Salamanders in forest-floor food webs: Complex outcomes from long-term studies

Plethodontid salamanders are hypothesized to be important regulators of invertebrate communities in temperate North American forests. Densities of red-backed salamanders, Plethodon cinereus, were manipulated for 5 years to test for effects on food web dynamics. Variability in salamander occupancy of 2 m diameter circular, open plots was manipulated with artificial cover objects. Within one year of plot establishment, surface densities were significantly greater on high cover plots (0.16 m²) in comparison to plots with 0.04 m² or no artificial cover.  Averaged over 2003-2006, salamander surface densities on high cover plots were 0.30 individuals/m² and 3-fold greater than plots lacking artificial cover. Salamander occupancy of plots was associated with statistically significant differences in the composition of the invertebrate community, but the strength and direction of P. cinereus effects varied among taxa and years. Some taxa (e.g., onychiurid Collembola, oribatid mites) showed early declines (2002-2004) in the presence of P. cinereus but subsequent increases (2005-2006) in comparison to plots without salamanders. Pseudoscorpions responded positively to salamanders, increasing in abundance by 2-5-fold in comparison to plots lacking salamanders. Psocids showed strong seasonal trends, increasing to particularly high densities in the presence of salamanders in fall. Our results indicate that terrestrial plethodontids have important, but complex, effects on detrital food-web dynamics that may not be detected by short-term experiments. Preliminary results regarding effects of P. cinereus on leaf-litter decomposition will also be discussed.