species, in biology, a category of classification, the original and still the basic unit in the demarcation of plant and animal types. The species marks the boundary between populations of organisms rather than between individuals. Because related species are not absolutely permanent (see evolution), a precise definition of the term is difficult. On the basis of genetics, scientists now include in a species all individuals that are potentially or actually capable of interbreeding and that share the same gene pool. The latter term refers to that collection of characteristics whose combination is unique in the species, although each individual of the group may not display every single one of the characteristics (see genetics). In the few cases where members of different species can interbreed, the offspring are usually sterile (e.g., the mule). Groups distinguished by lesser differences than those marking a species are called variously subspecies, varieties, races, or tribes.

endangered species, any plant or animal species whose ability to survive and reproduce has been jeopardized by human activities. In 1999 the U.S. government, in accordance with the U.S. Endangered Species Act (1973), classified 935 native species as endangered or threatened, including animals such as the Florida panther, the Key deer, the San Joaquin kit fox, the northern spotted owl, the chinook salmon, the Karner blue butterfly, the snail darter, and the cave crayfish and plants such as the Hawaiian nehe and the clover lupine. Over 500 more species were so classified worldwide. The official list of endangered wildlife and plants in the United States is kept by the Fish and Wildlife Service; the National Marine Fisheries Service oversees marine species. In addition, many states keep their own lists. The International Union for the Conservation of Nature and Natural Resources maintains an international list, published as the Red Data Book.

Causes of Endangerment

Hunting, trapping, and poisoning to protect livestock have taken a great toll among predatory mammals and birds. Overharvesting is currently threatening species worldwide, especially food fish species such as the cod. A large number of species are threatened by introduced species, or "exotics," plants or animals that are introduced into a habitat and bring with them diseases or the ability to compete more effectively than native species. The now ubiquitous European starling, for example, purposely introduced into the United States in the 1890s, is displacing the native American bluebird and other species, and the brown tree snake, native to Australia and introduced to Guam during World War II, has preyed on native species of that island to the extent that nine bird species are now extinct. Another danger is hybridization with other species and subspecies.

Another important threat is destruction of habitat by chemical pollutants. For example, bird populations have suffered great losses because of insecticides. The chemicals they contain, such as DDT, accumulate in birds' bodies and interfere with calcium metabolism. As a result, the females lay eggs with extremely thin shells or no shells at all, so the embryos do not survive to hatching. Acid rain has destroyed the habitats of many North American fish and amphibians by lowering the pH of surface waters. It is also changing the soil chemistry and harming many tree species.

Most serious of all, the destruction of physical habitat—by the drainage and filling of swamps and marshes, by the damming of rivers, by the leveling of forests for residential and industrial development, by strip mining, and by oil spills and water pollution—has left many creatures with literally no room in which to live and breed. For example, only 5% of the original forests in the 48 coterminous states, i.e., those forests that were present at the time of the first European settlement, are still standing.

Efforts to Protect Species

Many local, national, and international organizations, such as Greenpeace, the Sierra Club, and the National Audubon Society, work to preserve habitats and heighten public awareness. Conservationists have pressed for habitat preservation through the establishment of new wildlife refuges and wilderness areas and for public and private land-use planning that would provide for development without habitat destruction. Some wildlife conservation organizations try to keep seriously endangered species viable with captive breeding programs, releasing new offspring into the species' native habitat when breeding is successful.

U.S. legislation affecting endangered species includes the various federal antipollution laws, the banning of DDT, the National Environmental Policy Act (NEPA) of 1969, and the Endangered Species Acts of 1966, 1969, 1973, 1978, 1982, and 1988. The landmark 1973 Endangered Species Act prohibits any trade in endangered species or their products and requires that federal agencies assess the impact on wildlife habitat of proposed projects—much as NEPA requires an environmental impact statement. These laws are often the only tool that conservationists have to prevent the development or other exploitation (e.g., logging or mining) of important habitats, but enforcement is also hampered by litigation and a lack of funds. Despite these problems, in the years since 1973 the status of a number of species, including the bald eagle, American alligator, and black-footed ferret, became stable or improved.

The protection of species in the United States has, however, become highly politicized. Asserting that the enforcement of environmental rules unfairly burdens business, the Republican 104th Congress prevented any further species from being added to the U.S. list of endangered and threatened wildlife and plants for 13 months from 1995 to 1996. Despite the perception that enforcement of the laws affects the economy and impedes progress, only 1% of the 50,000 projects that raised endangered-species questions between 1976 and 1986 required further investigation because of possible serious impact on a species; most of those moved forward after some modification.

On the international scene, efforts have been made to halt the trade in spotted cats and crocodiles and to curtail whaling and the taking of porpoises in tuna seines. A conference in Washington, D.C., in 1973, attended by 80 nations, drew up the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), which protects more than 600 species of animals and plants. By the early 1990s some success had been achieved in banning the trade in rhinoceros horn, elephant ivory, South American parrots, bird eggs, and rare orchids, but poaching—for the high profits that can sometimes be gained from these items—continues to be a serious threat. In addition to CITES, the 1992 United Nations Conference on Environment and Development (the "Earth Summit") produced an agreement to stem the depletion of the world's diverse species (see biological diversity). See also conservation of natural resources.

Quantity of plant and animal species found in a given environment. Sometimes habitat diversity (the variety of places where organisms live) and genetic diversity (the variety of traits expressed within a species) are also considered types of biodiversity. The estimated 3–30 million species on Earth are divided unequally among the world's habitats, with 50–90percnt of the world's species living in tropical regions. The more diverse a habitat, the better chance it has of surviving a change or threat to it, because it is more likely to be able to make a balancing adjustment. Habitats with little biodiversity (e.g., Arctic tundra) are more vulnerable to change. The 1992 Earth Summit resulted in a treaty for the preservation of biodiversity.

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Subdivision of biological classification composed of related organisms that share common characteristics and can interbreed. Organisms are grouped into species according to their outer similarities, but more important in classifying organisms that reproduce sexually is their ability to interbreed successfully. To be members of the same species, individuals must be able to mate and produce viable offspring. Because genetic variations originate in individuals which then pass on their variations only within the species, it is at the species level that evolution takes place (see speciation). The international system of binomial nomenclature assigns new species a two-part name.

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Dying out or termination of a species. It occurs when a species can no longer reproduce at replacement levels. Most past extinctions are thought to have resulted from environmental changes that the doomed species was either unable to adapt to or that caused it to adapt so thoroughly that it became a distinctly new species. The effect of humans on the environment, through hunting, collecting, and habitat destruction, has become the principal factor in plant and animal extinctions.

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Any species of plant or animal threatened with extinction. International and national agencies work to maintain lists of endangered species, to protect and preserve natural habitats, and to promote programs for recovery and reestablishment of these species. The Species Survival Commission of the International Union for Conservation of Nature and Natural Resources (IUCN) publishes information online about endangered species worldwide as the Red List of Threatened Species. Separate books for animal and plant species are also published. In the United States, the Fish and Wildlife Service is responsible for the conservation and management of fish and wildlife, including endangered species, and their habitats. Its list now consists of about 1,200 domestic species of endangered or threatened animals and plants, and some 200 recovery programs are in effect.

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