Seeing Double: Size-Based and Taxonomic Views of Food Web Structure

Here, we investigate patterns in the size structure of one marine and six freshwater food webs: that is, how the trophic structure of such ecological networks is governed by the body size of its interacting entities. The data for these food webs are interactions between individuals, including the taxonomic identity and body mass of the prey and the predator. Using these detailed data, we describe how patterns grouped into three sets of response variables: (i) trophic orderings; (ii) diet variation; and (iii) predator variation, scales with the body mass of predators or prey, using both a species- and a size-class-based approach. We also compare patterns of size structure derived from analysis of individual-based data with those patterns that result when data are "aggregated" into species (or size class-based) averages. This comparison shows that analysis based on species averaging can obscure interesting patterns in the size structure of ecological communities. Specifically, we found that the slope of prey body mass as a function of predator body mass was consistently underestimated and the slope of predator prey body mass ratio (PPMR) as a function of predator body mass was overestimated, when species averages were used instead of the individual-level data. In some cases, no relationship was found when species averages were used, but when individual-level data were used instead, clear and significant patterns were revealed. Further, when data were grouped into size classes, the slope of the prey body mass as a function of predator body mass was smaller and the slope of the PPMR relationship was greater compared to what was found using species-aggregated data. We also discuss potential sampling effects arising from size-class-based approaches, which are not always seen in taxonomical approaches. These results have potentially important implications for parameterisation of models of ecological communities and hence for predictions concerning the dynamics of ecological communities and their response to different kinds of disturbances.