Cannibalism and competition can increase parasite abundance for parasites with complex life history strategies

Understanding how parasite transmission is affected by ecological interactions among hosts is critical for predicting disease dynamics. For instance, theory predicts that intraguild predation (IGP) and cannibalism negatively impact parasite populations, but this is based primarily on assumptions of simple or single host life cycles. Here we investigate the effects of cannibalism on two digenean trematodes that have complex multi-host life cycles. Contrary to theory, high cannibalism among paratenic hosts produced higher parasite abundances, whereas cannibalism among obligate hosts reduced parasite abundances as predicted. We attribute this difference to trophic transfer of parasites among paratenic hosts via cannibalism. Moreover, we found evidence of indirect competitive interactions between parasites that can also increase infections at small scales. Our results show that there are multiple mechanisms through which high cannibalism environments can benefit parasites that use paratenic hosts and trophic transfer to complete their life cycles.