Within communities, pathogens and parasites have the potential to indirectly influence predator-prey interactions. For instance, prey that exhibit pathology or alter traits (e.g., behavioral shifts) following exposure could be more prone to predation, which is known as parasite-induced vulnerability to predation (PIVP). PIVP has been documented for pathogens with trophic transmission because predators are often critical in the pathogen’s life cycle. However, for pathogens without trophic transmission, PIVP can lead to a healthy herds effect, reducing transmission in the system. In this study, we explored whether the pathogen ranavirus (Family Iridoviridae) enhances vulnerability of four species of larval amphibians, Spring Peepers (Pseudacris crucifer), Gray Treefrogs (Hyla versicolor), American Toads (Anaxyrus americanus), and Northern Leopard Frogs (Lithobates pipiens), to two common tadpole predators, larval Green Darners (Anax junius) and adult water bugs (Belostoma flumineum). For each anuran species, we conducted short-term microcosm experiments to assess predation rates on unexposed or exposed individuals to virus. To evaluate whether the change in predator success is mediated by a behavioral shift in ranavirus-exposed tadpoles, we performed a behavioral videography experiment using larval Wood Frogs (Lithobates sylvaticus). The larvae were exposed or unexposed to ranavirus and recorded daily to observe behavioral changes across fourteen days.
Results/Conclusions
For three of the four species, we found that exposure to ranavirus decreased survival rates with Anax by 66 to 77%. However, we did not see the same trend with Belostoma, which indicates that predator identity is important to this interaction. More specifically, the higher efficiency of Anax in capturing and consuming prey, relative to Belostoma, may allow them to capitalize on trait changes induced by virus exposure and enhance the PIVP effect. This study indicates that trait-mediated indirect effects could play a role in creating healthy “herds” in amphibian communities. In our behavioral videography experiment, we found that exposed tadpoles had a 45% increase in percent activity and distance traveled compared to unexposed tadpoles. These behavioral changes induced by ranavirus could increase the detectability of tadpoles by movement-based predators such as Anax. Collectively, our work suggests that behaviorally-mediated PIVP could play an important role in driving the healthy herds effect in amphibian communities.