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.
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