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 assemblage s, the form of the interspecific relationship between abundance and bo dy size is still contentious. At least three different patterns have b een suggested, which can broadly be characterized as negative and line ar, negative but non-linear, and polygonal. At least eight different m echanisms have been suggested whereby the linear (or non-linear) and p olygonal patterns can be reconciled. 2. We collated data from the lite rature on over 500 interspecific plots of the abundance-body size rela tionship with two aims. First, to examine the extent to which publishe d studies support the different forms proposed for the relationship; a nd secondly, to test whether any of the mechanisms that have been sugg ested to reconcile linear and polygonal patterns actually do so. 3. Th e 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 mec hanisms that have been suggested to reconcile linear and polygonal pat terns, we were able to test five. Of these, only the measure of densit y used by a study explains none of the observed variation in abundance -body size relationships. Variation in the regression slope between st udies is only explained by the type of data used(compilations vs. samp les) and the scale of study (local vs. regional): compilation studies at regional scales show more linear negative relationships, while samp le studies performed at local scales show more polygonal patterns. Gen eral 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 artefac tual component, resulting from sampling methodology inadequate to eluc idate the abundances of the less common species in any given assemblag e. 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 ex cluded. 6. Linear and polygonal patterns are not necessarily mutually exclusive, but may both indicate the 'true' abundance-body size relati onship at different spatial scales. We conclude by suggesting that muc h more attention be paid to the effect of spatial scale on this relati onship, especially given that scale of measurement can have subtle but severe consequences.