Understanding how anthropogenic stressors, such as chemical contaminants, shape species interactions will become increasingly important as human populations continue to grow. The effects of contaminants on host-parasite interactions have received significant recent attention. Many studies have demonstrated that contaminants can alter host-parasite outcomes via direct and indirect effects on both hosts and parasites. However, the majority of this work has focused on evaluating the effects of contaminants on host-parasite interactions in isolation. In nature, hosts and their parasites are embedded within complex ecological communities, and interactions with other community members (i.e. predators) may lead to density- or trait-mediated effects that modify the effects of contaminants on host-parasite interactions. Therefore, using an amphibian-trematode model, we conducted an experiment asking 1) Does NaCl exposure alter tadpole susceptibility to a common parasite? and 2) Do density- and trait-mediated effects of predators modify the effects of NaCl exposure on infection outcomes? To address these goals, we conducted a 2 (tadpole exposed to 0 g L-1 NaCl vs. 1 g L-1 NaCl) x 4 (no predator, dragonfly cue, damselfly, both dragonfly cue and damselfly) factorial experiment. We quantified tadpole activity levels and trematode infection across all individuals.
In the absence of predators, tadpoles exposed to NaCl were 170% more susceptible to trematodes than tadpoles not exposed to NaCl. Similarly, in the presence of damselflies, tadpoles exposed to NaCl were also more susceptible to trematodes than tadpoles not exposed to NaCl (200% increase in trematode susceptibility). However, in the damselfly treatment, we found a 52.5% reduction in overall trematode susceptibility compared to tadpoles not exposed to predators. This reduction was due to consumption of trematodes by the damselfly. In contrast, in the presence of the dragonfly cue, exposure to NaCl did not alter tadpole susceptibility to trematodes. This effect was largely driven by the dragonfly eliciting a 75% increase in overall trematode susceptibility compared to the no predator treatment. This increase was driven by a 26% reduction in tadpole activity in the dragonfly cue treatment compared to tadpoles not exposed to predators. Similarly, NaCl exposure did not influence trematode susceptibility in tadpoles exposed simultaneously to both predators. Tadpoles exposed to both predators exhibited an intermediate susceptibility to parasites compared to tadpoles individually exposed to damselfly or dragonfly cue. Collectively, these findings underscore the importance of considering community-level interactions toward understanding the effects of contaminants on host-parasite interactions.