Extinction

Extinction is the death of all members of a species and thus, of the species itself. Extinction may occur as a result of environmental changes (natural or human-caused) or competition from other organisms. A species confronted by environmental change or competitors may (1) adapt behaviorally (e.g., shift to a different diet), (2) adapt by evolving new characteristics, or (3) die out. At the present time, human impact on the environment is the leading cause of extinction. Habitat destruction by development and resource extraction, hunting, pollution, and the introduction of alien species into environments formerly inaccessible to them are causing the greatest burst of extinctions in at least 65 million years.

Extinction as such is a normal event; the great majority of all plant and animal species that have ever lived are now extinct. Extinction has always occurred at a fluctuating background rate. However, the fossil record also reveals a number of exceptional mass-extinction events, each involving the simultaneous or near-simultaneous disappearance of thousands of species of animals, plants, and microorganisms: five major mass extinctions, and about 20 minor ones, have occurred over the past 540 million years. The extinctions that occurred some 225 million years ago, at the end of the Permian period (the most severe of all mass extinctions, wiping out, for example, over 90% of shallow-water marine species), and some 65 million years ago, at the end of the Cretaceous period, are particularly well known. Almost half of all known species disappeared in the Cretaceous extinction, including all remaining dinosaurs and many marine animals.