Spider effects on prey: tests for superfluous killing in five web-builders

New impetus for considering the potential of spiders to control insect prey comes from foodweb models indicating that generalist predators can limit prey populations through the establishment of stable population equilibria between predators and associated prey (De Angelis et al., 1975; Tanner, 1975; Post & Travis, 1979). Spiders are generalist feeders (Riechert & Ĺuczak, 1982) that meet the criteria for equilibrium point control of prey as they are: (1) selfdamped through territorial behaviour and cannibalism (Riechert, 1982), and (2) can both recognize and respond to patterns of prey availability in a positive density-dependent fashion (Riechert, Provencher & Lawrence, in press). Recent work indicates that the maintenance of spider diversity within a system significantly increases the limiting effect these predators may have on associated prey populations beyond that which a single, prominent species might exert: spider species assemblages limit assemblages of insect species (Provencher & Riechert, 1994; Riechert & Lawrence, 1997). Though an assemblage of predatory species is not a necessary condition of equilibrium point control, the presence of numerous spider species exhibiting varied foraging strategies, phenologies, and sizes significantly augments the limiting influences spiders might have on prey populations over what a single spider species would have. Other factors that may increase the limiting effect spiders may have on associated insect populations include: (1) their numerical prominence in habitats, and (2) the potential for spiders to capture more prey than they consume. Nyffeler & Benz (1987), reviewing studies conducted worldwide on spider population numbers in natural habitats, estimated that they attain densities of up to 1000 individuals m-2 with a mean of 130.8 individuals m-2. The introduction of a mulch cover to agricultural systems overcomes the impoverished spider fauna (Foelix, 1996) typically found in the agroecosystem and produces spider densities and diversities more common to natural habitats (e.g. 30 times those Spider effects on prey: tests for superfluous killing in five web-builders