Conclusions
Our results urge caution when implementing gape-limited biocontrol in parasite transmission systems that meet similar conditions to these prawns, snails, and schistosomes. Resource uptake by intermediate hosts that increases transmission, intense resource competition among hosts, and size-selective predators are all ingredients for potential biocontrol failure that may also worsen with greater effort. Here, we illustrate how resource competition as an underlying mechanism pervades trophic levels and population changes to drive potentially counterintuitive outcomes in the control of a major human parasite. Overcompensation as a key element of these systems is emerging as a recurrent theme in the ecology of pest control (Choisy and Rohani 2006; Bolzoni, Real, and De Leo 2007; De Roos et al. 2007), resulting in a growing list of scenarios that suggest ‘do or do not’ management responses. Importantly, we emphasize that not adopting this strategy does not suggest predatory biocontrol is hopeless, but rather weak or inconsistent control efforts seem unlikely to succeed. One critical management suggestion is that control programs should start small so that effort can be sustained and gradually expanded, rather than spread too thin. Ultimately, we stress the clear need for more experimental testing of predator biocontrol of human schistosomes and encourage more physiology and ecology detail in pest management more broadly. These details legitimise the biological data needed for successful control design that can extend to other host-parasite systems.