Study: Math glitch wildly underestimated extinction risk

Endangered species may face an extinction risk that is up to 100 times greater than previously thought, according to a study published in the July 3 issue of Nature.

Brett Melbourne, a biologist at the University of Colorado at Boulder, found that current mathematical models used to estimate the threat of extinction overlook random differences between individuals in a given population. Such differences, which include size, behavioral characteristics, and the ratio of males to females, influence an individual's reproductive success. According to the study, these variations have an unexpectedly large effect on extinction-risk calculations.

"When we apply our new mathematical model to species-extinction rates, it shows that things are worse than we thought," said Mr. Melbourne in a press release from the University of Colorado. "By accounting for random differences between individuals, extinction rates for endangered species can be orders of magnitude higher than conservation biologists have believed."

"There has been a tendency to misdiagnose randomness between individuals in a population by lumping it with random factors in the environment, and this underestimates the extinction threat," said Melbourne.

According to Melbourne, current extinction-risk models, such as those used by the International Union for Conservation of Nature, whose "red list" is generally recognized as the world's authority on the conservation status of endangered species, are based on two factors: the number of random events that could kill an individual within a given population, and environmental conditions, such as temperature or rainfall fluctuations, that could influence birth and death rates.

These models, says Melbourne, overlook other factors that have a huge impact on small populations.

To test their new mathematical model, Melbourne and his colleague, Alan Hastings of the University of California, Davis, monitored populations of beetles in lab cages.

By factoring in the beetles' sizes and the sex ratio of the population, the researchers discovered that they could explain large swings in a specie's population that were unaccounted for by the older model.

"The results showed the old models misdiagnosed the importance of different types of randomness, much like miscalculating the odds in an unfamiliar game of cards because you didn't know the rules," said Melbourne.

The Guardian, a British daily, quotes Craig Hilton-Taylor, who manages the IUCN's red list. "We are certainly underestimating the number of species that are in danger of becoming extinct because there are around 1.8m described species and we've only been able to assess 41,000 of those," he said.

According to the IUCN's 2007 red list, more than 16,000 species worldwide are threatened with extinction, including 1 in 4 mammal species, 1 in 8 bird species, and 1 in 3 amphibian species.

In May, the World Wildlife Fund reported that populations of animals have declined by one quarter since 1970, an extinction rate that is 10,000 times faster than has been tracked in the fossil record.

You can view Melbourne and Hasting's paper in Nature here (a subscription is required, but you can read the abstract).