Cannibalism in the prey: Emergent indirect interactions, mortality risk reduction, and consequences for predator-prey dynamics

Intraspecific interactions within the prey population can strongly affect the dynamics of predator-prey systems. Cannibalism is a common intraspecific interaction and cannibalistic species are commonly subject to predation themselves. Yet, previous studies have generally focused on cannibalism in the predator and the consequences of cannibalism in the prey are virtually unknown. This study used a field experimental approach with two species of dragonfly larva to test what indirect interaction effects emerge from cannibalism in the prey and how cannibalism alters the impact of a predator on prey mortality. Cannibalism in the prey reduced the impact of the predator on the prey mortality by 47% due to behavioral (trait)- and density-mediated indirect interactions resulting from cannibalism. Based on the treatments, a mechanistic predation model was developed and parameterized to determine the relative importance of each indirect effect and to predict the observed mortality. The model showed that the non-lethal effect of cannibals on the predation rate and the non-lethal effect of the heterospecific predator on the cannibalism rate each explained two times more of the observed reduction in mortality rates than the consumption of cannibals. Only a full model that accounted for all three indirect interactions explained 100% of the observed mortality. This indicates that it is important to account for the trophic structure within the prey population resulting from cannibalism and to treat such systems as a multi predator-prey interaction with all the possible indirect interactions and their emergent effects and non-linearity.