Extinction is actually a normal process in the course of evolution. To date, many species of animals have become extinct rather than the total number that exist. These species slowly disappeared because of climatic changes and the inability to adapt to such conditions as competition and predation. Since the 1600s, however, the process of extinction has accelerated rapidly through the impact of both human population growth and technological advances on natural ecosystems. Due to the rapid changing of the environment by fast growing human technology, many animals unable to adapt to these changes fast are dying a relatively fast death.

Species become extinct or endangered for a number of reasons, but the primary cause is the destruction of habitat. Drainage of wetlands, changing of shrub lands to grazing lands, removal of forests (especially in the Tropics, where the rain forests will be gone by AD 2000 if destruction continues at its present rate), urbanization and highway and dam construction have seriously reduced available and natural habitats. The commercial exploitation of animals for food and other products has caused many species to become extinct or endangered. The slaughter of great whales for oil and meat, for example, has brought them to the brink of extinction; the African rhinoceros, which is renowned for its medicinal value, is killed for its horn, is also endangered. Also, due to overhunting the great auk became extinct in the 19th century. The same reason was for the perishing of the Carolina parakeet. Predator and pest control also have undesirable effects. Excessive control of prairie dogs, for example, has nearly eliminated one of their natural predators, the black-footed ferret.

Toxic chemicals-especially chlorinated hydrocarbons such as dichloro-diphenyl-trichloroethane (DDT) and polychlorinated biphenyls (PCBs)-have become concentrated in food webs, affecting most strongly those species at the end of the chain. Both DDT and the PCBs, interfere with the calcium metabolism of birds, causing soft-shelled eggs and deformed young. PCBs also impair reproduction in some carnivorous animals. Water pollution and increased water temperatures due to the greenhouse effect have wiped out several types of fishes.