Effects of Moderators
Given the level of variation in the effect of predators on parasites in prey we were interested in identifying attributes of the study or study system that were most important for explaining variation in effect sizes across studies. To do this, we fit mixed effects models (MEM) to the prevalence and intensity effect size data sets, including a series of moderators: predator effect type manipulated (non-consumptive vs. all interaction types), predator-spreader identity (identified as predator-spreader or not), and parasite type (macro vs. micro) and all two-way interactions. We note that because studies in which the predator was described as a viable host for the parasite were excluded all predator-spreaders are non-host spreaders. Study design (experimental vs. observational) was also included as a moderator to control for variation in responses but without a particular hypothesis. We also included study as a random effect. We note that while we were interested in the distinction between non-consumptive and consumptive effects, most consumptive effect studies technically allowed for both non-consumptive and consumptive interactions due to limitations in experimental design. Therefore we draw the distinction between studies which manipulate only non-consumptive interactions and those which include consumptive interactions (all interaction types). From this model, we generated candidate sets of all possible MEMs for each data set and used the Akaike information criterion corrected for sample size (AICc) to compare model fit. We calculated the importance (on a scale from 0 to 1) of each moderator as the summed model weights for all MEMs in which a given moderator occurred. We then fit univariate models for each moderator to identify the direction of the effect. When reporting the results of univariate models for the most important variables, we provided the direction of the effect of the moderator and the results of a test for residual variation. Because parasitoid studies were uniformly terrestrial, experimental, and consumptive, we did not fit MEMs with moderators to these data.