To eat or not to eat infected food: A bug's dilemma
Interactions between parasitism and predation are important drivers of host population dynamics. Parasites can adversely affect host population densities, but predators can regulate disease by reducing the density of susceptible hosts and by consuming parasites contained in infected hosts. Parasites that rely on predation of their intermediate hosts to reach their definitive hosts often induce phenotypic modifications in their intermediate hosts that potentially lead to increased predation. In this case, selection is expected to act on definitive hosts towards avoidance of predation on infected intermediate hosts. However, theoretical and empirical evidence for such avoidance are rare. Here we investigated the role of parasite-modified appearance in the interaction among Daphnia magna, its bacterial parasite Pasteuria ramosa, and its predator, the Backswimmer Anisops sp. Our aim was to test the Backswimmer's prey selectivity between infected and uninfected D. magna under light and dark conditions.
In the light treatment the Backswimmers avoided infected prey and consumed almost twice as many uninfected individuals than infected ones. In contrast, the Backswimmers in the dark treatment preferred infected D. magna and consumed almost twice as many infected individuals than uninfected ones. This reversal in prey preference suggests that the modified appearance is the cause of this selectivity. Such anti-parasite selectivity by the predator could alter and perhaps strengthen parasite-mediated selection by increasing the density of infected hosts. In this case, selective predation by Anisops sp. could strongly influence Daphnia population dynamics as loss of fecundity, mortality from disease and predation may be additive or synergistic, resulting in a decline in the Daphnia population.