Selective predation, parasitism, and trophic cascades in a bluegill-Daphnia-parasite system

As disease incidence increases worldwide, there is increased interest in determining the factors controlling parasitism in natural populations. Recently, several studies have suggested a possible role of predation in reducing parasitism, but this idea has received little experimental attention. Here, I present the results of an experiment in which I manipulated predation rate in large field enclosures to test the effects of predation on parasitism using a bluegill predator-Daphnia host-yeast parasite system. Based on previous work showing high bluegill sunfish selectivity for infected over uninfected Daphnia, I anticipated that predators would reduce infection levels. Contrary to expectations, predation did not reduce infection prevalence. Instead, there were large epidemics in all treatments, followed by reductions of host density to very low levels. As Daphnia density decreased, phytoplankton abundance increased and water clarity decreased, suggesting a parasite-driven trophic cascade. Overall, these results suggest that selective predation does not always reduce infection prevalence, and that parasites have the potential to drastically reduce host densities even in the presence of selective predators.