Extinctions

According to a 1998 survey of 400 biologists conducted by New York's American Museum of Natural History, nearly 70 percent of biologists believe that we are currently in the early stages of a human-caused mass extinction, known as the Holocene extinction event. In that survey, the same proportion of respondents agreed with the prediction that up to 20 percent of all living species could become extinct within 30 years (by 2028). Biologist E. O. Wilson estimated  in 2002 that if current rates of human destruction of the biosphere continue, one-half of all species of life on earth will be extinct in 100 years. More significantly the rate of species extinctions at present is estimated at 100 to 1000 times "background" or average extinction rates in the evolutionary time scale of planet Earth; moreover, this current rate of extinction is thus 10 to 100 times greater than any of the prior mass extinction events in the history of the Earth.

Owing to the incompleteness of our understanding of the fossil record, mass extinctions are harder to pin down than it might seem, and the task becomes more difficult the farther one goes back in time. Very ancient rocks are poorly represented today, so we cannot say with surety than a given assemblage went extinct within a geologically short interval or not; the critical horizon may simply not be available for sampling. So it is not presently known for sure how many mass extinctions have occurred throughout the history of life on earth, and different authors offer varied interpretations.

Some of the characteristics of mass extinctions seem to be:

   1. More intense extinction of tropical forms.
   2. Extinction strikes in both the land and the sea, though higher rates are generally cited among marine forms.
   3. On the land, while animals suffer repeatedly, plants seem to be more resistant to mass extinctions.
   4. There has been a suspicion that fish might also be more than usually resistent to mass extinction events, but this idea has been rejected as an artefact of preservation (see below).

However, species mortality alone may not tell the full story. Ecology may be important too: in other words, not all taxa are equal. Mary Droser of UC Riverside and her colleagues have compared the late Devonian and end-Ordovician extinctions. The Ordovician extinction was significantly the greater in terms of number of taxa killed, but did not disrupt global ecology to the extent of the end-Devonian extinction.

An unresolved question is whether mass extinction events represent a vastly increased rate of natural selection, in which the least well-adapted organisms are killed preferentially, or whether they are the result of catastrophic change that randomly eliminates taxa regardless of adaptation.