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|Risk of extinction|
All species become extinct sooner or later. The end of a species may happen for many reasons. It may be caused by habitat loss or by being overhunted, or by a major extinction event. An example of an animal that is now extinct is the Dodo, from over-hunting.
Duration of species
The average duration of species varies according to the groups studied. A study of benthic (deep water) foraminifera showed an average of 16 million years if they lived above 200 metres, but 25my if they lived below 200 metres. On the other hand, the average survival time for mammalian species over the past 20my has been 2.33my.
It is estimated that 99.9% of all species that have ever lived are now extinct. More than half of the plants and animals that have become extinct in the last 200 years have been in Australia. There the arrival of Europeans bringing (accidentally or deliberately) foreign species has damaged the native biota. Especially, the introduction of eutherian mammals has caused the extinction of many marsupial forms.
Duration of higher groups
Naturally, higher categories such as genera, families, orders, classes and phyla last longer, because they are composed of more species. As far as we know, no phylum has gone completely extinct since the Cambrian, but quite a few classes went extinct in one or other of the 'big five' extinction events. Examples of higher categories of animal now extinct are: placoderm fish, trilobites, ammonites, pelycosaurs, dinosaurs, pterosaurs, ichthyosaurs, plesiosaurs...
Darwin thought that most extinctions occurred because some organisms became more effective, and replaced their less advanced competitors. No doubt that does occur, but the view now is that environmental change is more important. Organisms become adapted to the environments they live in. The environment certainly does change, and sometimes quite dramatically. When it does, some organisms can adapt to the changes, and some cannot. The ones that cannot become extinct.
The Earth's environment can stay rather stable for long periods of time. Then the rate of extinction is usually quite low. Then changes occur, sometimes quite rapidly. In these times the rate of extinction can be much higher. The rate of extinction is therefore a useful thing to measure.
- When talking about volcanos, "extinct" means that the volcano is no longer active. When talking about languages, "extinct" mans that it is no longer in use.
- Bozas M.A. & Culver S.J. 1984. Species duration and evolution: Benthic forms on the Atlantic continental margin of North America. Science 225, 829–230.
- K.J. Willis, E.S. Vrba and D. DeGusta 2004. Do species populations really start small? New perspectives from the Late Neogene fossil record of African mammals. Phil Trans Roy Soc B359, 285–293. 
- Vrba E.S. 2000. Major features of Neogene mammalian evolution in Africa. In T.C. Partridge & R. Maud (eds) Cenozoic geology of southern Africa. Oxford. pp 277–304
- Newman, Mark. 1994. "A mathematical model for mass extinction". Cornell University. Retrieved July 30, 2006.
- Raup, David M. 1991. Extinction: bad genes or bad luck? W.W. Norton, New York.
- Lawton J. and May R. 1995. Extinction rates. Oxford University Press. 
- Peripatus.gen.nz