The importance of food and space in limiting web-spider densities; a test using field enclosures

The relative roles of prey availability and habitat structure as possible limiting factors of web-spider density are investigated. Spiders belonging to one of three foraging classes (orb-web weavers, sheet-web weavers, or tangle-web weavers) were placed in field enclusures under constant regimes of prey and habitat complexity. Initially the density of spiders in the enclosure dropped due to cannibalism. After six to eight days, however, the enclosures appeared to reach an equilibrium number of spiders that they could support for the particular conditions provided. The equilibrium density increased if the amount of prey supplied daily was held constant but the substrate available to which webs could be attached was increased. Likewise, if the substrate for web attachment was held constant and prey was increased in separate experiments, the density of spiders also increased. These results provide strong evidence that field densities of web-spiders are determined by some combination of habitat structure and prey activity.Orb and tangle weavers responded with higher intensity to a change in either factor than did sheet weavers. This difference in response level may be due to differences in total web investment between the three web types. The requirements of an appropriate web site may also play a role in creating this difference.At extremely high prey densities, three species of tangle weavers and two species of orb weavers displayed social tendencies. They intertwined their webbing and shared prey from common caches. This result has implications in social evolution in spiders.