Abstract
The relationship between body mass and abundance is a major focus for research in macroecology. The form of this relationship has been suggested to reflect the partitioning of energy among species.
We revisit classical datasets to show that size - density relationships vary systematically among taxonomic groups, with most variation occurring at the order level. We use this knowledge to make a novel test of the ''energy equivalence rule'', at the taxonomic scale appropriate for the data.
We find no obvious relationship between order-specific exponents for abundance and metabolic rate, although most orders show substantially shallower (less negative) scaling than predicted by energy equivalence. This finding implies greater energy flux among larger-bodied animals, with the largest species using two orders of magnitude more energy than the smallest.
Our results reject the traditional interpretation of energy equivalence as a predictive rule. However, some variation in size -density exponents is consistent with a model of geometric constraints on foraging.