Conditions favoring group living in web-building spiders in an extreme desert environment

Colonial spiders construct individual capture webs within a matrix of shared supporting frame threads. Cyrtophora citricola is a colonial orb-weaving spider with a complex three-dimensional web. Colonies may contain a few to several hundred individuals, but individuals may also occur solitarily. Local conditions such as food supply and substrate availability are likely to influence colony formation. In this study we explored the influence of local conditions and dispersal behavior on colony establishment in a desert population of C. citricola in the hyper-arid Arava Valley in Israel. Colonies in the Arava occur primarily on scattered Acacia trees and less commonly on shrubs. The spatial distribution of colonies was clustered and was not influenced by the condition of the Acacia trees (leaf flush, flowering, or fruiting). In a controlled experiment, we showed that dispersing spiders remained longer and built webs faster in trees that contained conspecific webs than in trees without webs. We propose that spiders benefit from establishing webs in the proximity of other spiders, while dispersal to another tree may not result in arrival at an improved habitat. These two factors may promote colony living even in prey-poor environments such as the extreme desert.