Energy, taxonomic aggregation, and the geography of ant abundance

Ecological communities and their component populations vary geographically in abundance. Energy theory posits that abundance (the number of individuals area-1) should increase with the ratio of available energy (as net primary productivity, NPP) to individual energy use (i.e. metabolic rate). Most tests of energy theory evaluate the assumption that population abundance decreases as body mass-0.75 (a proxy of metabolic rate). Using 664 ant populations from 49 communities we examine how both NPP and body mass individually and as a ratio predict abundance (colonies m-2) at these different levels of taxonomic aggregation. Energy theory best predicts ant abundance when populations are aggregated into communities. At the population level, abundance formed a unimodal scatter plot vs all three drivers colony mass, NPP, and NPP mass-0.75 suggesting that the majority of populations exist below energetic limits set by the ecosystem. At the community level, however, abundance scaled as predicted for mass-0.75 and NPP1.0, (b =-0.75 and 1.0, respectively) and was a positive decelerating function of their ratio (i.e. [NPP mass-0.75]0.61, r2= 0.68). Since geographic trends in colony mass and abundance are largely reciprocal deserts tend to support few large colonies, and tropical rainforests support many small colonies the geography of ant biomass (g m-2) is remarkably invariant.