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.密度依赖扩散是一种常见的扩散策略，其核心作用是规避因种内竞争加剧而导致的适合度下降。然而，对于群居物种而言，较高的集群密度对个体是有利的，至少在达到某一临界阈值之前是如此。维持最优集群密度的潜在机制之一，便是负密度依赖扩散。为验证这一假说，我们以群居性蜘蛛——柑橘络新妇（Cyrtophora citricola，Forskål, 1775，金蛛科Araneidae）为研究对象，探究了不同饵料条件下，集群密度对其生长、扩散及猎物捕获能力的影响。柑橘络新妇的集群往往能达到较高密度，但该蜘蛛同样可独立存活。既往研究表明，尽管不存在直接合作，但群居生活可带来与猎物捕获及天敌防御相关的间接收益。本研究发现，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，幼蛛的栖居持久性随集群中成体雌性密度的升高而增强。上述两项结果均支持负密度依赖扩散策略的存在。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（阿利效应（Allee effect））。高密度集群内的个体体重方差高于独居蜘蛛，由此推测集群内蜘蛛的猎物捕获成功率存在差异，进而导致体重分化。阿利效应、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具有重要意义。