Competition and host size mediate larval anuran interactions with trematode parasites

How parasites influence individual host traits and survival often depends on the ecological context of the host-parasite interaction, such as the presence of competitors or predators and trait variation among hosts. We examined the effects of three key components of ecological context - host density, size structure and predator cue - on interactions between larval frogs and trematode parasites (Digenea: Echinostomatidae) in mesocosms. We found that effects of parasites on host growth could be either negative or positive, depending on host size and overall growth rate, but not on predator presence. A surprising positive effect of parasites on host growth under some conditions could represent an adaptive host life history response, whereby enhanced growth allows escape from a smaller, less tolerant size class that experiences more negative fitness effects of infection. Notably, only host size class was a strong predictor of infection intensity, but not host density or predator cue. Overall, these results suggest that parasitism, competition and host size interact to influence host fitness. Ecological context thus mediates the interactions between parasites and their hosts, with implications for parasite effects in nature.