Ecological theory suggests that predators keep herds healthy by decreasing parasitism in prey. This phenomenon can result from selective predation on sick individuals and declines in host densities due to predation. However, empirical tests of this prediction paint a more complicated picture. Not only do predators often appear to have minimal effects on parasitism in their prey, but predators also frequently increase the abundance and prevalence of parasites in prey. These opposing effects are likely due to the wide range of non-consumptive effects that predators can have on prey which may increase or decrease parasitism. For example, non-consumptive effects such as the elevation of prey stress levels can result in suppression of immune function and changes in prey space use behavior can increase parasite transmission in some systems but decrease it in others. We used a meta-analysis of 71 empirical comparisons from 55 studies to determine both the overall effect of predators on parasite prevalence or abundance in prey and the effect of predator interaction type on this response. These differences, and any consistent trends in this response, are likely to be useful for predicting the effects of predator loss or reintroduction in natural systems.
Results/Conclusions
Preliminary results indicate that the average effect of predators on parasites in prey, across studies, was not significantly different from zero (p = 0.6). This result masked an enormous amount of residual variation between studies (I2 = 81.75%; Q = 231.78, p < 0.0001), some of which can be explained by a single important feature of study design: whether predators were given physical access to prey (QM1 = 11.396; p = 0.0007). When predators were not able to directly eat prey, the net effect of their indirect effects on parasites was positive. However, when predators had full access to prey individuals, the combined direct and indirect effects on parasites was negative. These results match our general understanding of the basic ecology of predator-prey systems in that we find consumptive effects to be primarily (though far from universally) negative. While the net positive effect of non-consumptive interactions suggests that processes such as predator induced immunosupression, which increase parasite susceptibility and transmission, are the most common results of non-consumptive interactions.