The role of body size in predator-prey interactions and community structure

Abstract

Body size is a fundamental property of an organism. Consistent body size-related patterns, relevant to food webs, have been found at the individual (e.g. metabolism), population (e.g. predator-prey interactions), and community (e.g. body-size scaling with abundance) levels of ecological organisation. These patterns represent key components of food webs, so body-size distributions should be representative of a wide range of food web processes. Therefore, knowledge on the controls of the distribution of body sizes within a community should aid in the understanding of community structure and stability. I focused on two key body size-associated relationships in food webs: the relationship between body mass/size (M) and abundance (N), and relationships between body size and predator-prey interaction strengths. Analysis of M-N data, collected from stream communities spanning habitat size and flooding disturbance gradients, indicated that habitat size limited top predator size, whereas disturbance limited their abundance. These results highlight how M-N relationships, and changes in body size patterns across communities in general, are particularly useful tools for understanding influences on community structure. Mesocosm experiments were used to investigate how relationships between predator-prey body mass ratios (PPMR) and predator-prey interaction strengths differed when prey defences and density varied. Larger predators generally had stronger per-capita interaction strengths however, prey morphological defences and prey abundance significantly altered the relationship between body size and per-capita interaction strengths. Defended prey were eaten less than undefended prey and the relationship between PPMR and interaction strength was steeper at higher prey densities. These results indicated that while PPMRs are generally good predictors of interaction strengths, the incorporation of other general traits, beyond body size, into current theory would improve prediction of interaction strengths in food webs. An in-stream channel experiment was conducted to investigate the population, compared to per-capita, effects of top-predator body size and abundance on the strength of top-down interactions in food webs. By manipulating top-predator size and abundance, but keeping top-predator (brown trout) biomass constant, I showed that smaller, more abundant top predators had greater top-down effects compared to, fewer large predators of equivalent biomass. Overall, my results indicate that body size-related relationships can be used to describe changes in predator-prey interaction strengths, community structure and by extension possibly community stability. However, the usefulness of these relationships could be improved by incorporating traits over and above body size, which would aid in the prediction of community stability as communities face ongoing anthropogenic pressures.