BERKELEY (3/9/00) -- The Earth needs, on average, about 10 million years to recover from a mass extinction of the planet's species, far longer than most scientists thought, according to a new study by scientists at the University of California, Berkeley, and Duke University.
Moreover, the recovery time is the same whether the global die-off involves the loss of most life on Earth or wipes out far fewer species.
This unexpected finding has major implications for the Earth's fate as human activity threatens species around the globe.
"People have argued that we only have to worry about human-caused extinctions if we do something that causes the loss of 80 or 90 percent of species on the planet," said UC Berkeley environmental scientist James W. Kirchner.
"Our analysis shows that even if the human impact is much smaller than that - 20 or 30 or even 50 percent of species - it's still going to take 10 million years for the Earth to recover. That is well past the expected life span of the human species, or even of the genus Homo."
"Extinctions caused by humans don't have to be large to have an effect that reverberates in the ecosystem for tens of millions of years," said paleontologist Anne Weil, a former doctoral student at UC Berkeley and now a research associate in the Department of Biological Anthropology and Anatomy at Duke University.
Weil and Kirchner, a professor of geology and geophysics at UC Berkeley, report their findings in the March 9 issue of Nature.
During the past half billion years, life on Earth has blossomed and crashed many times - some die-offs the result of geologic cataclysm, but most of unknown cause.
Paleontologists have known that the Earth needed a long time to recover from large extinctions, such as that which occurred at the end of the Permian 250 million years ago, when more than 90 percent of all species died out. The debate was over the smaller, background extinctions that pepper the fossil record, in which 10 to 20 percent of species died out.
"The presumption has been that while big extinctions require a long recovery, the biosphere should bounce right back after smaller extinctions," Kirchner said.
One critical reason for wanting an answer to this question is that human activity now is eliminating many species each year in what some see as a major, human-caused extinction event.
"We don't know what our current level of extinction is, but some biologists estimate that eliminating 90 percent of tropical rain forests would lead to the extinction of half the species on Earth," Kirchner.
Scientists despaired of an answer to the question because the fossil record is full of holes. Kirchner, however, came across an analysis technique used in astrophysics that can be used to compare the rates of extinction and of evolution in a spotty fossil record.
Using a database compiled by the late University of Chicago paleontologist Jack Sepkoski, Weil and Kirchner compared the rate of extinction of fossil marine organisms with the rate of evolution or "origination" over the past 530 million years.
They found that evolution rebuilds biodiversity very slowly after extinction events. Looking at all the biotic crises during this period, they found an average of 10 million years between an extinction and a subsequent flourishing of life.
When they eliminated the five mass extinctions from that period, including the extinction 65 million years ago when the dinosaurs died out, the smaller background extinctions also averaged a 10 million-year recovery period.
"People are really excited about this from an evolutionary perspective," Weil said. "This shows that long recovery time is a previously unrecognized feature of the fossil record, evidence of an evolutionary dynamic we didn't suspect before."
Weil said that the long recovery period is perhaps not surprising. While a species' extinction does not leave an empty niche, it destroys all or part of other niches. For example, it removes opportunities for predators, parasites and other organisms that would normally rely on that species. Extinction also is a lineage termination, which removes a potential ancestor from which other species could evolve.
"Extinction is a double whammy. You not only take out the ecological niche as you take out the species, but you take away the evolutionary potential for radiation, too," Weil said. "Diversification takes a long time to ramp up."
Such complexity may also be the reason that the size of the extinction is relatively unimportant in determining the time it takes the ecosystem to recover. Extinction totally alters the ecosystem, making recovery a slow process no matter what the scale.
"The post-extinction ecosystem is not anything like the pre-extinction ecosystem, so life will never recover to the same place," Weil said. "Life reaches a new but different plateau."
Kirchner notes that their sobering findings don't necessarily imply a stark fate for the Earth.
"It is not preordained that high levels of human-caused extinction have to happen," Kirchner said. "Our future depends on what we choose to do on a national and international level, as a society. Those decisions are critical because they will have very long-lasting consequences.
"If we deplete Earth's biological diversity, we will leave a biologically impoverished planet, not only for our children and our children's children, but for all the children of our species that there will ever be."
The work was supported by grants from the University of California and the National Science Foundation.
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