The effects of prey availability and conspecific density on emigration of the wolf spider Pardosa milvina

Optimal foraging theory often focuses on the decisions made by individuals concerning whether to stay in a particular patch and feed or to move on. These choices may be mediated by factors such as the amount of food in a patch, distance between patches, and the hunger of the individual, as well as presence of potential competitors or predators. We measured the effects of two of these factors, prey availability and competition (conspecific density), on emigration of the wolf spider Pardosa milvina.  This species is commonly found in high densities in agroecosystems across the Midwest.  We used factorial experiments to examine the possible interactive effects of predator density (high/low) and prey density (high/low) in enclosures placed in a soybean agroecosystem. Prey were suction sampled and placed along with marked spiders in circular metal enclosures (1m diameter) with four exit holes allowing emigration into pitfall traps outside. Twenty-four hours later, the number of spiders remaining in each enclosure was recorded along with the number of individuals that had tried to emigrate.  Results showed that reduced prey availability increased the proportion of spiders leaving.  However, density of conspecifics had little effect on emigration. Also, there was no interaction between predator and prey density. This indicates that prey density may be a driving role in emigration of these spiders in the agricultural fields, while competition (conspecific density) is secondary. Therefore, maintaining high levels of alternative prey in agroecosystems may be important in retaining these predators.