Top-Down Regulation of Litter Invertebrates by a Terrestrial Salamander

Terrestrial plethodontid salamanders are abundant predators within the forest floor litter of eastern North America, and are hypothesized to regulate soil and litter invertebrate density and species composition. I tested this hypothesis during a 6-yr study of the effects of the Eastern Red-backed Salamander (Plethodon cinereus) on the invertebrate community of a forest site in northeast Ohio. Salamander surface density, invertebrate abundance, and community composition were monitored within 30 open, circular plots. Variation in plot occupancy by P. cinereus was achieved by supplying plots with differing amounts of artificial cover (0, 1, or 4 ceramic tiles) that served as refuges for the salamanders. Salamander plot occupancy, invertebrate density, leaf litter mass, and leaf litter moisture were quantified each spring and fall from 2003 through 2008. Statistically significant effects of salamander plot-occupancy on invertebrate densities were found for several taxa of mesofauna, including several Collembola taxa, oribatid mites, pseudoscorpions, and psocoptera. The strength and direction of salamander effects varied among taxa and included negative, positive, and no effects on invertebrate densities. The magnitude and sign of salamander effects on invertebrate densities were predicted by seasonal and interannual variation in leaf litter mass and, to a lesser extent, litter moisture content. Salamander effects decreased with increasing litter mass and were more often negative when litter mass was high, whereas positive effects on invertebrate densities were more likely when litter mass was low. For several taxa, the positive effect of P. cinereus also increased with litter moisture. I propose two mechanistic hypotheses for these dynamics that integrate behavioral ecology of salamander prey selection and territorial defense with variation in litter mass and litter moisture.