Parasites are ubiquitous in nature, and species that are hosts to parasites are locked in an endless eco-evolutionary battle with these enemies. Although tightly coupled, host-parasite interactions are embedded within a rich mosaic of other interspecific interactions such as predation, competition, and mutualisms, in addition to intraspecific interactions such as competition and reproduction. The fitness consequences of these species interactions are thought to vary in response to the effects of parasites on host organisms, with parasitized individuals suffering negative effects relative to their non-parasitized counterparts. We conducted a meta-analysis focusing on the effects of parasites on species interactions, to test the hypothesis that parasites negatively affect the outcome of species interactions. First, we searched the literature for all studies investigating differences between control and parasitized treatments in experiments. Second, we calculated effect sizes to understand the magnitude and direction of the effect of parasites on the fitness components (survival, competitiveness, and fecundity) of organisms involved in species interactions.
Results/Conclusions
The magnitude and direction of the effect of parasites on species interactions varied and was dependent on the type of species interaction, host organism, and parasite type. Despite this variation, we found a general trend of parasites negatively impacting all fitness components of infected organisms. Parasitized individuals tended to be less competitive, less fecund, and have lower survival than controls. Non-parasitized control treatments performed better than their parasitized counterparts across all study types (predation, competition, mutualisms, and reproduction). These results provide insight into how parasites mediate species interactions, and as such they provide a framework for predicting the outcome of diverse species interactions when parasitism is occurring concomitantly. We caution that further research must account for the effects of parasites on focal organisms in any study of species interactions.