Do generalists always like edges? Habitat structure influences generalist predator responses to habitat edges in a salt marsh system

Background/Question/Methods:

One of the main generalizations that has emerged from the fragmentation literature is that generalists tend to increase in fragmented habitat, while specialists decline.  This response is associated with access to resources, which are often more widely separated for generalists.  In a previous study of the arthropod community between two intertidal salt marsh grasses: Spartina alterniflora (SA) and S. patens (SP), we found that, in line with general theory, a generalist predator increased along the habitat edge, while all specialists (predators and prey) declined.  Furthermore, this response by generalist predators was not driven by increased prey abundance near edges.  Edges can also differ in structural complexity which may offer non-consumptive resources, and greater habitat complexity has previously been shown to reduce intraguild predation and cannibalism among salt marsh predators.  We therefore designed a manipulative experiment to examine the role of habitat complexity on multiple generalist predators at habitat edges.   We removed or added thatch (dead grass that increases complexity) to both sides of the SA/SP habitat edge and examined generalist predator responses.  Since the density of most prey is lower at these edges, we predicted that generalist predators that increase near edges would respond positively to thatch addition. 

Results/Conclusions:

Responses of generalist predators to habitat edges differed according to hunting strategy, and groups that demonstrated a positive edge response also responded positively to thatch addition.  Hunting spider density increased along habitat edges, and similar to our predictions, we found that hunting spider densities increased in thatch addition relative to removal plots.  While hunting spiders demonstrated positive edge responses, web-building spiders were either unaffected or negatively affected by habitat edges, and we found that thatch additions did not impact web spider densities.  Predator hunting strategy was a much stronger predictor of edge responses relative to microhabitat usage.  Aerial and ground-dwelling predators either demonstrated no response or a negative response to habitat edges, and we found that thatch addition either did not affect or only weakly affected predator densities in these groups.  Prey densities generally declined along the habitat edge and in thatch addition plots, so the positive response of hunting spiders to habitat edges and increasing structural complexity is likely to be driven by a reduction in antagonistic interactions.  While numerous studies have found that generalist predators positively respond to habitat edges, here we demonstrate that non-consumptive resources such as protection from intraguild predation and cannibalism could drive this response.