Spider predation on forest-floor Collembola and evidence for indirect effects on decomposition

Spiders are major predators in forest-floor leaf litter, yet little is know n about their impact on prey populations and on forest-floor processes such as litter decomposition. This experiment investigated the effect of removi ng spiders on Collembola densities. We also examined the potential indirect effect of spider removal on rate of litter disappearance. Twenty-eight 1-m (2) plots were randomly assigned to one of four treatments: O - open, no ma nipulations; F - fenced, no litter sifted, no spiders removed; FS - fenced, litter sifted, no spiders removed; and FSR - fenced, litter sifted, spider s removed. In early August, we sifted the litter in the FS and FSR plots, r emoving all encountered spiders from the FSR treatment. A month later, we p laced into each plot one 15 x 15-cm litterbag filled with a known amount (c a. 3 g) of dried straw. After six weeks, litterbags were collected and faun a were extracted in a Kempson-McFadyen apparatus. Decreasing spider predati on increased densities of Collembola, and increased the rate at which straw disappeared from litterbags. These results indicate that spider predation may reduce Collembola densities enough to lower rates of litter disappearan ce on the forest floor.