More than More Individuals: The Nonequivalence of Area and Energy in the Scaling of Species Richness

One of the primary ecological hypotheses put forward to explain patterns of biodiversity is known as the more-individuals hypothesis of species-energy theory. This hypothesis suggests that the number of species increases along the global energy gradient primarily as a result of an increase in the total number of individuals that can be supported along that gradient. Implicit in this hypothesis is that species richness should scale with energy in the same way in which it scales with area in species-area relationships. We developed a novel framework for thinking about the interaction of area and energy, and we provide the first global test of this equivalence assumption using a data set on terrestrial breeding birds. We found that (1) species-energy slopes are typically greater than species-area slopes, (2) the magnitude of species-area and species-energy slopes varies strongly across the globe, and (3) the degree to which area and energy interact to determine species richness depends on the way mean values of species occupancy change along the energy gradient. Our results indicate that the increase in richness along global productivity gradients cannot be explained by more individuals alone, and we discuss other mechanisms by which increased productivity might facilitate species coexistence.