Predation on parasites and its consequences for transmission

Predation and parasitism are each important ecological processes within communities and ecosystems. However, the interactions between predation and parasitism may have significant consequences for transmission dynamics and disease in host populations. Predators play multiple roles including changing host densities, behavior, and morphology that could lead to different disease outcomes. Furthermore, direct predation on parasites can potentially lead to reduction in disease risk to host populations. Here, I used a combination of mathematical modeling, small-scale laboratory studies, and a semi-realistic mesocosm experiment to characterize parasite transmission dynamics, provide a predictive framework for the role of direct predation on parasites, and evaluate the direct and indirect (trait-mediated interactions) effects of multiple predators on transmission and infection. My study system included the trematode parasite, Ribeiroia ondatrae, larval amphibian hosts, and a suite of invertebrate and vertebrate species that co-occur in nature. Using laboratory experiments and maximum likelihood approach, I characterized non-linear functions as the most accurate representation of transmission. These models capture the saturation of infection at high exposure levels and subsequent experiments with anesthetized hosts suggest that parasite behavior may be an underlying mechanism for nonlinear relationships. Next, I identified damselfly larvae and juvenile mosquitofish (Gambusia affinis) as predators of R. ondatrae and California Newts (Taricha torosa) as alternative hosts. In transmission trials, both damselflies and newts reduced transmission by ~50% through the independent mechanisms of consumption and infection. Additional experiments including a wider range of parasite species showed that predation on parasites is based on predator foraging mode (active vs. ambush) and body size, parasite size and light availability. In more realistic aquatic communities with multiple trophic levels, I found that the trait-mediated indirect effects of predators of parasites and predators of hosts, including reduced activity and morphological changes were associated with higher infection compared to communities with predators absent. My research demonstrates both the direct and trait-mediated roles of predation on disease dynamics and identifies direct predation on parasites as an important factor in transmission dynamics. Future research should evaluate patterns of predator diversity and abundance with the prevalence and pathology due to R. ondatrae in nature