Distribution of Energy Use and Biomass Among Species of North American Terrestrial Birds

Abstract

The distribution of biomass and energy use among species with different body sizes provides an empirical basis for studying ecological processes that determine species diversity. Biomass and energy use distributions were determined for North American terrestrial birds from data on population density and body mass of 380 species and data on energy use obtained from the literature. Using these data, several hypotheses regarding the specific form of biomass (summed for all species in a body size category) and energy use distributions were evaluated. Biomass continued to increase in successive log body mass intervals, but this was not due simply to increasing species numbers. Energy use initially increased in these same intervals but leveled off above a body mass of °80 g. Energy used by average populations of individuals species was uniformly distributed between the lower and upper bounds of each log body mass interval. In addition, the upper boundaries on biomass and energy use for individual species paralleled closely the biomass and energy use distributions. Qualitatively similar patterns were obtained for plant— and animal—eating birds considered separately, and for birds in 14 arbitrarily defined subregions of the North American continent. There were important quantitative differences among energy use distributions for the 14 subregions. Subregions at lower latitudes had energy use distributions that were nearly an order of magnitude higher than those of regions at higher latitudes. These results imply that previous hypotheses to explain biomass and energy use distributions were not of sufficient generality to account for both similarities among distributions of very different systems (e.g., birds and aquatic plankton) and spatial variation among systems composed of similar species. A more general hypothesis should consider the importance of inherent physiological constraints on energy use and environmental limitations on energy availability. The process that influence resource allocation in a large assemblage of many species may result in statistical patterns of energy use and biomass that tend to maximize ecological quantities analogous to entropy in statistical physical systems.