A critical assessment of the form of the interspecific relationship between abundance and body size in animals

1. Despite a wealth of studies for a wide variety of animal assemblages, the form of the interspecific relationship between abundance and body size is still contentious. At least three different patterns have been suggested, which can broadly be characterized as negative and linear, negative but non-linear, and polygonal. At least eight different mechanisms have been suggested whereby the linear (or non-linear) and polygonal patterns can be reconciled. 2. We collated data from the literature on over 500 interspecific plots of the abundance-body size relationship with two aims. First, to examine the extent to which published studies support the different forms proposed for the relationship; and secondly, to test whether any of the mechanisms that have been suggested to reconcile linear and polygonal patterns actually do so. 3. The data revealed that abundance-body size relationships commonly assume both linear negative and polygonal forms. Around 25% of all plots of the relationship show a positive regression slope. 4. Of the eight mechanisms that have been suggested to reconcile linear and polygonal patterns, we were able to test five. Of these, only the measure of density used by a study explains none of the observed variation in abundance-body size relationships. Variation in the regression slope between studies is only explained by the type of data used(compilations vs. samples) and the scale of study (local vs. regional): compilation studies at regional scales show more linear negative relationships, while sample studies performed at local scales show more polygonal patterns. General linear modelling indicates that study scale is the most important factor influencing when different relationships are likely to arise. 5. Our results show that different patterns tend to arise at different scales of study, but say nothing about whether the patterns are real or artefactual. Polygonal relationships potentially contain an artefactual component, resulting from sampling methodology inadequate to elucidate the abundances of the less common species in any given assemblage. However, the presence of a sampling artefact does not indicate the shape of the underlying relationship from which a sample is taken, or indeed whether samples would exhibit any other shape were artefacts excluded. 6. Linear and polygonal patterns are not necessarily mutually exclusive, but may both indicate the 'true' abundance-body size relationship at different spatial scales. We conclude by suggesting that much more attention be paid to the effect of spatial scale on this relationship, especially given that scale of measurement can have subtle but severe consequences.