The importance of phylogeny in predator-prey body-size relationships

Body size is a major factor constraining the structure and functioning of organisms. It affects growth, reproduction, space use, abundance, and trophic position of organisms. Moreover, the ratio of body sizes between predators and their prey plays an important role in explaining who eats whom. Using a unique global database on consumer and resource body sizes, Brose et al. (Ecology 2006) showed that the relative size differences between predators and prey rise with increasing size of the species. Taking into account the phylogeny of predators and prey we reanalyzed the database. First of all, average predator-prey body-size ratios differ among and within taxa of predators and prey. Second, the relative size differences between predators and prey decreased, instead of increased, with increasing body size in predator taxa. However, the size differences between predators and prey increased with larger prey taxa. This asymmetry between predators and prey was confirmed by the analysis of intermediate species, i.e. taxa that preyed and were preyed. The body-size range of intermediate species’ prey was also smaller than that of their predators. Altogether, this indicates that within taxonomic groups predators are more constrained by the size of their prey than vice versa. Our results emphasize the importance of species’ phylogeny to understand food web structure. Organisms’ evolutionary history in concert with their current ecological environment explain the organization of natural communities.