Foraging activity of a dominant epigeal predator: molecular evidence for the effect of prey density on consumption

Despite the widely held assumption that generalist predators consume most prey available to them, there is a growing body of evidence suggesting otherwise. Generalists are expected to perform well in disturbed areas because they can switch between prey pathways when one food source becomes depleted. Indeed, these predators have the potential to promote diversity by switching to prey in a frequency dependent manner and consume prey groups in relation to local abundance. It is therefore important to understand how predation rates fluctuate as local availability changes. We performed open-field and mesocosm experiments in a corn and soybean agroecosystem to delineate the role prey density plays in determining predation frequency of a dominant epigeal predator. To track trophic pathways, molecular gut-content analysis using enzyme-linked immunosorbent assay (ELISA) was performed to track foraging behavior of the wolf spider Pardosa milvina feeding on dipterans, flies. Extensive monitoring of foraging activity and prey populations revealed that predation varied temporally. Importantly, the frequency of individuals testing positive for flies was lower than predicted when flies were extremely abundant but higher when they were scarce, relative to the prey community as a whole. Furthermore, isolating predators in mesocosms revealed an effect of Diptera density on the likelihood of consumption, as determined by ELISA, only when flies were at low levels (12.5% of prey provided). The molecular results suggest that these spiders do not appear to be consuming flies in a frequency-dependent manner where the decision to switch between different prey pathways is driven by relative abundance. Rather, selectivity of prey is somewhat independent of variation of other prey groups, which is indicative of their consistent reliance on dipterans and may be related to nutritional requirements and/or capture success.