While trophic cascades are relatively rare in most terrestrial ecosystems, theory predicts that where trophic cascades occur, predators are likely to be subsidized by sources external to the focal food chain. Previous studies in an intertidal salt marsh dominated by the grass Spartina alterniflora have found that the intraguild predator Pardosa littoralis (a hunting spider) induced trophic cascades. We sought to determine whether prey subsidies from channels outside of the Spartina food web provided Pardosa with both spatial and temporal subsidies that could either increase Pardosa densities, or sustain Pardosa when Spartina prey resources were not available. To determine whether Pardosa utilized different prey resources through space or time, we examined the trophic niche of Pardosa using stable isotopes of nitrogen and carbon. We used stable isotopes of nitrogen to determine whether Pardosa utilized prey from different trophic positions over: 1) the course of the growing season and 2) across an elevational gradient in the salt marsh. Likewise, we used stable isotopes of carbon to determine whether Pardosa utilized prey from different channels outside of the Spartina food chain.
Results/Conclusions
While δ15N values for Pardosa remained relatively consistent through time and across different habitats, we found significant temporal and spatial variation in Pardosa δ13C values. Early in the season, Pardosa δ13C reflected both algal and Spartina-based prey resources, and as the season progressed, Pardosa began to utilize prey resources from Spartina more exclusively. Across the elevational gradient, the diet of Pardosa reflected spatial differences in resource distribution, with the importance of the algal food base declining at higher elevations on the salt marsh. These results demonstrate that the intraguild predator Pardosa littoralis is utilizing multiple sources of prey, both on a temporal and a spatial scale. Our results thereby provide support for the prediction that predators capable of inducing trophic cascades in terrestrial systems are subsidized by prey resources from outside the focal food web.
