Extinction is the termination of an entire species. It occurs when a species can no longer reproduce at replacement levels. Extinction usually occurs as a result of major environmental or climactic changes. The affected species responds to the change in one of three ways: (1) by adapting and successfully continuing to prosper at current levels; (2) by not adapting and quickly perishing; or (3) by adapting in such a way as to become a distinctly different species. Another cause of extinction is the effect of humans on the environment. Habitat destruction, hunting and pollution have become significant factors in plant and animal species survival in the last few millennia. The introduction of new species may also serve as a major stress to indigenous organisms when the new organism competes for food and/or space.

Extinction is an ongoing feature of the Earth's ever-changing ecosystem. Most of the plant and animal species...