Spillover predation describes predators traversing adjacent habitats to take advantage of multiple prey sources, a large-scale form of apparent competition. Bats and spiders are consumers in agroecosystems and it is unclear if these same predators eating emergent insects along agricultural ditches spillover into adjacent corn fields to take advantage of crop pests. Another important question is how predator mobility and energy demand influence spillover? In this study, we hypothesized that predators capable of mobility like bats and wolf spiders exhibit spillover from agricultural ditches into fields while non-mobile predators like web spiders feed in one habitat at a time. A series of surveys were conducted at ten different farms where bats were recorded, spiders counted, and insects captured at ditches, at 100 and 200 meters into adjacent corn fields to understand how active and abundant the predators were at increasing distances into the fields. In addition to these surveys, we conducted stable isotope analysis on wolf and web spiders (collected at all distances) and bat guano collected from nearby barns. Since ditch and crop insects come from vegetative communities with distinct photosynthetic pathways (corn=C4 vs ditch plants=C3), carbon isotope ratios of tissues should reflect what prey communities predators are consuming.
Results/Conclusions
Carbon-13 ratios from bat guano were mixed, indicating that the highly mobile predators may consume a combination of crop and ditch insects. It is expected that bat calls will be recorded at all distances, potentially tapering off with increasing distance into the field, if spillover is occurring. Spiders from deep in the cornfield had high enrichment of carbon-13, showing heavy consumption of crop pests. Ratios for spiders at ditches were mixed, indicating that ditches may have more diverse prey than assumed. Nonetheless, spiders appear to capitalize on whatever prey is within their immediate vicinity. The isotopic data in this study suggests that highly mobile predators exhibit spillover predation, while less mobile predators eat what’s closely available. If improved ditch management fosters insect emergence, it may increase subsequent spillover of bats into adjacent cornfields, reducing both crop pests and the need for pesticides. This would lessen the pollution of agricultural ditches that flow into waterways leading directly into Lake Erie.