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Global species extinction isn't quite so dire, study finds

Scientists using a new method to calculate the rate of extinction say the crisis is being overestimated by as much as 160%.

May 21, 2011|By Amina Khan, Los Angeles Times
  • A Hawaiian monk seal basks on the beach in Hau'ula. The species is critically endangered.
A Hawaiian monk seal basks on the beach in Hau'ula. The species is critically… (Jim Collins, Associated…)

Hit the snooze on that ecological doomsday clock for a minute: The world's species may not be going extinct quite as fast as we thought they were. Scientists may be overestimating the crisis by as much as 160%, according to a recent study.

The research was published online Wednesday in the journal Nature.

While stressing that the global extinction crisis is still indeed a crisis, the study's two authors called for a better mathematical model to predict how fast the world's diversity is disappearing.

The massive loss of species occurring today may constitute the sixth mass extinction of life on Earth. It is caused in large part by habitat destruction, which can be blamed on human encroachment. The rate of biodiversity loss, however, is difficult to estimate, said study coauthor Stephen Hubbell, an ecologist at UCLA.

One of the models that scientists use reverses what's called the species-area relationship: This starts with the number of species in a certain area and then measures how many new species pop up as the surveyed area increases.

Extinction rates, at first glance, should be the opposite of that relationship — losing species as land is lost. So researchers run the equation backward to calculate how fast species are disappearing.

But that's not an entirely accurate way to measure extinction, Hubbell said. The species-area relationship counts the first member of each species to appear — so running it backward would count the first member of each species to disappear. But just because one individual dies doesn't mean all of them have, he said.

He and coauthor Fangliang He, an ecologist at Sun Yat-sen University in Guangzhou, China, used a different model in their study. Rather than counting the disappearance of the first member of a given species, their method relied on finding the last member of a species — a better fit with the definition of extinction.

Using range maps of North American bird species assembled by Natureserve, a nonprofit conservation organization, and data from the Center for Tropical Forest Science's network of rainforest plots in Asia, Africa and the Americas in which every tree in a plot is tagged, the scientists charted both models to see how well each fit the real-life data.

They found that the model they used closely matched the known data, whereas the other model overestimated the extinction rate by as much as 160%.

The authors and other scientists hastened to add that extinctions were still occurring at an alarming rate.

"This does not mean that there's not an extinction crisis — just because it's slightly less disastrous than we think doesn't mean it's not disastrous," said Robert Colwell, an ecologist at the University of Connecticut who co-wrote a commentary on the study. "We're narrowing in on a more accurate way of estimating something that's difficult to predict.... It's a refinement, not a revolution."

But some scientists questioned the study's findings. "There are dozens of studies that have appeared over the past 15 years that show a surprisingly good match between the theoretical predictions and what you actually observe," said Stuart Pimm, a conservation ecologist at Duke University who has used the standard method of predicting extinctions in his own research.

These new findings, Pimm argued, might be fine for assessing those extinctions that occur right after habitat is destroyed, but not for those species that fade from existence over a longer period of time. For example, he said, even if a particular species survives destruction of most of its habitat, it still may not be able to reproduce, and thus could die out when the current generation does.

"Dozens of people have been using the right method for quite a long time, and it works very well, thank you very much," Pimm said.

amina.khan@latimes.com

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