Researchers devise new comparison of mass extinction events
mongabay.com
September 2, 2008




Researchers have created a new way to compare historical mass extinction events.

The scoring system, presented in the early online edition of the journal Proceedings of the National Academy of Sciences, works by multiplying the number of taxa — species, genera, and families — that went extinct by the inverse of the time it took to produce a measure dubbed "greatness", which represents the magnitude of the event.

Using the system, Celâl Sengör and colleagues from Istanbul Teknik Üniversitesi of Turkey rank the Cretaceous-Tertiary (K-T) event — when the dinosaurs were extinguished in a flash by an asteroid strike or mass oceanic poisoning by a spasm of volcanic activity — as the greatest mass extinction in history. The Permian, which featured a greater loss of species (90-95 percent of marine species and 70 percent of land species) but played out over a longer period to time some 250 million years ago, ranks third after the Silurian, which occurred around 440 million years ago. The Permian, Pennslyvanian, and Ordovician lag behind on their scale.

The "greatness" of an extinction is the product of its magnitude (i.e. biodiversity loss during the event) times the its intensity (the duration of the extinction event). Thus the red column is what researchers consider the most complete measure of extinction events.

The authors then attempt use the methodology to compare past mass extinctions to the current extinction event that scientists say is being driven by human activities, including habitat destruction, pollution, over-exploitation, introduction of alien species, and global warming. In the end they conclude that the present-day mass extinction — dubbed the Holocene extinction event — bears little resemblance toward any individual historical event, but shares characteristics of several and could someday prove to be the greatest event the planet has ever seen.

"The present extinction, which seems greater than any thus far, has elements of those of both the Cretaceous and Permian," they write. "It represents a virtual Pangea formation by providing human-caused transfer of organisms world-wide simulating a world with no oceanic barriers, leading to increased competition without opening new niches and the introduction of a global annihilating agent, namely humans, that creates functional deserts for much of the rest of the biosphere (human dwellings) at a rate unknown in the history of the biosphere during the Phanerozoic. It is a Lyellian event accelerated and magnified to Cuvierian dimensions."

"If unchecked, the present extinction threatens to be the greatest killer of all time," the authors conclude.

A. M. Celâl Sengör, Saniye Atayman, and Sinan Özeren (2008). A scale of greatness and causal classification of mass extinctions: Implications for mechanisms. PNAS September 9, 2008  vol. 105 no. 36.






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mongabay.com (September 02, 2008).

Researchers devise new comparison of mass extinction events.

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