Parasites are ubiquitous, and species that are hosts to parasites have evolved myriad defenses against them. Parasites in turn have evolved enhanced offenses to parasitize their hosts. Although tightly coupled, these pair-wise interactions are embedded within a rich mosaic of other species interactions. Consequently, host-parasite coevolutionary dynamics may vary spatially in response to variation in the intensity of other species interactions, such as competition and predation. Here we use a host (damselfly (Odonata: Zygoptera)) parasite (water mite (Arrenurus spp.)) system focusing on three target species, Enallagma basidens, E. signatum, and Ischnura posita, to test the hypothesis that variation in competition and predation affect host-parasite interactions. First, we estimated the probability of parasitism for each species across natural predator densities, and for Enallagma species, conspecific density gradients. Second, we estimated levels of anti-parasite immune defenses across these same gradients.
Results/Conclusions
E. basidens was more likely to be parasitized at high larval density, though no such relationship existed between fish density and infection status. E. signatum and I. posita parasitism decreased from low to high fish density lakes. Like E. basidens, increases in larval density had a positive, though stronger correlation with infection status for E. signatum. Increased infection at higher densities may be driven by increased intraspecific competition, as high larval densities are associated with lower resource availability and increased cannibalism - odonates wound one another when attempting to feed, activating the immune response. Activation of the immune response during the larval stage may leave the hosts unable to mount a strong immune defense against parasites as adults. The lower parasitism in high fish density lakes is likely due to increased predation releasing the hosts from intraspecific competition and reducing host density. These results provide insight into how competition and predation mediate host-parasite interactions. As such, they will lead to development of a framework for predicting parasitism based on complex food web interactions.