Predators and pathogens (i.e. natural enemies) are fundamental components of ecological communities that have been extensively studied in ecology. Although these groups have traditionally been examined in isolation, natural enemies have the potential to influence each other within the community via their interactions with hosts or prey and initiate density- and trait-mediated effects. We examined the interactive effects of predatory dragonflies and a viral pathogen, ranavirus, on larval amphibians (gray tree frogs and northern leopard frogs) with a semi-natural mesocosm experiment. First, we examined whether caged predators reduced the activity level and contact rates of larvae (trait-mediated effect) thereby reducing ranavirus transmission. Then we examined whether free-ranging predators reduced ranavirus transmission by reducing host density and reducing host behavior (density- and trait-mediated effects). Lastly, we examined how natural enemies influence trophic cascades in our system. More specifically, we quantified the response of periphyton to changes in the density and traits of hosts (primary consumers).
Results/Conclusions
In the presence of caged predators, we observed a 48% reduction in tadpole activity compared to the controls. This behavioral alteration influenced the probability of ranavirus infection. In the presence of caged predators, we detected a 20% and 54% reduction in infection prevalence in tree frogs and leopard frogs, respectively, compared to tanks exposed to virus and without caged predators. Our results demonstrate a trait-mediated effect of predators on this host-parasite interaction. The presence of free-ranging predators had strong effects on tadpole behavior and survival. We observed a 70% reduction in tadpole activity compared to the controls. The survival of both species was reduced by 80% with predators compared to the control. As a consequence of behavioral and density changes associated with free-ranging predators, infection prevalence was 61% and 89% lower in surviving tree frogs and leopard, respectively. Lastly, we detected cascading effects of natural enemies on periphyton biomass. The presence of predators (caged or free-ranging) and ranavirus resulted in 62–132% greater periphyton biomass. Collectively, these results demonstrate that the presence of multiple natural enemies in a community can initiate trait- and density-mediated effects that alter host-parasite interactions and influence the magnitude of trophic cascades.