How do species really divide resources?

We show that the relationship between variance in population energy use and variance in species abundance depends critically on the model of causal relationships among metabolic rate, body size, and population density assumed, provided that they specify alternative paths of error propagation. It has been claimed that the allometric relationship between population density and body size indicates that species within communities use resources less equitably than would be inferred from a particular species abundance distribution. Our analysis of 41 local bird communities shows that this claim is supported only if it is assumed that both body size and metabolic rate are a consequence of population density. A more realistic model that assumes a causal role for body size as affecting metabolic rate and population density provides estimates of variance in population energy use that closely match the pattern of variance in population density. This implies that the apportionment of individuals and resources, among species, are equivalent processes.