Background/Question/Methods The ability to maintain high densities of natural enemies in managed systems may depend on the strength of agonistic interactions both within and among species.  One explanation is that interference between predators decreases predator performance. There are few empirical tests of the interaction between prey and predator density in determining patch use and subsequent impacts on a predator’s functional response.  Spiders experience fluctuation in prey availability in the field leading to food limitation, which may increase the frequency or intensity of predator-predator interactions such as cannibalism and intraguild predation. We explored the tendency of the common hunting spider, Pardosa milvina (Lycosidae:Aranaea), to reside in a patch by manipulating levels of predator and prey in field enclosures and measuring the spider’s subsequent emigration. To understand the consequences of concurrent variance in prey and predator density on foraging rate, we conducted a functional response experiment in the laboratory to estimate mutual interference. 

Results/Conclusions In the field, spiders emigrated at a lower rate and consumed more prey in treatments where prey was added.  Furthermore, a greater proportion of spiders emigrated from enclosures containing higher spider density.  In the laboratory, the functional response was consistent with ratio-dependence, where interference between spiders negatively influenced attack efficiency.  Combined, these results indicate that both prey and predator density influence the decision of this generalist predator to remain in a patch.  The proportional abundance of prey to predators potentially sets an interference threshold, at which point these predators leave in search of a better patch.