Species Extinction Rates Have Been Overreported, New Study Claims

The most widely used methods for calculating species extinction rates are "fundamentally flawed" and overestimate extinction rates by as much as 160 percent, life scientists report May 19 in the journal Nature. However, while the problem of species extinction caused by habitat loss is not as dire as many conservationists and scientists had believed, the global extinction crisis is real, says Stephen Hubbell, a distinguished professor of ecology and evolutionary biology at UCLA and co-author of the Nature paper.

"The methods currently in use to estimate extinction rates are erroneous, but we are losing habitat faster than at any time over the last 65 million years," said Hubbell, a tropical forest ecologist and a senior staff scientist at the Smithsonian Tropical Research Institute. "The good news is that we are not in quite as serious trouble right now as people had thought, but that is no reason for complacency. I don't want this research to be misconstrued as saying we don't have anything to worry about when nothing is further from the truth."

Because there are very few ways of directly estimating extinction rates, scientists and conservationists have used an indirect method called a "species-area relationship." This method starts with the number of species found in a given area and then estimates how the number of species grows as the area expands. Using that information, scientists and conservationists have reversed the calculations and attempted to estimate how many fewer species will remain when the amount of land decreases due to habitat loss.

"There is a forward version when we add species and a backward version when we lose species," Hubbell said. "In the Nature paper, we show that this surrogate measure is fundamentally flawed. The species-area curve has been around for more than a century, but you can't just turn it around to calculate how many species should be left when the area is reduced; the area you need to sample to first locate a species is always less than the area you have to sample to eliminate the last member of the species.

"The overestimates can be very substantial. The way people have defined 'extinction debt' (species that face certain extinction) by running the species-area curve backwards is incorrect, but we are not saying an extinction debt does not exist."

How confident is Hubbell in the findings, which he made with ecologist and lead author Fangliang He, a professor at China's Sun Yat-sen University in Guangzhou and at Canada's University of Alberta?

"100 percent," he said. "The mathematical proof is in our paper."

There were predictions in the early 1980s that as many as half the species on Earth would be lost by 2000.

"Nothing like that has happened," Hubbell said. "However, the next mass extinction may be upon us or just around the corner. There have been five mass extinctions in the history of the Earth, and we could be entering the sixth mass extinction."

Hubbell and He's mathematical proof addresses very large numbers of species and does not answer whether a particular species, such as the polar bear, is at risk of extinction.

"We have bought a little more time with this discovery, but not a lot," Hubbell said.