Interplay between individual growth and population feedbacks shapes body-size distributions

Body size in contemporary ecology Body size and variation in body size have formed the focus of many studies in ecology, ranging from the study of individual performance to large-scale communities and ecosystems (Werner & Gilliam, 1984, Gaston & Lawton, 1988, Werner, 1988, Cohen, Johnson & Carpenter, 2003, Brown et al., 2004, Loeuille & Loreau, 2005). This focus is well-founded given the large variation in body size that exists among organisms from micro-organisms to large mammals (Gaston & Lawton, 1988; Werner, 1988). Body size is also the most important trait that affects the performance of individuals. Basic ecological capacities such as foraging rate and metabolic requirements are close functions of body size (Peters, 1983; Kooijmann, 2000; Brown, et al., 2004) affecting, for example, competitive abilities of differently sized organisms (Wilson, 1975; Persson, 1985; Werner, 1994). Body size strongly influences the diet of consumers with mean prey size, but also the variation in the size of prey eaten, increasing with predator size (Wilson, 1975; Werner & Gilliam, 1984; Cohen et al., 2005; Woodward & Warren, this volume; Humphries, this volume). Furthermore, the risk for an organism being preyed upon is heavily influenced by its own body size as well as the body size of its potential predator (Polis, 1988; Werner, 1988; Claessen, De Roos & Persson, 2000). Given its influence on basic individual ecological processes, body size has been an important variable in the investigation of larger ecological entities including communities, food webs and ecosystems. For example, predator–prey size ratios have formed the basis for food-web models such as the cascade model (Chen & Cohen, 2001), and for estimating interaction strengths in food webs (Emmerson & Raffaelli, 2004). Body size has also been the key variable in the analysis of food-web patterns with regard to numerical and biomass abundance at different trophic positions (Cohen et al., 2003; Cohen, this