Intraguild predation among snails drives human schistosome amplification despite susceptible host regulation

Competitors and predators of hosts have the potential to alter transmission dynamics within host- parasite systems. Biocontrol aims to harness these effects to mitigate diseases of human or agricultural importance, but without a detailed understanding of the mechanisms of how hosts and non-hosts interact over time these attempts may backfire. We investigated the role that two resource competitors that have been employed in biocontrol, Melanoides tuberculata and Physa acuta , play in the Biomphalaria glabrata - Schistosoma mansoni system . We created experimental communities with differing compositions in mesocosms and tracked resource availability, host snail abundance, size, reproduction, and parasite production over 16 weeks. We found that Melanoides acted as a typical, albeit weak, resource competitor of Biomphalaria, reducing host body size and per capita cercarial production but with no appreciable influence on host abundance. Physa , however, reduced host abundance but increased resource availability and cercarial production, which represents biocontrol failure and is inconsistent with solely acting as a resource competitor. In follow-up experiments, we determined that Physa is a voracious consumer of host eggs. We then built a model representing this intraguild predation effect, which was able to explain these initially counterintuitive results. These divergent results from two putative resource competitors of host snails underscore the importance of establishing the mechanisms through which hosts, non-hosts, and parasites interact, especially for anticipating the potential outcomes of biocontrol interventions.