A new theory suggests the worst mass extinctions result from a combination of events such as volcanic eruptions, meteor impacts, epidemics and climate change.
Credit: U.S. National Parks Service
SYDNEY: Mass extinctions may have no single cause but may be a complex interplay between sudden events and long-term effects, according to a new theory.
Whether you favour 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, U.S. scientists say. That may be because big extinctions aren't simple events.
The new 'press/pulse' theory, devised by researchers at Hobart and William Smith Colleges in Geneva, New York, gets around the controversy by rejecting the all-or-nothing approach to mass extinction. Instead, it cites a combination of deadly sudden catastrophes (or 'pulses'), and longer, steadier pressures on species (or '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, co-author of the study.
West and palaeontology professor Nan Crystal Arens presented their new theory at the Annual Meeting of the Geological Society of America in Philadelphia last week.
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 per cent of genera became extinct every million years; 8.3 per cent of genera became extinct in stages characterised by volcanism alone. When neither press nor pulse were active, 8.2 per cent of genera became extinct. These averages are statistically indistinguishable.
"Statistically speaking, extinction rates are not significantly higher at times of impact or volcanism versus no geologic events," West said. In contrast, when press and pulse events coincided, an average of 12.8 per cent 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 volcanic 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 industrialisation 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.
According to 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."
with The Geological Society of America
