Resource distribution and species interactions drive trophic responses to a habitat edge

Although many studies have described species responses to habitat edges, experimental studies to elucidate the mechanisms driving such responses are rare. We experimentally manipulated the habitat edge between two dominant grass species in the intertidal salt marsh, Spartina patens (SP) and S. alterniflora (SA), in order to understand the mechanisms leading to increased predator abundance, and decreased prey abundance, along the habitat edge. Because a decline in herbivores along the habitat edge could not be explained by a change in host plant quality, we manipulated habitat structure (by adding or removing dead Spartina material or “thatch”) to determine whether such declines were driven by increased abundance of hunting spiders (Pardosa littoralis). Thatch offers Pardosa a structural refuge from cannibalism and decreases intraguild predation. We found that Pardosa density increased along the habitat edge and with thatch addition, but thatch was only important early in the season when the risk of cannibalism was greatest. The densities of planthopper herbivores declined along the habitat edge, but thatch removal only increased the densities of SP specialists. While habitat edges did not affect the densities of Tytthus egg predators, thatch removal significantly decreased Tytthus alboornatus densities. The risk of predation by Pardosa therefore increased in more structurally complex habitats for SP planthopper herbivores, but decreased for intraguild prey such as Tytthus egg predators. Because edge-associated interactions predominate as habitats shrink in size, our research has general significance for understanding how habitat loss will affect species interactions and diversity in these critically important intertidal habitats.