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Extinction (the complete disappearance of a species from Earth) is an important part of the evolution of life on Earth. The current diversity of species is a product of the processes of extinction and speciation throughout the previous 3.8 billion year history of life. Raup (1991) assumed that there might be 40 million species alive today, but between 5 and 50 billion species have lived at some time during the history of the Earth. Therefore, Raup estimated that 99.9% of all the life that has existed on Earth is now extinct); a species is assumed to be extinct when there is no reasonable doubt that the last individual has died (IUCN, 2002). However, 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. The timing of these mass extinctions is shown in Figure 1.

Figure 1
Figure 1 (extinction.png)

Each of the first five mass extinctions shown in Figure 1 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 some tens of millions of years (Jablonski, 1995).

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 predated 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. These mass extinctions, and their historic and modern consequences are discussed in more detail in the modules on Historical perspectives on extinction and the current biodiversity crisis, and Ecological consequences of extinctions..

Glossary

Extinct:
a species is assumed to be extinct when there is no reasonable doubt that the last individual has died (IUCN, 2002)
Extinction:
the complete disappearance of a species from Earth
Mass extinction:
a period when there is a sudden increase in the rate of extinction, such that the rate at least doubles, and the extinctions include representatives from many different taxonomic groups of plants and animals

References

  1. Raup, D.M. (1991). Extinction: bad genes or bad luck? New York, New York, U.S.A.: W.W. Norton and Co.
  2. [IUCN] International Union for the Conservation of Nature and Natural Resources. (2002). UCN Red List of Threatened Species. [Available from: http://www.redlist.org (accessed March 11, 2003)].
  3. Jablonski, D. (1995). Extinctions in the fossil record. In J.H. Lawton and R.M. May (Eds.), Extinction rates. (pp. 25-44). Oxford, U.K.: Oxford University Press.

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Definition of a lens

Lenses

A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

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