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How long does it take species to go extinct? Longer than previously thought.

Habitat destruction drives species extinct more slowly than previously thought, according to a new model described in this week's Nature. 'We have bought a little time for saving species,' says scientist.

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Some suggested that the gap may be due to what became known as "extinction debt," where endangered plants or animals may linger after losing a significant portion of their habitat, but whose eventual disappearance has become inevitable.

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Now, Drs. He and Hubbell say they have found the reason for the gap and offer up a method that more closely matches extinction rates seen in the field. While the new approach doesn't rule out the possible existence of an extinction debt, the duo says their method doesn't need it to explain the gap.

Mass extinctions then and now

The world still faces a significant extinction problem, the two emphasize. They cite a study published in March, for instance, that compares extinction rates written in the fossil record with those recorded by the International Union for the Conservation of Nature, the world's scorekeeper on threatened, endangered, and extinct species today.

The analysis, by a team led by Anthony Barnosky of the University of California at Berkeley, concluded that species extinctions over the past few thousand years are higher than the typical rate seen in the fossil record.

If currently threatened species become extinct within the next 100 years and the pace continues unchecked, within 240 to 540 years extinctions will rise to a level not seen since the last five major mass extinctions in Earth's history, Barnosky found.

The major drivers in the five mass extinctions were prolonged volcanic eruptions, changing climate, or even comet or asteroid collisions with Earth. The main driver today is widely seen as the impact of human activities on climate, landscape, and oceans.

The previous approach to modeling extinction

He's and Hubbell's work tries to get at an important aspect influencing the pace of extinction.

At the heart of the discussion is the the "species-area relationship," a long-established relation between the amount of area scientists look at and the number of new species they discover as they expand the patch of land or ocean they explore.

As the search area expands, Hubbell explains, scientists add a new species when they spot even one specimen of a previously unknown, undescribed, or reclassified organism.

This has led to the development of a mathematical relationship between the amount of new territory explored and the number of new species one could expect to find.

But nothing similar existed for extinctions. So researchers figured that one approach might be, in essence, to throw this "discovery" calculation into reverse to see how many species one would lose as habitat shrank.

That's where the conceptual problem arises, He and Hubbell say. Extinction is a different beast from the discovery approach to ecology.

Instead of relying on the presence or absence of one specimen, "the extinction problem requires that all of the individuals be lost and be included in the area of habitat loss," Hubbell explains.

Figuring out this distinction was relatively easy, the pair acknowledges. But it took eight years for the two scientists to work out the math behind the approach and use it to demonstrate why and how the previous approach to estimating extinctions so often failed to match field observations.

Testing the new model


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