Scientists have been overestimating animal and plant extinction rates for decades and need a new method of counting populations to better manage animal survival, a study in the journal Nature showed.

Animals and plants are dying out about 2.5 times more slowly than previously thought, according to the authors of a study published today. The finding doesn’t mean species won’t become extinct, the authors said.

Previous estimates of extinction rates suggested that 33 percent to 50 percent of all species would be lost between the 1970s and 2000. That hasn’t happened, according to the study. The Millennium Ecosystem Assessment, published in 2005 on request by the United Nations, may also be inaccurate, the authors wrote.

"People realized there was an overestimation but, for years, no one could pick out why,” said Fangliang He, a study author and professor of biodiversity and landscape modeling at the University of Alberta in Canada.

Ecologists and conservationists have been using a flawed a model called “species-area relationship” to estimate populations, said Stephen Hubbell, a professor of evolutionary biology at the University of California, Los Angeles, and a paper author.

By starting with the number of species found in a given area, scientists can estimate how the number would grow as the area expanded. Scientists and conservations reversed the method to determine how many would be lost as a habitat decreases.

Species Count

This method was inaccurate because, when a scientist encounters a member of a species for the first time, the species count goes up by one. However, for extinction, all members of the species must be lost, Hubbell said.

"I couldn’t see any problem right off the bat, but then we sampled some large, mapped forest plots and we could see a big problem,” Hubbell said.

Individual members of a species may migrate away from lost habitat or continue to live in a much smaller area, Hubbell said. Also, the inability to find a species in a given area doesn’t mean it has died out there.

"This new paper does not alter the bottom line of the Millennium Ecosystem Assessment,” said Janet Ranganathan, the vice president of science and research for the Washington-based World Resources Institute, one of the main groups behind the assessment, in an e-mail.

About two-thirds, or 15 of 24 ecosystems assessed in the report, “have been degraded in the past 50 years,” Ranganathan said. Today’s finding “does not dispute the urgent need to stem habitat loss in order to preserve biodiversity,” she said.