The negative effects of prey pathogens on predators
Intraguild predation (IGP) occurs when two predators competing for the same resource consume one another. Theory predicts that low quality resources drive communities to exclusion of both predators, while high quality resources may exclude the bottom predator. This suggests many communities exist with medium quality resources. While IGP commonly occurs in nature and has been used in biocontrol, previous research and meta-analyses focused on strict predator-predator interactions. However, a large number of potential IGP communities may occur where predators and pathogens compete for a shared resource. In these systems, infection may alter resource quality.
Specifically, we asked how IGP predators were affected by pathogen-infected prey (low quality) compared to healthy prey (high quality). We conducted a meta-analysis to investigate how healthy or infected prey affected predator life-history traits (e.g., survival). Using the ISI Web of Knowledge, we found 50 studies with lepidopteran pests and their predators interacting with pathogens. We used lepidopterans because a large body of work is available from biocontrol efforts. For each life-history metric considered, we recorded the mean, standard deviation, and sample size for predators exposed to healthy and predators exposed to infected prey. We also recorded the type of predator and the type of pathogen.
The results showed that predators were negatively affected by consuming the low quality pathogen-infected prey. Overall, life span was reduced by 26%, fecundity was reduced by 30%, and survival was decreased by 13%. Results varied within predator- and pathogen-type. When consuming infected prey, parasitoids suffered reduced life spans and fecundity. Given the consequences of consuming low-quality resources, parasitoids showed a preference for healthy over infected prey. Predators had reduced life spans and survival, but showed no preference for or against pathogen-infected prey. Pathogen type also affected which life history metric was impacted. Fungus-infected prey increased developmental time and decreased life span, while virus-infected prey decreased fecundity and survival.
Prey quality has varying impacts on predators, which may have long-term consequences with regards to system stability. Parasitoids and pathogens are likely to be simple competitors rather than intraguild predators, as parasitoids preferred healthy prey. Predators have no preference and may decrease in pathogen abundance. Since pathogen-infected prey represent low-quality prey, while healthy prey are high-quality prey, the system may ultimately be stable given that the resources on average are of medium quality. While the meta-analysis points to potential outcomes in IGP communities, work needs to be done to test these predictions.