By overlooking random differences between individuals in a given population, researchers may have badly underestimated the perils confronting threatened wildlife, it said.

"Many larger populations previously considered relatively safe would actually be at risk," Brett Melbourne, a professor at the University of Colorado and the study's lead author, told AFP.

There are more than 16,000 species worldwide threatened with extinction, according to the International Union for Conservation of Nature (IUCN).

One in four mammals, one in eight birds and one in three amphibians are on the IUCN's endangered species "Red List".

In a study released on Wednesday by the journal Nature, Melbourne said the current models used draw up such lists typically look only at two risk factors.

One is the individual deaths within a small population, such as Indian tigers or rare whales.

When a species dwindles beyond a certain point, even the loss of a handful of individuals can have devastating long-term consequences, Melbourne explained.

There are less than 400 specimens of several species of whale, for example, and probably no more than 4,000 tigers roaming in the wild.

The second commonly-used factor is environmental conditions that can influence birth and death rates, such as habitat destruction, or fluctuations in temperature or rainfall, both of which can be linked to climate change.

Melbourne and co-author Alan Hastings from the University of California at Davis argue that these factors must be widened in order to give a fuller picture of extinction risk.

They say that two other determinants must be taken into account: male-to-female ratios in a species, and a wider definition of randomness in individual births and deaths.

These complex variables can determine whether a fragile population can overcome a sudden decline in numbers, such as through habitat loss, or whether it will be wiped out.

"This seems subtle and technical, but it turns out to be important," Melbourne said in an email. "Population sizes might need to be much larger for species to be relatively safe from extinction."

The new mathematical tool will be most useful for biologists who want to assess the survival prospects of species such as marine fish whose numbers can suddenly fluctuate and for which data is limited, the authors say.

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