Abstract
Predation and competition can induce important density‐ and trait‐mediated effects on species, with implications for community stability. However, interactions of these factors with parasitism remain understudied. Here we investigate interactions among competition, predation and parasitism by crossing tadpole density ( Bufo americanus ), presence of a caged predator ( Notophthalmus viridescens ), and Echinostoma trivolvis trematodes, experimentally partitioning their effects on tadpole exposure and susceptibility to infection. Predation did not affect E. trivolvis infection but accelerated tadpole development and growth, and decreased activity. The presence of E. trivolvis caused the opposite effects on these three responses and reduced tadpole survival. High conspecific density reduced tadpole survival, growth, and development, and increased tadpole activity. Effects of predation and parasitism on activity were only evident at low tadpole density. High‐density mesocosms also had twice the number of E. trivolvis infections as low‐density mesocosms, despite a lack of evidence for stress‐induced immunomodulation. Instead, this effect was explained by high density delaying tadpole development, which increased both the duration of exposure to cercariae and susceptibility to infection, because tadpoles spent more time in highly susceptible early stages. These results highlight the importance of accounting for trait‐mediated effects, host plasticity, and exposure vs. susceptibility in parasite ecology.