Forest floor food web dynamics are complex and include top-down, bottom-up, and intraguild interactions. Within the forest floor community of the eastern United States, salamanders and spiders are important generalist predators that influence invertebrate populations and community dynamics. Two species in particular have become model organisms in the study of forest floor food webs. Plethodon cinereus (Amphibia, Caudata, Plethodontidae) is a euryphagic predator and one of the most abundant vertebrates within its range. Schizocosa ocreata (Arachnida, Araneae, Lycosidae) is a common and often abundant member of the forest floor wandering spider community that preys upon a wide variety of invertebrates. Both species have been shown to impact both predators and detritivores directly and indirectly through consumptive and/or behavioral interactions. To date, however, few data are available for direct comparison of these two predators within the same system. Herein we present the results of a laboratory microcosm experiment in which we manipulated the presence/absence of P. cinereus and S. ocreata in a factorial design to resolve how each predator affects various forest-floor invertebrate populations. In a parallel study, we tested whether salamander feces contribute to bottom-up regulation of meso-detritivores by serving as a supplemental resource.

Results/Conclusions

Our results show that salamanders significantly reduced the density of snails and predaceous mites, whereas spiders had no detectable effect on these taxa. Snails also exhibited a negative response to salamander feces, and we suggest that this is an example of behavioral avoidance of predator cues by snails. Isotomid springtails showed a positive response to salamanders but a negative response of similar magnitude to spiders, with no significant statistical interaction between salamander and spider treatments. Fecal subsidies did not result in greater isotomid densities, thus we rule out salamander feces as a significant resource for this group. The opposing roles of salamanders and spiders in the regulation of isotomid populations reflects the complexity of the forest floor food web and highlights the need for further study to understand the dynamic interactions within this community.