Individual and combined effects of natural enemies on amphibian communities
Natural enemy ecology strives to integrate the fields of disease ecology and community ecology to forge a broader understanding of how pathogens and predators structure communities. To advance this field, we need a greater emphasis on: 1) quantifying pathogen-mediated effects on community structure and comparing these effects to those observed with predators and 2) determining the interactive effects of combined natural enemies on communities. I conducted a mesocosm experiment designed to assess the individual and combined effects of predators (dragonfly larvae and adult water bugs) and a pathogen (ranavirus) on a larval amphibian community. Additionally, I conducted laboratory experiments to assess whether ranavirus exposure increases the vulnerability of tadpoles to predation. In my laboratory experiments, I found that virus exposure increased predation rates with dragonflies, but not water bugs. For tadpoles in the dragonfly treatments, the probability of survival for virus-exposed tadpoles was 66-77% lower compared to unexposed tadpoles. This data suggests that predators may selectively remove infected individuals from the population, which can enhance the magnitude of the healthy herds effect. I found that the risk level of the predators largely explained effects on the community. For instance, high-risk dragonflies reduce overall survival to 30% whereas low-risk water bugs only reduced survival to 67%. Additionally, I found that virus reduce survival to 62%, which was comparable to effect of the low-risk predator. Interestingly, all three natural enemies influenced community structure (i.e. species relative abundance) in unique ways. These results demonstrate that pathogens can have effects similar to predators on communities, and that natural enemy identity is important when considering impacts on community structure. When predators were combined with the virus, I found that mortality was relatively unchanged from the predator-only treatments suggesting less than additive effects of combined natural enemies. This result was driven by the healthy herds effect; the presence of dragonflies reduced overall infection prevalence in the community to 7% compared to 30% in the virus-only treatment. This effect was observed in the water bug treatments, to a lesser degree, suggesting that predator risk or efficiency contributes the magnitude of the effect. Collectively, my work demonstrates the importance of examining the individual and combined effects of natural enemies on ecological communities.