Salamanders in forest-floor food webs: Invertebrate species composition influences top–down effects

SummaryThe red-backed salamander, Plethodon cinereus, is an abundant predator in northeastern North America, where it is hypothesized to play an important role in the regulation of forest-floor invertebrate communities. We tested the hypothesis that the top–down effects of P. cinereus are influenced by the species composition of invertebrates of the forest-floor. In a month-long microcosm experiment, we compared salamander effects on invertebrate species composition in leaf-litter from two sites that differed in species and size composition of invertebrate detritivores (an urban forest site at which isopods and millipedes were numerous and a rural forest site at which Collembola were the most numerous detritivores, but isopods and millipedes were few). Our results were consistent with our initial hypothesis. The presence of salamanders within microcosms was associated with decreased densities of oribatid mites, pseudoscorpions, isopods and millipedes in comparison to salamander-absent controls, but negative effects of salamanders on isopods and millipedes were most strongly expressed within urban litter microcosms. Salamanders were also associated with a positive, indirect effect, i.e., an increase in density of podomorphic Collembola in comparison to salamander-absent controls, but within urban litter exclusively. Our results suggested that plethodontid salamanders exerted positive, indirect effects through consumption of an intermediate predator, i.e., pseudoscorpions, and macroinvertebrate competitors, i.e., isopods and millipedes, of podomorphic Collembola. We hypothesize that differences in salamander-mediated dynamics between the two litter sources resulted from prey selection by P. cinereus based on differences in prey size and abundance between the rural and urban forest litter.