Cannibalism is a prevalent form of trophic interaction (Fox 1975; Polis 1981) that is commonly observed in predator populations. The outcome of cannibalistic interactions is expected to strongly influence the dynamics and structure of ecological communities (Rudolf 2007a,b), but our understanding of the ecological impacts of cannibalism and the underlying mechanisms remains limited. A better understanding of the ecological significance of cannibalism requires greater attention to the factors affecting the intensity of cannibalistic interactions. Several factors influence the intensity of cannibalism including population density, the availability of alternative prey and the size structure of interacting individuals (Elger & Crespi 1992; Rudolf 2008; Wissinger et al. 2010). In many cannibalistic interactions, size structure can be potentially important because the success of cannibals strongly depends on the size asymmetry between interacting conspecifics (e.g. the capturing organ of potential cannibals and the body size of potential victims (Kusano, Kusano & Miyashita 1985; Ohdachi 1994; Sogard & Olla 1994; Qin & Fast 1996)). Indeed, most theory on population and evolutionary ecology of cannibalism explicitly assumes that size structure plays a key role in cannibalistic interactions (Claessen, De Roos & Persson 2004; Rudolf, Kamo & Boots 2010).
Although the importance of size structure to cannibalism is well appreciated, it is important to note that cannibalism itself may operate to enhance size structure within populations (DeAngelis, Cox & Coutant 1979; Sogard & Olla 1994; Fangan & Odell 1996; Huss, Van Kooten & Persson 2010). For example, the energetic benefits of cannibalism can enhance size structure by increasing the growth rates of cannibals vs. noncannibals. This seems to be likely, when conspecific prey represent a richer nutrient source that is also easy to assimilate (Meffe & Crump 1987; Wildy et al. 1998) or when heterospecific prey are scarce in the environment. In addition, size-selective cannibalism can increase size structure within populations. For example, selective cannibalism on individuals in intermediate size classes can produce a bimodal size distribution within population. Hence, we suggest that while size structure promotes cannibalism, cannibalism may further drive asymmetries in size structure and such dynamics are fundamentally important to spatiotemporal variation in the size structure of cannibalistic populations. However, few empirical studies have explored the ecological importance of this positive feedback to short-term population dynamics (DeAngelis, Cox & Coutant 1979; Fangan & Odell 1996).
A growing body of work also demonstrates that the risk of being eaten can strongly influence trophic dynamics in natural communities via trait-mediated indirect interactions (reviewed by Schmitz et al. 2008). Such nonconsumptive predator effects are ubiquitous and can play an important role in cannibalistic systems by altering prey foraging behaviour (Rudolf 2008) and reducing the development of trophic polyphenisms (Kishida, Trussell & Nishimura 2009).
This study examined how cannibalism dynamics through time were influenced by the interaction between temporal changes in salamander size structure and predation risk imposed by dragonfly larvae. We found that cannibalistic interactions strongly depended on whether predation risk from larval dragonflies was present. In the absence of dragonflies, cannibalism was initially intense and the changes in salamander size structure caused by this interaction further intensified cannibalistic interactions through time. In contrast, this positive feedback was not observed in the presence of dragonfly predation risk. Hence, early behavioural responses to predation risk and subsequent developmental responses were keys to reducing the intensity of cannibalism.