Another clue to where the species are.

From Greenland to Guatemala, Antarctica to Africa, one of the most striking and consistent patterns of life on this planet is the greater profusion of species the farther one gets from the poles. Though the pattern may be obvious, the explanation for it is not-climate, available space, and the vagaries of geologic history have all entered into discussions of species richness, with no clear resolution.

Several analyses, particularly a large-scale study in 1991 of species richness in North America, have focused on energy as the key. Closer to the equator, more solar energy is available for photosynthesis, the first link in the food chain.

A new statistical analysis of the data from that study has come up with an interesting twist: The energy explanation applies only about as far south as the Canadian-U.S. border. For the rest of the continent, at least for distributions of mammal species, local differences in habitat exert the greater influence.

"The energy-species richness relationship breaks down in warm regions," says biologist Jeremy T. Kerr of York University in North York, Ontario. Kerr and York biologist Laurence Packer report their findings in the Jan. 16 Nature. The researchers noticed a marked change in the scatter of data points on a graph relating mammal species richness to potential evapotranspiration, a measure of energy based on how much water would evaporate from a surface. The change corresponds to a zone just south of the Canadian-U.S. border. North of this zone, in energy-limited Canada and Alaska, evapotranspiration remained the statistically most important determinant of mammal species diversity. On the rest of the continent, other environmental variables became key-namely, topography and local (rather than continental) variations in evapotranspiration.

The result makes sense not only statistically but intuitively, says Kerr. "At the same latitudes in Canada, the diversity is [generally] the same, east to west." In the United States, where solar energy is more abundant, the mammals of the Southwest are more diverse than those of the Florida panhandle. The Southwest, for example, has many more rodent species than Florida does. When you go to Arizona or Florida, it's quite hot, says Kerr. "There's no shortage of energy for organisms to use." So local habitat differences such as elevation become more important for the greater species richness. John Downing, an ecologist at Iowa State University in Ames, finds the new analysis to be an "interesting articulation" of the 1991 data, but he's not convinced that the strong energy-species richness connection of the earlier analysis can be discounted, particularly since it considered several groups of organisms, not just mammals.

"Some of the species richness has to do with spatial variability, but it's not the whole story," Downing says. That will come only with more scanning of the "wide screen" in ecology.