Polar bears may be the poster children for the havoc that climate change could wreak on sensitive species, but animals and plants in the tropics could actually be in the greatest peril from global warming, a new study suggests.

While temperature changes in the tropics are expected to be much less extreme than those at higher latitudes, tropical species actually have a far greater risk of extinction from just a degree or two of warming, according to the results of the study, detailed in the May 5 issue of the journal Proceedings of the National Academy of Sciences.

Tropical species are accustomed to living within a much smaller temperature range than species at temperate and higher latitudes, so once temperatures exceed that range, many hot-zone species might not be able to cope, the authors said.

"There's a strong relationship between your physiology and the climate you live in," said study team member Joshua Tewksbury of the University of Washington. "In the tropics many species appear to be living at or near their thermal optimum, a temperature that lets them thrive. But once temperature gets above the thermal optimum, fitness levels most likely decline quickly and there may not be much they can do about it."

This threat to tropical species is particularly worrisome because, "unfortunately, the tropics also hold the large majority of species on the planet," said study team member Curtis Deutsch of the University of California, Los Angeles.

Tewksbury and Deutsch, who was a postdoctoral researcher at UW when the study was done, took temperature records from 1950 to 2000 and climate model projections from the Intergovernmental Panel on Climate Change for the first years of the 21st century and compared them to data describing the relationship between temperature and fitness for a variety of temperate and tropical species, including insects, frogs, lizards and turtles. Their research was funded in part by the National Science Foundation and the UW Program on Climate Change.

How a given species would fare in a warming world appeared to depend more on how a temperature change would affect population levels and a species' physical performance than the actual amount of warming predicted for where they lived.

Some tropical species can now shield themselves from the heat of the day by sitting under a shady leaf or burrowing into the soil. But if they are already living close to their critical high temperature, a slight increase in air temperature could make staying out of the sun a futile exercise, Tewksbury said. Warming may simply come too fast for the creatures to adapt.