Unexpected indirect effect of spiders on the rate of litter disappearance in a deciduous forest

Previous experiments in meadow and forest-floor communities have uncovered a negative indirect effect of spiders and other large epigeic predators on decomposition, presumably through depression of densities of microbi-detritivores. We report the results of a 17-month field experiment in a deciduous forest in which spiders exerted the opposite effect on decomposition. In autumn we collected falling leaves of oak (Quercus), maple (Acer) and hickory (Carya), and placed three bags of mixed litter, and three groups of tethered leaves, in 1-m² fenced plots. We determined the disappearance rate of these leaves through April of the second year. Spider densities were reduced by >50% in the spider-removal treatment. The rate of leaf-litter disappearance was not higher in spider-removal plots, but instead was ca. 20% lower (P=0.057). Thus, spider predation clearly was not decreasing the rates of litter decomposition in our experiment; on the contrary, the results suggest that spiders were indirectly enhancing decomposition. Possible hypotheses to explain this unexpected result are discussed.