Ecological interactions within and between species determine the structure and dynamics of ecological communities and their responses to the environment. Understanding and predicting the community-level consequences of size-dependent interactions such as competition, predation and cannibalism is therefore central to ecological theory and ecosystem management. Nevertheless, it has rarely been studied how intraspecific processes can affect interspecific interactions with other species in the food web. Here, we investigate the community-level effetcs of cannibalism in structured populations with density-dependent maturation and reproduction. We model a stage-structured consumer population with an ontogenetic diet shift to analyze how cannibalism alters the conditions for the invasion and persistence of predators that are specialized on a specific consumer life-stage or competitors that are specialized on a shared resource population.
We show that cannibalistic interactions can facilitate coexistence with other species at different trophic levels. This effect of cannibalism critically depends on the food dependence of the demographic processes, and the presence of stage structure in the consumer population. The underlying mechanism is a cannibalism-induced shift in the biomass distribution between life-stages. Thus, cannibalism in stage-structured populations that undergo ontogenetic diet shifts can alter the structure of ecological communities through its effects on species coexistence. These findings suggest that explicit consideration of intraspecific interactions at the level of life-stages may be crucial for better understanding the functioning of ecological communities, for instance in the context of fisheries management.