Habitat edges have interested ecologists since ecological theory was in its infancy. Although many studies have described species responses to habitat edges, mechanisms driving such responses are poorly understood. Moreover, experimental manipulations of edge variables are extremely rare, yet such studies are essential for elucidating the mechanisms that govern species’ responses to habitat edges. In the intertidal salt marsh, two dominant grass species, Spartina alterniflora (SA) and S. patens (SP) grow in natural monocultures, leading to extensive habitat edges that form between these two species. Contrary to many habitat edges, host plant quality naturally increases along the SA/SP habitat edge; however, we do not find a corresponding increase in herbivore abundance even though herbivores in this system are severely nitrogen-limited. This dampening of herbivore response was likely driven by the accumulation of generalist predators along the habitat edge; in particular, we found higher densities of hunting spiders along the habitat edge relative to interior habitats. While SA provides hunting spiders with higher densities of prey which they are more effective in capturing, the resources provided by SP appeared to be primarily structural. The dense layer of dead plant material or thatch in SP provides a structural resource that decreases cannibalism and intraguild predation in hunting spiders. We therefore tested whether predator accumulation along the habitat edge was the result of complimentary prey and structural resources between SA and SP. We manipulated habitat structure by removing or adding thatch to adjacent SP and SA plots along the habitat edge and examined effects on hunting spiders, herbivores, and overall arthropod community structure.
Results/Conclusions
We found that the addition of thatch positively affected hunting spider density, negatively affected herbivore density, and altered arthropod composition. Additionally, we found that the habitat edge had a stronger effect on the arthropod community found in SP relative to SA. These results have important implications for the SP community where habitat loss is increasing, primarily due to a rise in mean tidal height. Because edge-associated interactions predominate as habitats shrink in size, our research also has general significance for understanding how habitat loss will affect arthropod diversity and composition.
