WASHINGTON (AP) - While global warming is expected to be strongest at the poles, it may be an even greater threat to species living in the tropics, scientists say.

Tropical species are accustomed to living in a small temperature range and thus may be unable to cope with changes of even a few degrees, according to an analysis in Tuesday's edition of Proceedings of the National Academy of Sciences.

"There's a strong relationship between your physiology and the climate you live in. 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," said Joshua J. Tewksbury, an assistant professor of biology at the University of Washington.

Concern over global warming has largely focused on arctic species like the polar bear. But such animals are accustomed to living in a wide range of temperatures, while there tends to be little change in the tropics, so there has been no need for species there to adapt.