Crowding leads to fitness benefits and reduced dispersal in a colonial spider

Density-dependent dispersal is a common dispersal strategy, mainly as a mechanism of escaping decreased fitness associated with high intra-specific competition. However, in group-living species, high density is expected to be beneficial for the individual, at least up to a certain threshold. A possible mechanism for maintaining an optimal density is negative density-dependent dispersal. In order to examine this hypothesis, we studied the effect of colony density on growth, dispersal and prey capture under different diets in the colonial spider Cyrtophora citricola (Forskål, 1775) (Araneidae). Colonies of C. citricola often reach high densities but the spiders are also capable of living solitarily. Previous studies showed that indirect benefits related to prey capture and predator defense may arise from colony-living, despite the lack of direct cooperation. We found that dispersal propensity of spiders decreased with increasing colony density, and that the effect was strongest when prey abundance was high. Additionally, site tenacity of spider hatchlings increased with greater density of adult females in colonies. Both results support a negative density-dependent dispersal strategy. As expected, body mass of spiders increased with density, suggesting that fitness increases with density (Allee effect). Variance in body mass was higher within dense colonies than among solitary spiders, therefore it is likely that spiders in the colony differ in their prey capture success, and consequently in body mass. This interplay between Allee effect, dispersal strategy and individual fitness may have an important role in the life history and distribution of colonial spiders and of other group-living species.