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Scientists call for update of extinction-risk models

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The extinction risks for many endangered species could be significantly underestimated because of a flaw in the way these risks are mathematically modeled, according to a new study in this month’s Nature.*

Extinction-risk models are based on accounting for ‘randomness’ and currently involve looking at two factors: 1) Accidents or chance events, such as the drowning of a rock wallaby, and 2) External or environmental variability, such as changes in weather or natural disasters.

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‘Randomness is very important when populations get very small,’ said Brett Melbourne, an assistant professor at the University of Colorado. ‘Because then, at that point, a string of unfortunate coincidences is what ends up finishing them off... Things you couldn’t predict.’

The study, by Melbourne and UC Davis Professor Alan Hastings, took into account two additional factors of randomness: Population sex ratios (the number of females versus number of males) and physical variation between individuals, such as size, which can relate to the number of offspring produced in certain species.

‘Our current theories of extinction don’t include enough things,’ Melbourne said. ‘This is really the way that science operates. You go along with a theory, and then at some point, somebody else realizes that something else should be in there.’

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The researchers monitored populations of beetles in lab cages to test their new mathematical models and found that the old models had misdiagnosed or didn’t take into account the different types of randomness.

For example, an individual difference could be classified as an environmental effect, but Melbourne said individual differences have a much larger affect on populations than do environmental effects. This misdiagnosis may mean the extinction risks of certain endangered species could be underestimated by as much as a factor of 100.

And because natural populations are likely to have a larger sex ratio and differences in physical characteristics, the effects could be larger, Melbourne said.

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‘Our results have less implication to species we know quite well,’ Melbourne said. ‘... but there are many species we don’t know much about and the only data we have to access them is data we know about like fluctuations in the population size... For those species it’s easy to misdiagnose one randomness for another.’

Melbourne said that this research is likely to be a springboard for more complex and elaborate models that will take into account probability of individual characteristics reappearing in offspring and more specific differences between types of species, among other criteria.

The report suggests that extinction risks for many populations be ‘urgently re-evaluated with full consideration of all factors’ of randomness. The research was funded by the National Science Foundation.

A 2007 report by the International Union for Conservation of Nature found that there were more than 16,000 species worldwide that are threatened with extinction.

‘There are a lot on the list,’ Melbourne said, ‘but there are probably a lot more that should be.’

-- Tami Abdollah

*Payment required to access study.

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