Explaining the global biodiversity gradient: energy, area, history and natural selection

Hubbell's neutral theory of biodiversity is used to investigate the decline in species richness from the tropics to the poles. On this basis, biodiversity should correlate with productivity or climate (there is strong statistical evidence for this), with the latitudinal width of the continents (insufficiently investigated as yet), and with the speciation rate (which may not vary in such a way as to produce a planetary gradient). According to the neutral, model, biodiversity will vary with the area of the metacommunity: it is suggested that at higher latitudes species disperse most readily east-west, within their climatic belt, but that the relatively uniform temperature across the intertropical belt allows isotropic dispersal there. Metacommunities within the tropics may therefore be an order of magnitude larger than those at other latitudes. This could explain the extra bulge in the gradient in the tropics. It is further possible that long-term and cyclical climate change generates a tropic-pole gradient. Niche assembly models wilt also explain tropical biodiversity, but the enhanced division of habitat may be the result, not the cause, of the species richness. The neutrality-competition debate in ecology closely parallels the neutrality-natural selection debate in evolution and may be equally hard to resolve. (C) 2004 Elsevier GmbH. All rights reserved.