Temperature and diversity

The diversity (species richness) of plants and animals is typically highest in the tropics and the strongest environmental correlate of species richness is often climate. The energy for plant production is sunlight, but the rate is governed jointly by temperature and the availability of water (as captured by actual evapotranspiration, AET). Greater production is then linked to higher diversity because larger population size protects against stochastic extinction (the more individuals mechanism). A greater biomass and diversity of plants allows for a greater diversity of herbivores and so on through the food web, though the correlation with climate (AET) gets progressively weaker at higher trophic levels. This is the basis of the species-energy theory of diversity. The Metabolic Theory of Biodiversity posits a mechanistic explanation for higher diversity in warmer places mediated through an enhanced generation of mutations as a by-product of the faster metabolic rate associated with a higher body temperature. Evidence for this is equivocal, and this mechanism cannot explain the strong association between endotherm species richness and climate. The striking differences between the northern and southern hemispheres point to an important role for history, particularly recent glacial history, in influencing current patterns of diversity. We still lack a comprehensive theory of biological diversity, but evidence points to a complex series of factors being important, with the dominant ones being energy and time (history).