Patch shape alters spider community structure: links between microhabitat choice and sensitivity to increased edge habitat

Increased edge effects in fragmented habitats can affect the abundance of edge-dwelling organisms, but these impacts may depend on the biological attributes of species. Microhabitat choice, a species characteristic that reflects combinations of biological traits, may affect the ability of peripheral species to take advantage of increased edge habitat in the presence of edge effects. In this field study, we built artificial shrub modules designed to encourage web spiders to build webs on the periphery. While modules were identical in volume, they differed in shape (cubic and elongated), so that elongated modules had more edge habitat and were subject to enhanced edge effects. Given that the tangle-web spiders Theridion and Dictyna built webs on module edges and strongly differed in terms of concealment and substrate generalization, two habitat characteristics associated with lower vulnerability to habitat modification, we tested the hypothesis that Theridion, which built webs in more concealed locations and on a greater diversity of substrate configurations in the modules compared to Dictyna, would take better advantage of increased edge habitat. As predicted, Theridion was significantly more abundant on elongated modules whereas the abundance of Dictyna did not respond to shape, even though the change in module shape entailed a similar increase in favored substrate for both spider groups. Our results suggest that the microhabitat associations of organisms may be linked to their propensity to be sensitive to edges, and that a better understanding of these links can improve our ability to predict the effects of habitat modification on biodiversity.