Professor University of South Carolina Columbia, South Carolina, United States
Abstract: Trophic interactions can be critical in shaping population dynamics. Their inherent nature may be amplified by weather, thus mediating the strength of trophic interactions. Intraguild predation is a widespread, yet overlooked, trophic interaction that ties together competition and predation. For instance, the cascading effects of herbivory by large mammals have been well documented to affect herbivore population dynamics via changes in plant chemistry. However, a key component that is often overlooked in this system is that mammalian herbivores unintentionally consume insects as they feed on plant parts. We used piecewise structural equation modelling (PSEM) to answer the question, what are the direct and indirect effects of intraguild predation and weather, respectively, on the vital rate of pre-diapause survival for Euphydryas gillettii (Lepidoptera: Nymphalidae). We included the proportion of egg masses eaten and ungulate population estimates each year. We also included weather variables hypothesized to drive variation in the vital rate, intraguild predation, and ungulate population size. All variables were then run through an automatic model selection tool and tested for temporal autocorrelation before being used in the analysis. The results of our PSEM (Fisher’s C = 28.84, p = 0.91) provide support for our hypothesis that the direct consumptive effects of intraguild predation are greater than those of weather. Further, ungulate population density and instances of intraguild predation may respond to weather differently. For instance, intraguild predation could not be explained by any hypothesized weather variable, researcher impact, nor a density-dependent effect. Pre-diapause survival is, however, affected by intraguild predation and ungulate population density independently of each other. There is no clear trend suggesting ungulate population density is a consistent driver of variation in the vital rate. We have shown here that a single trophic interaction, independent from weather, has a stronger effect on a vital rate than abiotic factors. A holistic approach that examines the relevance of both biotic and abiotic drivers is required to understand what factors drive population persistence and growth.
