Gaps in Mammalian Body Size Distributions Reexamined

Holling suggested that discontinuities in the body size distributions among species of animals are a universal feature of terrestrial biomes. We compared the magnitudes of body size gaps of mammal communities of North America and Australia to those generated by a simple random null model. In most biomes, no gaps were significantly larger than random, so discontinuities in body size distributions are the exception, not the rule. We also made intraand intercontinental comparisons of size distributions to test two alternative hypotheses: (1) Holling's Textural-Discontinuity Hypothesis, that body size distributions reflect structural characteristics of the habitat; and (2) the Core-Taxa Hypothesis, that body sizes reflect the distributions of widespread taxa. We found that the gaps in body size were similar in structurally dissimilar but adjacent biomes that shared the same or closely related species. We conclude that body size distributions of biomes are not highly discontinuous, and their structure reflects taxonomic constraints on body size.