Hypotheses and trends on how body size affects trophic interactions in a guild of South American killifishes

A chief structuring force in food webs is the hierarchy of trophic interactions, where bigger animals feed on smaller ones. The anatomic and physiological explanations of why body size determines this hierarchy are embodied within the concept of gape limitation. The relaxation of gape limitation and an increase in energetic demands due to predators' larger body size determine the size and diversity of prey species. However, these patterns may be related to further trends in trophic interactions with body size, which have been less considered. Specifically, the passive incorporation of prey should involve a nested distribution of prey among predator size classes. However, predators avoid smaller resources because of their low energy return, with a clumped distribution of prey potentially generating modular organization with qualitative changes in prey identity (e.g. zooplankton, macroinvertebrates and fishes). Finally, size‐mediated interactions (such as direct and indirect competition) may cause predators of similar body size to differentiate among prey organisms, resulting in a checkerboard distribution of prey identity. Consequently, nestedness, modularity and checkerboard distributions of prey among predators of different size classes should form emergent network structures that are directly related to clear ecological mechanisms. We analyse these predictions in a killifish guild, where trends in trophic positions, prey richness, evenness and the number of energy sources systematically scale with body size. We found significant nestedness and segregation in diet among different size classes, supporting the progressive incorporation of prey items coupled with prey differentiation among similar classes. However, we also detected an ‘anti‐modular’ trend, which contradicts theoretical expectations and previous results. We hypothesize that this anti‐modularity is determined by the high biodiversity of the system and the continuous representation of prey size classes. These results reinforce the concept of size‐mediated interactions and its connection with community biodiversity as a main structuring force of food webs.