Biodiversity May Need Millions Of Years To Recover

by Cat Lazaroff

BERKELEY, California, January 3, 2002 (ENS) - The worldwide decimation of wildlife by humans could be "permanent on multi-million year timescales," warns James Kirchner of the University of California at Berkeley. Kirchner's analysis of long term trends in the fossil record suggests that natural speed limits constrain how quickly biodiversity can rebound after waves of extinction.

UC Berkeley geologist James Kirchner says current reductions in the planet's biodiversity could last for millions of years. Over the last 500 million years, life on Earth has experienced a series of booms and busts. The busts, or mass extinctions, can be gradual, occurring over thousands or millions of years, or they can happen suddenly in response to a natural catastrophe.

But the booms of diversification, in which hundreds or thousands of new organisms appear, rarely happen quickly, writes Kirchner in this week's issue of the journal "Nature."

His statistical analysis of the rates of extinction and diversification in the fossil record shows that life seldom rebounds rapidly after an extinction.

The results imply that the diversification of life obeys so called speed limits set by evolutionary processes, said Kirchner, a professor of earth and planetary science at UC Berkeley.

Some of the most biologically diverse areas on earth are also home to the hungriest humans. "There seem to be biological mechanisms that limit diversification of new organisms and control which ones become successful enough to persist," he said. "Biodiversity is slow to recover after an extinction."

This apparent speed limit on the rate at which surviving organisms evolve and diversify has major implications for present day extinctions - caused not by natural catastrophes but by human sources such as pollution, alteration of natural habitats, and unsustainable hunting and fishing.

"If we substantially diminish biodiversity on Earth, we can't expect the biosphere to just bounce back. It doesn't do that. The process of diversification is too slow," Kirchner said. "The planet would be biologically depleted for millions of years, with consequences extending not only beyond the lives of our children's children, but beyond the likely lifespan of the entire human species."

Kirchner has been mining a fossil database created by the late University of Chicago paleontologist Jack Sepkoski, who catalogued the genera and families of fossil marine animals over the past 530 million years, from the Cambrian to the present. Using a technique called spectral analysis, Kirchner looked for patterns in the rates at which new organisms appear or disappear.

Last year, Kirchner and colleague Anne Weil reported that the Earth needs, on average, about 10 million years to recover from global extinctions, whether they involve the loss of most life on Earth or wipe out far fewer species. This was much longer than most scientists had believed.

The new results come from asking a related question: How do rates of extinction and diversification vary, and how are they related? This is important because, if rapid diversification is possible, biodiversity might be able to rebound quickly from a global extinction.

Biologists are still discovering new species, like the tiny Jaragua lizard from Beata Island, off the coast of the Dominican Republic. Far more species are being lost to extinction every day, biologists warn.

Kirchner's analysis found that extinction rates and diversification rates are about equally variable over long spans of geological time. Over shorter periods, however, diversification rates vary much less than extinction rates do.

That means that evolution does not accelerate quickly in response to rapid bursts of extinction.

One possible explanation for why diversification takes so long to speed up after an extinction is that extinction eliminates not merely species or groups of species, but removes ecological niches: the roles which organisms play within ecosystems.

Recovery becomes more complicated because specialized roles, such as parasites that live on just one species, or animals that consume just one kind of food, do not evolve until their hosts are already well established.

"This shows that extinction is not like knocking chess pieces off a chessboard, with the empty squares ready for you to plunk down new pieces," Kirchner said. "Extinction is more like knocking down a house of cards. You only have places to put new cards as you rebuild the structure of the house."

96 percent of Madagascar's forests have been destroyed, leaving little habitat for species like this lemur, which live nowhere else on Earth.

"For a new kind of organism to evolve and survive long enough for us to notice it - for it to become common enough to leave a fossil record -requires that it have an evolutionary niche," he explained. "The organism has to have some role in order to succeed in its ecosystem. As a result, the ecosystem must first increase in complexity so there are niches for new organisms to fill, which is probably a very complicated process.

"At a fundamental biological level it takes time to build niches, evolve new organisms and filter out unsuccessful ones, although it's not yet clear what all the limiting factors are."

Kirchner's work was supported by grants from the National Science Foundation and the University of California.