Habitat edge effects alter ant-guard protection against herbivory

Edge effects are among the most important drivers of species interactions in fragmented habitats, but the impacts of edge effects on multitrophic interactions are largely unknown. In this study we assess edge effects on species interactions within an ant–plant mutualistic system—where ants protect plants against herbivory—to determine whether habitat edges alter the amount of protection ants provide. We focus on a single species of myrmecophytic plant, Solanum americanum, and experimentally manipulate ant access to study plants in large-scale fragmented habitat patches at the Savannah River Site National Environmental Research Park, USA. In this system, S. americanum commonly hosts honeydew-producing aphids that are tended by ants, and grasshoppers are the primary herbivores. We measured edge effects on the per-plant abundance of aphids and protective ants as well as the abundance of grasshoppers in each habitat patch, and we evaluated levels of ant protection against herbivory near and far from habitat edges. We found that ants provided significant protection to plants far from edges, where herbivory pressure was highest, despite the fact that aphids and ants were least abundant on these plants. Conversely, ants did not provide significant protection near edges, where herbivory pressure was lowest and aphids and ants were most abundant. We conclude that a strong edge effect on grasshopper abundance was a key factor determining the amount of protection ants provided against herbivory. Future studies of the impacts of habitat fragmentation on ant–plant mutualisms will benefit from studies of ant behavior in response to herbivory threats, and studies of edge effects on other species interactions may also need to consider how species’ behavioral patterns influence the interactions in question.