Foraging success of solitary and aggregated spiders: insights into flock formation

Abstract

The normally solitary spider species, Achaearanea tepidariorum (C. L. Koch) (Aranae: Theridiidae), will cease cannibalism and forage in aggregations of webs when prey is very abundant. The circumstances of their shift from solitary to group foraging and the effect it has on capture efficiency and success were tested. Fabricated aggregations quickly dispersed at a site where captures were low, but these aggregations persisted for several months where the abundance of prey was high. When resources were abundant for the spiders, the number of prey captured per day by solitary individuals was not different from the number captured by individuals foraging in groups; however, the biomass of prey obtained per day was greater for those individuals in groups. The variance in daily capture was substantially less for grouped individuals. Prey introduction experiments using both house flies and fruit flies revealed that groups captured a greater proportion of the prey that entered their web because of local enhancement and ricochet effects. These factors are probably responsible for the different variances in prey capture observed between aggregated and solitary individuals because they tend to distribute insect prey across more individuals. The different variances in foraging success are consistent with the predictions of risk-sensitive foraging theory that group foraging should be the risk-averse (low variance) foraging mode and solitary foraging should be the risk-prone (high variance) foraging mode.