 
New
study suggests humans will not live long enough to see Earth
recover from a mass extinction
09
Mar 2000
By
Robert Sanders, Public Affairs
BERKELEY--
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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