Non-metabolic explanations for the relationship between body size and animal abundance

1. Metabolic constraints are the usual explanation for the relationship between body size and species abundance in natural assemblages of animals. In some assemblages, abundance scales with body weight to the -075 power. Metabolic rate scales as weight raised to the (plus) 0-75 power, therefore, on average equal amounts of energy are available to each species in a community. This equality has been taken as evidence that a species' abundance is limited by its energetic requirements. 2. Criticisms of these arguments notice that most species in samples of complete assemblages cannot be energy limited (although the most abundant species, at the 'upper bound' may be). These arguments also ignore the frequency distributions of species' body size and species' abundance which underlie abundance versus size plots. Both frequency distributions have theoretical explanations, which combined could explain abundance versus size patterns independently of metabolic arguments. 3. We test the hypothesis that concatenating the underlying frequency distributions of species' body size and species' abundance can directly account for observed patterns in plots of size versus abundance in assemblages of animals. We compare the negative slope of the upper boundary of plots of size against abundance from real assemblages, with the same slopes derived from models with no energy constraints. It is species at this upper boundary that are most likely to be energy limited. 4. Our models give estimates for the upper bound slopes that are very similar to slopes calculated from real assemblages. We produce realistic patterns in plots of abundance versus size without recourse to metabolic arguments. 5. We conclude that patterns of abundance versus body size in natural assemblages may not be constrained by species' energy requirements, and do not require explanations independent of those for the constituent frequency distributions of size and abundance.