Body Size, Population Density, and the Energetic Equivalence Rule

1. We critically evaluated the Energetic Equivalence Rule (EER) with regard to its underlying theoretical framework, its dependence of a particular relationship between population density and body size and the way it is calculated. 2. We found it to be theoretically incorrect, not supported by empirical data on maximum population density and body size relationships, and that its calculation is misleading. In particular, we found a general pattern underlying the allometry of maximum population density, which stresses the correspondence between the documented linear patterns and triangular relationships between both variables. 3. Two independent data sets, one for mammals and another for intertidal invertebrates, showed that medium-size species attain higher population densities, and that population density decreases towards larger and smaller sizes. 4. A more direct calculation of population energy use for mammalian primary consumers, based on measured metabolic rates, showed that populations' energy use fluctuates widely among species and that its upper limit is not independent of body size but peaks at a body size of about 100 g, and decreases toward both smaller and larger body sizes. 5. Diet also has a strong effect on this relationship. Omnivores show a positive relationship between population energy use and body size, while this is negative for carnivores and insectivores.