Whether you favor meteor impacts, volcanic eruptions, cosmic rays, epidemics, or some other cause for the worst mass extinction events in Earth's history, no single cause has ever satisfied all scientists all the time for any extinction event. That may be because big extinctions aren't simple events.
The new Press/Pulse theory gets around the controversy by rejecting the all-or-nothing approach to mass extinction, calling instead on a combination of deadly sudden catastrophes - "pulses" - with longer, steadier pressures on species - "presses."
"What we wanted to do is move away from the idiosyncratic approach to extinction mechanisms and look for what these intervals had in common. If you have A and B you will get a mass extinction," said Ian West, a 2006 graduate of Hobart and William Smith Colleges in Geneva, NY.
West and Hobart and William Colleges paleontology professor Nan Crystal Arens are scheduled to present their work on the Press/Pulse theory on Wednesday, 25 October, at the Annual Meeting of the Geological Society of America in Philadelphia.
Using databases that chart genera of marine organisms and their extinctions through the fossil record, West and Arens divided the last 488 million years of geologic history into four groups: times of suspected impact events (Pulses), times of massive volcanic eruptions (Presses), times when neither Presses nor Pulses occurred, and times when Press and Pulse coincided. They compared average extinction rates in geologic stages in each of these groups.
During stages when only impacts occurred, an average of 7.3% of genera became extinct every million years; 8.3% of genera became extinct in stages characterized by flood volcanism alone. When neither Press nor Pulse were active, 8.2% of genera became extinct. These averages are statistically indistinguishable. "Statistically speaking, extinction rates are not significantly higher at times of impact or volcanism vs. no geologic events," West said.
In contrast, when Press and Pulse events coincided, an average of 12.8% of genera became extinct per million years, statistically higher than the rate observed during other geologic stages.
"The goal of our work was to come up with a unifying theory of mass extinctions. We also wanted to make it applicable to what's going on now," said West, referring to rapid losses of biodiversity worldwide now underway as a result of climate change and destruction of habitats by human activities.
"Is this model, which seems to work for the big five mass extinction events in Earth's history, applicable today?" West asked.
At first glance the answer would appear to be 'no.' There is, after all, no massive flood basalt eruption underway today, nor have there been any recent meteor impacts. On the other hand, some very similar effects are being seen on Earth.
"We came up with the idea that humans themselves act as both Press and a Pulse," said West. "Humans began manipulating the environment - the Press - from the advent of agriculture. However, that alone did not trigger the current mass extinction. That seems to have been triggered by the pulse of industrialization and the demands for energy and resources that came with it."
The bottom line, says West is that it's extremely hard to pinpoint simple causes for Earth's great periods of extinction.
"We sought to rephrase the question," said Arens. "In the modern world, species are commonly endangered by some stress before the final death blow falls. It seems likely that biological systems in the past worked in similar ways. By demonstrating that the coincidence of long-term stress and catastrophic disturbance is needed to produce big extinctions, we hope to break down some of the polarization characteristic of many discussions of extinction. We hope to send people back to the data with a more inclusive hypothesis to test."
WHEN & WHEREPRESS/PULSE: A GENERAL THEORY OF MASS EXTINCTION?
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