23 May 2011

Wildlife extinction rates hugely over-reported

by Kate Melville

Calculating species extinction rates is a tricky business at the best of times, but a new research paper in Nature suggests the most widely used methods are "fundamentally flawed" and overestimate extinction rates by as much as 160 percent.

Paper co-author Stephen Hubbell, a distinguished professor of ecology at UCLA, explained why he believed extinction caused by habitat loss is not as dire as many conservationists and scientists believe. "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. 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."

Traditionally, 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. "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."

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. As we continue to destroy habitat, there comes a point at which we do lose a lot of species - there is no doubt about that. However, we have to destroy more habitat before we get to that point."

Humans are already using 40 percent of all the plant biomass produced by photosynthesis on the planet, a disturbing statistic because most life on Earth depends on plants, the paper notes. Some three-quarters of all species thought to reside on Earth live in rain forests, and they are being cut down at an estimated rate of about half a percent per year.

"We need much better data on the distribution of life on Earth," Hubbell said. "We need to rapidly increase our understanding of where species are on the planet. We need citizens to record their local biodiversity; there are not enough scientists to gather the information. We also need much deeper thought about how we can estimate the extinction rate properly to improve the science behind conservation planning. If you don't know what you have, it is hard to conserve it."

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Source: University of California - Los Angeles