Current methods used to assess species on the brink overlook some key factors, a team of scientists claims.

These include the ratio of males to females in a population, which can have a profound influence on survival.

For some species, the risk could be a hundred times greater than previously thought, the team calculates.

Currently, two key factors are used to estimate risk - the birth-death ratio, and environmental conditions such as habitat destruction.

But the US researchers - Brett Melbourne and Alan Hastings - believe other elements are under-weighted.

As well as the male-to-female ratio, they point to the physical size of individuals in a species, and some aspects of behaviour.

They found that when populations are small and vulnerable, changes in the sex ratio can have a huge impact on survivability.

New model

Overall, their new model of risk suggests that some species are at much greater likelihood of extinction than normally assumed.

Professor Alan Hastings from the University of California, Davis, told the BBC: "With some species, if the numbers are going up and we can protect them then we're probably doing OK.

"But there may be many species - and some will not be the large, charismatic ones, but things like insects and other smaller ones that are still very important - where we may be underestimating the risk by quite a bit."

The International Union for the Conservation of Nature (IUCN) has identified more than 16,000 species threatened with extinction.

One in three amphibians, one in four mammals, one in eight birds and 70% of plants so far assessed for its Red Lists of Threatened Species are believed to be at risk.

BBC