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Periodic mass extinctions at random.

Periodic mass extinctions at random

Gazing into the rock and fossil record, some geologists and paleontologists have found various periodicities to the mass extinctions that punctuate the earth's history. These extinctions wiped out 50 percent to 95 percent of the species living at the time, and they seem to repeat with an interval of 26 million to 30 million years. To explain the apparent pattern, scientists have invoked cyclic meteor showers, galactic rotation and other periodic events. But more recently, paleontologist Steven Stanley of Johns Hopkins University in Baltimore theorized that random catastrophic events could generate periodic mass extinctions.

Michael L. McKinney of the University of Tennessee in Knoxville now reports that computer models support Stanley's theory. McKinney's model simulates the diversity of species through time, and into the model he incorporates a random series of environmental upheavals.

Scientists have compiled a long list of events--such as comet impacts, rises in sea level and massive volcanic eruptions-- that could individually stimulate extinctions. Since these events are often unrelated to each other, says McKinney, it is safe to assume that the total list of catastrophic events would form a random pattern.

As expected, McKinney found that a single event drastically reduces the number of different species. But following this first event, the system is temporarily "immune' to random cataclysms. According to McKinney, the species that survive a mass extinction are particularly hardy and are "resistant' to subsequent random changes. Moreover, after an extinction, there are simply fewer species left to die out.

"Therefore,' he says, "until many species have evolved, including extinction-"prone' types, any intervening catastrophes would have little effect.' After each extinction, the biological community requires a distinct recovery period before it is again ready to go out with a bang.
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Author:Monastersky, Richard
Publication:Science News
Date:Nov 14, 1987
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