March 9, 2000

Study Jolts Views on Recovery From Extinctions

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    Over 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.

    Humans may be bringing on one of history's major animal die-offs.

    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."

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