Interactions among top-down regulators in forest-floor food webs: Results of a long-term predator removal study

Red–backed salamanders, Plethodon cinereus, are small forest-floor predators that directly affect the density of invertebrates in detrital food webs.  As a result of their large biomass, predation by individuals of P. cinereus can have cascading effects throughout forest-floor food webs thus affecting rates of leaf litter decomposition.  Recent behavioral studies suggest that P. cinereus may also interact with large predacious invertebrate guild members through competition and intraguild predation (IGP).  The synergistic relationship between competition and IGP among forest floor predators as a driving force in terrestrial trophic cascades within terrestrial detrital food webs has not been investigated.  Our research was conducted to explore how removal of focal predators from open field plots influences densities of guild members and of organisms at lower trophic levels.  288 artificial cover objects (ACO’s) were divided into eights replicated blocks containing three removal treatment groups: salamander removal (SR), centipede removal (CR) and all predator removal (PR) and one control group: no animals removed (NR).  To date we have removed 1715 individuals of P. cinereus, 1103 spiders, 693 centipedes and 423 predatory beetles.  Invertebrate abundance was decreased by 21% in removal plots and salamander abundance by 57% in removal plots.  Despite being the most abundant predator at our field site, removal of Plethodon cinereus, had no effect on the abundances of centipedes, carabid beetles or predatory flat worms.  However, by the third year, salamander removal had a significant positive effect on spider abundance.