Mass extinction
From Paleos
Mass extinction refers to the abrupt disappearance of a significant number of life forms in a relatively short time period. The surviving life forms then rapidly evolve to fill in the newly available niches. This leads to rapid speciation. Mass extinction is usually taken to be the result of a globally significant event, such as the impact of a large meteorite; the onset of an ice age; or a massive volcanic explosion. Classic examples in geologic history include the Permian - Triassic or PT-Boundary event in which some 90% of species became extinct and the Cretaceous - Tertiary or KT-Boundary event in which some 75% of species disappeared.
The current impact of humankind upon the biosphere is sometimes considered to rank alongside these examples. See: The Sixth Extinction : Patterns of Life and the Future of Humankind by Richard Leakey ( ISBN 0385468091 ).
More recent work on the topic (Bambach et al, 2004) suggests that differences in rate between extinction and origination of species may explain some events previously thought to be mass extinctions. The Ashgillian, Djhulfian, and Maastrichtian extinctions retain their characterization as mass extinctions in this perspective but the late Frasnian and Norian extinctions may now be considered as different. These stages show a dramatic decrease in origination rate below the mean while extinction rate remains constant. Other unknown processes appear to be at work supressing the rise of new species during the Frasnian and Norian stages. This process gives an appearance of extinction rate increasing when in fact it is a decrease of species originations that is responsible for the observed decline in biodiversity during these stages.
Mass extinctions are a key part of the Punctuated Equilibrium hypothesis of Stephen Jay Gould and Niles Eldredge. See: Time Frames: The Evolution of Punctuated Equilibria ( ISBN 0691024359 )
References
- Bambach, R., Knoll, A., Wang, S. 2004. Origination, extinction, and mass depletions of marine diversity. Paleobiology v30 p522-542
