Why does species diversity vary so much?

The diversity of life varies predictably with climate and is greatest where it is warm and wet (the humid tropics). But the question “why” has puzzled biologists for over a century. In the December issue of Ecology Letters, Currie and colleagues examine three hypotheses about the origin of climatic gradients of diversity.

The “Species-energy” hypothesis proposes that high tropical plant productivity allows more species to maintain populations large enough to escape extinction. However, there is little evidence that the tropics contain enough extra individuals for this mechanism to work. A second possibility suggests that more species tolerate forgiving tropical climates. Yet, many more species can tolerate harsh climates than observed. Something else limits richness. Finally, the tropics have been a hotbed of evolution of diversity.

Could this generate observed gradients? The evidence is still thin, but it suggests that higher rates of evolutionary diversification may magnify climatic gradients of richness.