That's because many species considered relatively safe today have the potential to leapfrog to the top ranks of endangerment in coming decades.
The authors call that potential for rapid future decline "latent extinction risk."
"Species with a high latent risk of extinction are ones that are less threatened today than their biology suggests they could be," Cardillo said.
Like a person carrying a latent or hidden disease, such animal populations are vulnerable even while appearing healthy, he explained.
To measure and compare latent risk among mammals, Cardillo's group created mathematical models using data on 25 biological traits from roughly 4,000 species.
The scientists assigned each species a single numerical score representing its degree of inherent, biologically based vulnerability.
They then assigned a second score reflecting each species' actual degree of endangerment, as shown by its position on a widely used conservation reference, the global Red List of Threatened Species, maintained by the World Conservation Union.
The difference between these two numbers was the animal's latent extinction riskthe gap between the species' potential vulnerability and its real-world status.
By this formula, species that are already seen as critically endangered received relatively low scores for latent risk. Only those having the potential to be severely affected by human disturbance, but still seen as relatively safe, scored high.
To portray how the scores were distributed geographically, the scientists produced their own hot spot map.
Among the 20 hot spots are the forests and tundra of northern Canada and Alaska, and the chain of islands stretching from Indonesia to the South Pacific.
Other hot spots include the Bahamas, Tasmania, and the Patagonian coast of Argentina.
A New Look at Hot Spots
Cardillo emphasizes that his study is not meant to supplant the existing concept of biodiversity hot spots.
Global conservation groups have used hot spot mapping to help direct funding to protect unique and threatened areas.
Still, some conservationists see a danger in highlighting areas with relatively low biodiversity as priorities for conservation funding.
Thomas Brooks, a biologist and planner for the Washington, D.C.-based group Conservation International, says that while the latent risk concept is important, mapping average scores across regions may be misleading.
"It gives the impression that polar regions with a few sensitive species are equivalent priorities to the tropical island hot spots in the Philippines and Indonesia that have hundreds of sensitive species and hundreds more already threatened," Brooks said.
Cardillo argues that it is important to know the sensitivity of different regions and mammal communities, regardless of the overall number of species present.
He says his approach can help complement other strategies by pointing out in advance what areas are likely to become future conservation battlegrounds.
"There are still large areas of the world with relatively intact habitats, which will almost certainly come under pressure in the future," Cardillo said.
"If human impact increases in coming decades, then we expect the latent risk to become realized and many species to become threatened."
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