Host dietary niche and site location on the river continuum shape trematode (Renifer aniarum) infection patterns in sympatric watersnakes (Nerodia spp.).

Although parasite probability of infection and intensity are central to understanding parasite distributions and their ecological and evolutionary impacts, the drivers of these parameters remain poorly understood. At a local scale, we examined how host dietary niche partitioning and river landscape processes influence infections of the trematode Renifer aniarum in a community of watersnakes (Nerodia spp.). Host-specific dietary preferences and body size (SVL) were associated with infection probability among sympatric congeners. However, contrary to predictions of the stream drift hypothesis, infection probability increased with distance upstream. Infection intensity, in contrast, was unrelated to stream position and less predictable across host species. Thus, infection probability and intensity were heterogeneous among host species and across the river system. These results highlight that, even at local scales, multiple factors can distinctly shape the infection dynamics of a generalist parasite across closely related, co-occurring hosts.