biodiversity. When species are lost, the quality of all life is diminished.

The Gallup Organization conducts an annual poll on environmental issues. In 2004 Gallup pollsters asked participants to express their level of worry about various environmental problems. As shown in Figure 1.5, more than 60% of those asked expressed a "great deal" or "fair amount" of concern regarding the extinction of plant and animal species. This placed extinction sixth in the listing of environmental problems in terms of amount of worry. Extinction was considered less pressing a problem than water and air pollution, but more worrisome than damage to the Earth's ozone layer, global warming, or acid rain. Sixty-one percent of those surveyed were also concerned about the loss of tropical rain forests—home to many of the world's imperiled species.

Are Some Species More Important than Others?

In general, the public places high value on some endangered species and not others. For example, whales and seals are popular animals for which protection measures receive widespread support. On the other hand, there are several species listed under the Endangered Species Act that are considered pests or predators, because they pose a threat to human livelihoods or safety. Utah prairie dogs are burrowing animals that produce networks of underground tunnels. The resulting holes and dirt mounds can ruin cropland and trip and injure livestock. The protection of Utah prairie dogs and other imperiled rodents is a source of contention for people who believe that the Endangered Species Act puts animal interests above human interests. The same debate rages over such predators as wolves and mountain lions that may prey upon livestock, pets, and even people.

From a scientific standpoint some species are more valued because they are the last remnants of biological groups that once flourished. Examples of these include the coelacanth, one of the few species (along with lung-fish) that help to document the transition from aquatic to terrestrial life in vertebrates, and the tuatara, a highly endangered reptile found only in New Zealand. The extinction of species that have no closely related species left on Earth represent particularly significant losses to the genetic diversity of the planet.

Biological Indicator Species

The rapid rate of species loss should also concern human beings because many are dying out due to pollution and environmental degradation, problems that affect human health and well-being as well. Species that are particularly useful in reporting on the health of ecosystems are called biological indicator species. Environmental scientists rely on sensitive indicator species just as coal miners once relied on canaries to check air safety in underground tunnels, where dangerous gases frequently became concentrated enough to be poisonous. Miners carried a canary into the mineshaft, knowing that the air was safe to breathe as long as the canary lived. If the bird started to sicken, however, miners evacuated the tunnel. In the same way, the sudden deaths of large numbers of bald eagles and peregrine falcons warned people about the dangers of DDT, a powerful pesticide in wide use at the time. The disappearance of fish from various rivers, lakes, and seas also alerted people to the presence of dangerous chemicals in waters.

During the final decades of the twentieth century, many scientists became concerned about the sudden disappearance of many amphibians, particularly frogs, all over the world. Most troubling was the fact that many species disappeared from protected parks and wildlife refuges, areas that appeared relatively pristine and undisturbed. Amphibians are believed to be particularly sensitive to environmental disturbances such as pollution because their skins readily absorb substances from the environment. Their decline is a suggestion that all may not be well.

FACTORS THAT CONTRIBUTE TO SPECIES ENDANGERMENT

Experts believe that the increasing loss and decline of species cannot be attributed to natural processes, but results instead from the destructive effect of human activities. People hunt and collect wildlife. They destroy natural habitats by clearing trees and filling swamps for development. Aquatic habitats are altered or destroyed by the building of dams. Humans also poison habitats with polluting chemicals and industrial waste. Indeed, human activity may be causing changes in climate patterns on a global scale.

With each passing day, humans require more space and resources. According to the U.S. Census Bureau in Global Population at a Glance: 2002 and Beyond (March 2004), the human population totaled about one billion at the beginning of the nineteenth century. It surpassed the two billion mark in 1922 and began a period of rapid increase that saw the world population triple to six billion by the turn of the twenty-first century. According to the Census Bureau, the world population as of May 2006 was more than 6.5 billion people, and the population of the United States was expected to reach 300 million during October 2006. The large numbers of human beings puts tremendous pressure on other species.

Habitat Destruction

Habitat destruction is probably the single most important factor leading to the endangerment of species. It plays a role in the decline of nearly all listed species and has had an impact on nearly every type of habitat and ecosystem.

Many types of human activity result in habitat destruction. Agriculture is a leading cause, with nearly half of the total land area in the United States used for farming. Besides causing the direct replacement of natural habitat with fields, agricultural activity also results in soil erosion, pollution from pesticides and fertilizers, and runoff into aquatic habitats. Agriculture has compromised forest, prairie, and wetland habitats in particular. Nearly 90% of wetland losses have resulted from drainage for agriculture. According to a study by Brian Czech, Paul R. Krausman, and Patrick K. Devers ("Economic Associations among Causes of Species Endangerment in the United States," Bioscience, vol. 50, no. 7, 2000), the role of agriculture in the endangerment of species is greatest in the Southeast and California. However, agriculture impacts threatened and endangered species throughout the country, contributing to endangerment in thirty-five states.

Urban expansion has destroyed wild habitat areas as well, and is a primary factor in the endangerment of many plant species. As with agriculture, urbanization leads to the direct replacement of natural habitat. It also results in the depletion of local resources, such as water, which are important to many species. According to Czech and his colleagues, urbanization contributes to the endangerment of species in thirty-one states. The greatest impact is in California, Florida, and Texas, the three states that are urbanizing most rapidly. In contrast, only two species are endangered by urbanization in Utah, Nevada, and Idaho. The authors argue that this is because a large proportion of land in these states is public land and therefore not available for private development.

Logging, particularly the practice of clear-cutting forests (removal of all trees in a designated area), destroys important habitat for numerous species. Clear-cutting or extensive logging can also lead to significant erosion, harming both soils and aquatic habitats, which become blocked with soil.

Numerous other forms of human activity result in habitat destruction and degradation. Grazing by domestic livestock has a direct impact on numerous plant species, as well as animal species that compete with livestock. Mining destroys vegetation and soil, and also degrades habitat through pollution. Dams destroy aquatic habitats in rivers and streams. Finally, human recreational activity, particularly the use of off-road vehicles, results in the destruction of natural habitat. Czech, Krausman, and Devers reported that recreational activity has a particularly detrimental effect on species in California, Hawaii, Florida, as well as species in the Mojave Desert, which includes portions of Arizona, California, Nevada, and Utah.

Habitat Fragmentation

Human land-use patterns often result in the fragmentation of natural habitat areas that are available to species. Studies have shown that habitat fragmentation is occurring in most habitat types. Habitat fragmentation can have significant effects on species. Small populations can become isolated, so that dispersal from one habitat patch to another is impossible. Smaller populations are also more likely to become extinct. Finally, because there are more "edges" when habitats are fragmented, there can be increased exposure to predators and increased vulnerability to disturbances associated with human activity.

Global Warming

Global warming is a phenomenon associated with the enhanced greenhouse effect. Gases such as carbon dioxide and methane in the atmosphere absorb and maintain heat in the same way that glass traps heat in a greenhouse. This natural greenhouse effect keeps Earth warm and habitable for life. (See Figure 1.6.)

An enhanced greenhouse effect refers to the possible increase in the temperature of Earth's surface due to the release of excessive amounts of greenhouse gases from the burning of fossil fuels. Figure 1.7 shows that the global average of carbon dioxide in the atmosphere has increased dramatically since the early 1980s. A global temperature increase has also been compellingly documented, as shown in Figure 1.8, and has already had important effects on ecosystems worldwide.

According to ecologist Chris D. Thomas in "Extinction Risk from Climate Change," (Nature, no. 427, January 2004), a study of habitats comprising 20% of the Earth's surface suggested that 15% to 37% of the world's species may be extinct by 2050 if recent warming trends continue. Summarizing his findings, Thomas said, "The midrange estimate is that 24% percent of plants and animals will be committed to extinction by 2050. We're not talking about the occasional extinc-tion—we're talking about 1.25 million species. It's a massive number."

Continued warming of the Earth would alter habitats drastically, with serious consequences for numerous species. In places like Siberia and the northernmost regions of Canada, habitats such as tundra—permanently frozen land supporting only low-growing plant life such as mosses and lichens—and taiga—expanses of evergreen forests located immediately south of the tundra—are shrinking. Deserts are expanding. Forests and grasslands are beginning to shift towards more appropriate climate regimes. Animal and plant species that cannot shift their ranges quickly enough, or have no habitat to shift into, are dying out. Some plants and animals that are found in precise, narrow bands of temperature and humidity, such as monarch butterflies or edelweiss, are likely to find their habitats wiped out entirely. Global warming is already endangering some of the most diverse ecosystems on Earth, such as coral reefs and tropical cloud forests. The impact on endangered species, which are already in a fragile state, may be particularly great.

Pollution

Pollution is caused by the release of industrial and chemical wastes into the land, air, and water. It can damage habitats and kill or sicken animals and plants. Pollution comes from a wide variety of sources, including industrial operations, mining, automobiles, and agricultural products such as pesticides and fertilizers. Even animals that are not directly exposed to pollution can be affected, if other species they rely on die out. According to Czech and his colleagues, pollution currently affects a large number of species in the Southeast, particularly aquatic species such as fish or mussels.

Hunting and Trade

Humans have hunted numerous animal species to extinction, and hunting continues to be a major threat to some species. In the United States, gray wolves were nearly wiped out because they were considered a threat to livestock. The Caribbean monk seal was viewed as a competitor for fish, and exterminated. Other animals are hunted for the value of their hides, tusks, or horns, including elephants and rhinoceroses. Many exotic species, such as parrots and other tropical birds, are taken from their natural habitats for the pet trade.

Invasive Species

Invasive species are those that have been introduced from their native habitat into a new, nonnative habitat, and which cause environmental harm. Most introductions of invasive species are accidental, resulting from "stowaways" on ships and planes. Invasive species harm native life forms by competing with them for food and other resources, or by preying on them or parasitizing them. As of 2005 approximately 50,000 species were believed to have been introduced into the United States alone. While there are sometimes beneficial effects from introducing nonnative species, most of the effects are harmful.

The introduction of invasive species can lead to genetic swamping. This is a condition that arises when large numbers of one species breed with a much smaller population of another related species. The genetic material of the invasive species becomes overwhelming, causing the resulting generations to lose many of the characteristics that made the smaller population a unique species in the first place.

Many species in peril are endangered partly or entirely because of invasive species. Similarly, the IUCN Red List suggested in 2000 that invasive species affect hundreds of species of threatened birds and plants. In fact, the International Union for the Conservation of Nature and Natural Resources found that the majority of bird extinctions since 1800 have been due to invasive species such as rats and snakes. In 2003 the IUCN reported that the unique flora and fauna of islands such as the Galapagos Islands, Hawaii, the Seychelles, the Falkland Islands, and the British Virgin Islands, have been devastated by invasive species. Human commensals—species that are used by and associated with humans—can be among the most destructive introduced species. In Hawaii, for example, grazing by feral pigs, goats, cattle, and sheep is responsible for the endanger-ment of numerous plants and birds.

The introduction of invasive species by humans has also taken a toll on mammalian wildlife. Australia is overrun with domestic cats whose ancestors were brought by settlers to the island continent 200 years ago. Stray domestic cats have driven indigenous species such as bandicoots, bettongs, numbats, wallabies, and dozens of other bird and mammal species, most of which are found nowhere else on Earth, towards extinction. Richard Evans, a member of the Australian Parliament, claims the feral cats are responsible for the extinction of at least thirty-nine species in Australia. He has called for total eradication of cats from the island by 2020, to be achieved by neutering pets and spreading feline diseases in the wild. The Australian National Parks and Wildlife Service reports that each house cat kills twenty-five native animals each year on average, and feral domestic cats kill as many as 1,000 per year.

Recognizing the threat posed by invasive species, President Bill Clinton signed Executive Order 13112 on Invasive Species in 1999. This order requires federal agencies to make every possible effort to control the spread of invasive species, and resulted in the formation of the Invasive Species Council, which drafted the first National Invasive Species Management Plan in January 2001. The plan emphasizes prevention of introduction of alien species, early detection of invasions, rapid response to them, and coordination of national and international efforts in management and control of these species.

In "Update on the Environmental and Economic Costs Associated with Alien-Invasive Species in the United States" (Ecological Economics, vol. 52, no. 3, February 15, 2005), researchers at Cornell University, including David Pimentel, reported that invasive species cause environmental damages and losses that cost the country nearly $120 billion per year. Invasive species are blamed, in part, for imperiling approximately 42% of the species on the list of threatened and endangered species in the United States.