Data from: 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.

密度依赖扩散（density-dependent dispersal）是一种常见的扩散策略，其核心作用为规避种内竞争加剧引发的适合度下降。然而，对于群居物种而言，较高的集群密度至少在达到某一阈值前，对个体是有益的。维持最优集群密度的潜在机制之一为负密度依赖扩散（negative density-dependent dispersal）。为验证这一假说，我们以群居蜘蛛Cyrtophora citricola（Forskål, 1775，园蛛科Araneidae）为研究对象，探究了不同饵料供给条件下，集群密度对其生长、扩散及猎物捕获能力的影响。该种蜘蛛的群居集群常可达到较高密度，但它们同样具备独居生存的能力。既往研究表明，尽管不存在直接的合作行为，群居生活仍可带来与猎物捕获及天敌防御相关的间接收益。我们发现，蜘蛛的扩散倾向随集群密度升高而降低，且这一效应在猎物丰度较高时最为显著。此外，幼蛛的栖息地忠实性（site tenacity）随集群中成体雌性密度的增加而提升。上述两项结果均支持负密度依赖扩散策略。正如预期，蜘蛛的体重随集群密度升高而增加，这表明适合度随密度提升而上升，即阿利效应（Allee effect）。相较于独居蜘蛛，高密度集群内个体间的体重差异更大，因此集群内蜘蛛的猎物捕获成功率可能存在差异，进而导致体重分化。阿利效应（Allee effect）、扩散策略与个体适合度之间的这种相互作用，可能对群居蜘蛛及其他群居物种的生活史与种群分布产生重要影响。