Intraspecific competition in a social spider

An individual-based model was developed to investigate the population dynam ics of a permanently-social spider, Stegodyphus dumicola (Eresidae), in rel ation to intraspecific competition for food. This further develops our init ial model that demonstrated that individual variability in body mass causes contest competition and is based on it. This confirmed field observations. In the present study, we examined whether intraspecific contest competitio n for food could be a mechanism for density regulation in S. dumicola. We i nvestigated the consequences of contest competition on the individual varia bility in body mass as well as on the individual fitness of female spiders. Results demonstrate that colony size stabilises above a threshold of food level. At low food levels the risk of colony extinction is high because onl y few individuals survive and reproduce. Contest competition for food leads to high variance in body mass under conditions of food scarcity. Those var iances tend to decrease with increasing food resources and a threshold of f ood level separates ranges of high and low variances. Model results indicat e that individual females differ significantly by their fitness. An optimum of fitness was found at intermediate food levels. Results suggest that con test competition is a mechanism of density regulation in spider societies w hich stabilises colony size as well as variability in body mass and fitness . (C) 1999 Elsevier Science B.V. All rights reserved.