The influence of cannibalism and size structure on the dynamics of aquatic communities

Cannibalism is a common ecological interaction in the animal kingdom, and it is ubiquitous in aquatic and terrestrial food webs. The role of cannibalism in food webs, however, has been largely neglected and we still lack the basic knowledge of how cannibalism affects the dynamics and structure of natural communities. Using multiple sets of structured mathematical models I show that the presence of cannibalism can completely alter the dynamics and structure of simple two- or three-species food webs depending on the trophic position where cannibalism occurs. In general, cannibalism changed the conditions for species coexistence and strongly stabilized the dynamics of simple communities. The results also show that the lethal and the behavioral-mediated indirect interactions resulting from cannibalism in the predator can strongly alter how top-down and bottom-up cascades affect the structure of communities. In two field experiments with salamander larvae I demonstrate that cannibalism in the top-predator results in both, density (lethal) mediated indirect interactions between conspecific and heterospecific prey and behavioral (non-lethal) mediated indirect interactions between large cannibalistic predators and their conspecific prey. Both interaction types significantly altered the mortality of the prey. The impact of cannibalism in the prey was studied in two field experiments using odonate larvae. The first experiment showed that the functional response of a predator was strongly altered by the size-structure and cannibalistic interactions in the prey. The functional response could only be predicted with a composite functional response that accounted for the individual size specific mortality rates of each prey size class and the cannibalistic interactions among prey size classes. The second experiment showed that cannibalism in the prey resulted in different density-and behavioral-mediated indirect interactions. When the predator was present these indirect interactions strongly reduced cannibalism and predation rates which reduced the net effect of the predator on the mortality of the prey. In conclusion, this study suggests that the indirect interactions resulting from cannibalism can strongly alter the dynamics and structure of communities. Only if we explicitly account for the consequences of size-structured cannibalism will we be able to reliably predict the structure and functioning of natural communities.

Spine title: Cannibalism and community dynamics