But in an entirely unexpected finding that has gotten the attention of
both paleontologists and conservationists, researchers reported today
that it takes a long time to recover
from these large-scale extinctions --
around 10 million years -- and more
intriguing, the recovery time appears to be the same, whether the
original destruction was one of the
grander or one of the more minor
events.
Many biologists say that by destroying tropical forests and other
habitats, humans are driving species
extinct at an accelerating rate that if
unchecked will result in one of the
major extinctions in history.
An ominous implication of the new
research, some scientists say, is that
humans may already or will soon
have destroyed enough species that
it will require a full 10 million years
for the planet to recover -- 20 times
as long as humans have already existed and longer than many scientists predict humanity itself is likely
to persist into the future.
Scientists say the new study will
also prompt paleontologists to rethink how life evolves after major
extinctions, events that have played
a pivotal role in shaping the evolution of life.
The study, by Dr. James W. Kirchner, an earth scientist at the University of California, and Dr. Anne Weil,
a paleobiologist at Duke University,
appears today in the journal Nature.
"I think it's a surprise to everyone," said Dr. Douglas Erwin, a paleobiologist at the National Museum
of Natural History at the Smithsonian Institution in Washington. "We're
just at the beginning of understanding biotic recoveries following extinctions."
Dr. Mike Foote, a paleobiologist at
the University of Chicago, said the
paper would change how people
looked at the fossil record, adding,
"It's going to be seminal."
Borrowing statistical techniques
from other areas of geology and astrophysics, the researchers examined an enormous set of data gathered by the late Dr. Jack Sepkoski, a
paleontologist at the University of
Chicago. The data set documents
when new kinds of marine animals
first appear in the fossil record and
when they disappear or go extinct.
Scientists expected that with every
additional species that gets knocked
out during an extinction, the longer
the recovery should take.
What researchers found instead
was that after extinctions, big and
small, it took about 10 million years
for the proliferation of new kinds of
organisms to peak and then begin to
tail off. The decrease in the proliferation of new creatures is seen as a
sign that ecosystems have recovered
and are so full of diverse life that
there is little opportunity left for new
organisms.
So unexpected was the outcome
that Dr. Weil said that when Dr.
Kirchner handed her the results, she
said, "Did you do that right?" She
added, "I made him do the analysis
again while I was looking over his
shoulder."
Scientists said the new study suggested that the evolutionary rebuilding after an extinction might work
differently than had been envisioned.
Many paleontologists had previously pictured extinctions as being
the equivalent of wiping the pieces
off a chessboard. In such a situation,
though the pieces are gone, the
spaces on the chessboard, the place
for a queen or rook -- or in nature,
the niche for an algae-eating fish or a
long-lived evergreen tree -- are still
there, each simply needing to be refilled by a newly evolved species.
But the new study, Dr. Erwin said,
suggests instead that "when you
wipe the pieces off the chessboard,
the size of the chessboard shrinks."
That is, with the loss of a species
comes the loss of opportunities for
the existence of other organisms as
well -- like those that would make
their living by preying upon or parasitizing that organism.
So when there are few types of
organisms left after an extinction,
there are few opportunities for new
species to evolve. Each new kind of
organism that does evolve creates
more opportunities for other species,
and each of those, in turn, creates
more opportunities in a kind of positive feedback loop, the dynamics of
which may be setting the recovery
clock at 10 million years.
While some scientists predict that
humanity will not live to see a recovery from the extinctions it has
wrought, many questions remain. It
is difficult to compare modern-day
extinctions with those viewed
through the lens of the fossil record.
As a result, scientists still do not
agree on whether and when humanity will have destroyed enough species to require a 10 million year
recovery, with some saying that
point has already been reached, and
others predicting it will be anywhere
from 50 years to a thousand years or
more in coming.
But if diversity is what provides
the fuel for the recovery from any
extinction, as the new work suggests,
then in terms of a practical application, Dr. E. O. Wilson, an evolutionary biologist at Harvard University,
said the message was clear.
"The
bottom line," Dr. Wilson said, "is
that we had better take care to hold
on to the biodiversity that still exists."
ver the past 500 million years,
there have been dozens of episodes of
extinction on the planet, the most
famous being the titanic event that
swept away the dinosaurs. Scientists
have assumed that the time it takes
for the living world to bounce back is
proportional to the damage done:
more time should be required to fill
all of the earth's empty corners with
newly evolved life after gargantuan
extinctions and less time should be
required after a more minor die-off.
