THE RELATIONSHIP BETWEEN ABUNDANCE AND BODY-SIZE IN NATURAL ANIMAL ASSEMBLAGES

1. Studies of the relationship between body weight and population abun dance for animal species based on pooling data from many taxa and asse mblages suggest that abundance scales with weight to the -0.75 power ( e.g. Damuth 1987). Since metabolic rate scales with weight as (plus) 0 .75, this result has been taken as evidence that all species in assemb lages used equal amounts of energy. The evidence for 'energetic equiva lence' is, however, equivocal, because within many individual assembla ges the scaling of abundance on weight differs significantly from -0.7 5. 2. Here, we examine the relationship between body size (weight and/ or length) and abundance in nine previously unpublished animal assembl ages, and five previously published assemblages for which data were av ailable, to answer three questions: (i) What form does the relationshi p between size and abundance within assemblages usually take? (ii) How good is body size as a predictor of abundance? (iii) Is there evidenc e that species abundances in assemblages may be constrained by per cap ita energetic requirements? 3. Twelve of the 14 assemblages show a neg ative relationship between log body size and log population abundance, but the proportion of variance in abundance which body size explains is always low. Plots of the relationship tend to be polygonal. 4. Regr ession slopes for most assemblages differ significantly from the predi ctions of energetic constraint models: most species in these assemblag es cannot be energy limited. However, the most abundant species near t he upper bound slopes of assemblages may be energy limited, because in only one assemblage does the upper bound slope differ from energetic model predictions. 5. Two of the assemblages were compiled from 12 sub samples. Tested individually, the subsamples show the same patterns as the assemblages themselves. 6. We conclude that within natural assemb lages, the relationship between body size and abundance is usually pol ygonal, with species body size being a very poor predictor of species abundance. Abundances of most species in assemblages are not, apparent ly, constrained by energetic requirements, but the commonest species i n assemblages may be.