Extinction occurs when there are no more individuals of a particular species left on Earth.
Extinction has not occurred at a constant pace through the Earth’s history. There have been at least five periods when there has been a sudden increase in the rate of extinction, such that the rate has at least doubled, and the extinctions have included representatives from many different taxonomic groups of plants and animals; these events are called mass extinctions. Earth's major extinction events are shown on the table below.
Each of the first five mass extinctions shown above represents a significant loss of biodiversity - but recovery has been good on a geologic time scale. Mass extinctions are apparently followed by a sudden burst of evolutionary diversification on the part of the remaining species, presumably because the surviving species started using habitats and resources that were previously "occupied" by more competitively successful species that went extinct. However, this does not mean that the recoveries from mass extinction have been rapid; they have usually required tens of millions of years.
It is hypothesized that we are currently on the brink of a "sixth mass extinction," but one that differs from previous events. The five other mass extinctions pre-dated humans and were probably the ultimate products of some physical process (e.g. climate change through meteor impacts), rather than the direct consequence of the action of some other species. In contrast, the sixth mass extinction is the product of human activity over the last several hundred, or even several thousand years.
Source: http://cnx.org/content/m12157/latest/ (CC BY)
Last modified: Monday, 31 October 2011, 5:11 PM