Although many studies have described species responses to habitat edges, experimental studies to elucidate the mechanisms that drive trophic responses to habitat edges are rare. We studied the effects of a habitat edge between two dominant grass species found in an intertidal salt marsh, Spartina alterniflora (SA) and S. patens (SP), on the dominant herbivore and predator species. Previous studies have demonstrated that although host plant quality naturally increases near habitat edges, the abundance of specialist, planthopper herbivores decline. Instead, planthopper responses to the habitat edge were likely driven by the positive response of generalist, intraguild predators (i.e., hunting spiders) to the habitat edge. In contrast to generalist hunting spiders, previous studies have shown that specialist egg predators (Tytthus vagus and T. alboornatus) demonstrate a negative response to SA and SP habitat edges, which was likely mediated by prey availability and/or avoidance of hunting spider predation. Because the presence of thatch shifts hunting spider predation from mesopredators (Tytthus egg predators) to herbivores, we manipulated thatch to examine the influence of hunting spider predation on trophic responses to the habitat edge.
We found that planthoppers demonstrated a negative response to habitat edges, and this effect was exacerbated in plots where thatch was present. Generalist hunting spiders responded positively to the SP/SA habitat edge and thatch addition, but this positive response diminished during the course of the growing season. This may be due to the fact that cannibalism in hunting spiders is more intense when spiderlings are more abundant earlier in the growing season, thus the need for structural refugia (i.e., thatch) is greater at this time. In contrast to generalist predators, specialist mesopredators demonstrated a negative response to the SP/SA habitat edge. The negative response of mesopredators was driven largely by prey availability; however, this response intensified in habitats where thatch removal left mesopredators exposed to increasing levels of intraguild predation. Our results demonstrate that both trophic interactions and resource availability are important predictors of species responses to habitat edge. Furthermore, because edge effects predominate as habitats shrink in size, our research has general significance for understanding how habitat loss will affect arthropod trophic composition in these critically important intertidal habitats.