Model and Field Test of Prey Control Effects by Spider Assemblages

Abstract

We investigated by computer simulations and a field experiment the hypothesis that a species assemblage of generalist predators imposes more mortality on prey populations than a single species does. The simulation tracked the energetics of individual spiders feeding on prey in a patchy and variable habitat. Simulation tests showed that increasing the number of spider species and the variability of prey body sizes contributed Significantly to greater prey limitation and spider survival. Higher variability in spider body sizes had the reverse effect and increasing the number of prey species had mixed effects. Large spiders, although less abundant than smaller ones, cropped disproportion ally more prey biomass by capturing large prey and should have injured, thus reduced the capture rate, of smaller spiders. Simulations also showed that as model habitats were fragmented prey biomass dropped, causing a reduction in spider success variables. Increasing fragmentation enhanced the contribution of built-in negative environmental effects on the growth rate of prey. We showed with a factorial field experiment that lower prey biomass resulted from greater spider species richness, as predicted, higher prey richness increased prey biomass only if spiders were present, and habitat fragmentation only decreased prey biomass in the absence of spiders. At equal spider biomass, larger spiders, which were present only in the highest richness treatment, probably fed more than smaller ones. Only larger spiders could capture the largest prey, which represented a greater fraction of the total biomass. Spider predation seemed indifferent to fragmentation.