Impact of intraguild predators on survival of a forest-floor wolf spider

We investigated the impact of variation in densities of a guild of generalist predators on survival of young wolf spiders of the genus Schizocosa. Numbers of other spiders and centipedes were reduced by >80% in fenced 4-m² plots in an experiment that was replicated twice in each of three forest locations. Schizocosa survival during the 1st month was low (<50%) in all three locations, but did not differ between predator-reduction and control plots. By the end of the 1st month, densities of the manipulated predators had converged in control and perturbed treatments, most likely because of reduced per capita mortality from lowered rates of intraguild predation and cannibalism in the experimental treatment. During the 2nd month of the experiment, centipedes and spiders other than Schizocosa again were removed from the experimental plots and, unlike the earlier period, numbers of intraguild predators in the predator-removal treatment remained lower than in control plots. Reducing densities of intraguild predators during the 2nd month improved survival of older juvenile Schizocosa by 75% in two of three locations on the forest floor. In addition to this evidence that intraguild predation can affect older juvenile Schizocosa, survival of Schizocosa during the last half of the experiment was negatively correlated with spatial variation in densities of gnaphosid and ctenid spiders. These two abundant families of cursorial spiders preyed on Schizocosa at a high rate in laboratory trials. Thus, variation in densities of intraguild predators did not influence the youngest Schizocosa, but did influence the survival of older juveniles, most likely due to variations in densities of other cursorial spiders. Received: 21 October 1998 / Accepted: 14 May 1999