An "endangered species" under United States law (the Endangered Species Act [1973]) is a creature "in danger of extinction throughout all or a significant portion of its range." A "threatened" species is one that is likely to become endangered in the foreseeable future.

For most people, the endangered species problem involves the plight of such well-known animals as eagles, tigers, whales, chimpanzees, elephants, wolves, and whooping cranes. However, literally millions of lesser-known or unknown species are endangered or becoming so, and the loss of these life forms could have even more profound effects on humans than that of large mammals with whom we more readily identify and sympathize.

Most experts on species extinction, such as Edward O. Wilson of Harvard and Norman Myers, estimate current and projected annual extinctions at anywhere from 15,000 to 50,000 species, or 50 to 150 per day, mainly invertebrates such as insects in tropical rain forests...