The United States Endangered Species Act of 1973 defines an endangered species as any plant or animal "which is in danger of extinction throughout all or a significant portion of its range." An extinct species is one in which living individuals of its kind no longer exist. The act identifies a threatened species as one "likely to become endangered within the foreseeable future throughout all or a significant portion of its range." A rare species has no legal definition but refers to any kind of plant or animal that occurs in low numbers in its natural range. About 1000 species in the world are now recognized as endangered or threatened with extinction. Plants and animals have become extinct and new species have evolved since life began.

Primitive human cultures may have eliminated some species, but the primary causes for species to become extinct have been natural ones. Major environmental changes resulted in the eventual disappearance of species unable to adapt to new conditions. Well-known natural extinctions include dinosaurs and many other species represented in the fossil record.

Natural forces are still at work, but human activities cause most of the rapid and widespread environmental changes that affect plants and animals today. Many species have been unable to make the biological adjustments necessary for survival. Thus more species than ever before are threatened with extinction.

Destruction of forests, draining of wetlands, and pollution are environmental changes that may eliminate species in an area. Some herbicides and pesticides can have severe effects on certain species. Many species have small geographic ranges, so habitat alteration may eliminate them entirely. The logging of tropical forests, with their tremendous diversity of species having specialized requirements, has caused a steady increase in the extinction rate. Excessive hunting and trapping for commercial purposes also cause major problems. Elephants have been reduced to critically low numbers because of uncontrolled killing for their tusks, used to make ivory earrings and other art objects. Plants also can be reduced to near extinction levels by extensive collecting. Many cactus species of the arid southwestern United States are now legally protected by state laws to prevent their removal.

The planned or accidental introduction of exotic species to a region can also lead to extinction. An introduced species often has no natural enemies to control its spread, and native species may have no natural protection against it. The introduction of Dutch elm disease to North America, mongooses to Jamaica, and pigs to Hawaii resulted in the loss of native species having inadequate defenses.

Only since the 19th century has there been worldwide concern about the plight of species in their natural environments. In earlier times, when human population sizes were small and modern technology was developing, the impact of human activities on natural populations seldom seemed significant. Protection of animal species on an international scale was initiated as early as 1916 with the Migratory Bird Treaty between the United States, Great Britain, Canada, and, later, Mexico. A far-reaching wildlife conservation measure came from a United States-hosted conference in 1973 that resulted in an international treaty known as CITES the Convention on International Trade in Endangered Species of Wild Fauna and Flora. This program involves more than 100 nations working together to protect endangered species through worldwide control of exports and imports. The United States Fish and Wildlife Service (FWS) of the Department of the Interior is authorized to assist in the development and management of endangered species programs in foreign countries.

An additional measure of concern for vanishing species is the upsurge in private organizations involved in educating the public about environmental issues. Many of these give major emphasis to species preservation through national and international magazines, lecture series, and television programs.

Species are officially added to the endangered list in the United States through an established administrative process. A proposed listing of a species is published in the 'Federal Register'. Scientists, conservationists, and government officials are asked to provide information about the biological status of the species. The FWS accumulates the data and makes a decision about the species within the guidelines specified in the Endangered Species Act. A species may be listed as endangered or threatened, removed from consideration, or requiring more information. Once a species has been listed, the FWS assesses what must be done to ensure protection.

Major responsibilities of the FWS include programs to protect the remaining members of the species and to develop a recovery program designed to return the species to a point at which it can function in a natural manner. Recovery programs may include direct involvement of biologists in research, management, and habitat manipulation. Programs for raising some species in captivity have been carried out in attempts to restore former population levels.

Birds provide several modern examples of how extinction can occur. One of the best known is the passenger pigeon, a species said to have occurred in greater numbers than any other bird or mammal for which there are records. Passenger pigeons looked very similar to mourning doves, a close relative that is still common. One distinction a requirement for nesting in colonies ultimately led to the downfall of the passenger pigeon. The birds, concentrated at nesting sites, were slaughtered for food by the millions. The extinction of the passenger pigeon is a commentary on the erroneous belief that if a species occurs in large numbers it is not necessary to be concerned about its welfare. Although John James Audubon reported seeing more than 1 billion of these birds in Kentucky in 1813, the last member of the species died in 1914.

In 1918 the last Carolina parakeet died. This colorful green bird with an orange or yellow head was reported throughout much of the eastern United States in the 1800s. Parakeets were shot for their attractive feathers and because they damaged crops. Little in the written record, however, is available concerning their behavior, so no one knows for sure whether hunting resulted in their extinction. Many other native birds were hunted without restraint but did not become extinct.