Resource distribution drives arthropod community responses to habitat edges

Although many studies have described species responses to habitat edges, community responses to habitat edges and experimental studies to elucidate the mechanisms driving such responses are rare.  We studied the effects of habitat edges between the two dominant grass species that are found in an intertidal salt marsh, Spartina alterniflora (SA) and S. patens (SP), on the dependent arthropod community.  Although host plant quality naturally increased near habitat edges, we did not find a corresponding increase in the herbivore abundance, even though herbivores in this system are severely nitrogen-limited.  This response is likely driven by the positive response of hunting spiders to the SA/SP edge and their subsequent top-down effects on herbivores.  Hunting spiders are far more effective in catching herbivore prey in SA and their positive response to the habitat edge was likely mediated by the structural resources (dead biomass or thatch) present in SP that provide protection from intraguild predation and cannibalism.  We therefore manipulated habitat structure (via removal or addition of thatch) to determine the effects of resource complimentarity (i.e. prey in SA, structure in SP) on hunting spider abundance and novel species interactions.  We found that thatch provided a critical resource for hunting spiders and where these intraguild predators were more abundant, they altered arthropod species composition. Because edge-associated interactions predominate as habitats shrink in size, our research has general significance for understanding how habitat loss will affect arthropod diversity and composition in these critically important intertidal habitats that act as buffers to adjacent estuaries.