Field experiments were performed to determine whether the spider Linyphia marginata C. L. Koch is food limited, and, if so, whether a shortage of prey acts as a density—independent or density—dependent limiting factor. The experiments uncovered the effects of food supply, spider density, and the interaction of food and density upon survival (combined effects of migration and mortality), rate of increase in weight, and fecundity. Replicate, unenclosed spider populations were established at two densities, and natural food levels were supplemented by fruit flies added to the spider webs in one—half of the populations at each density. Adding food to the webs of immature spiders did not improve survival at either low or high spider density. Survival was lower in the high—density populations. Supplementing the food supply of immatures did increase the rate at which they gained weight. Density, however, had no negative effect on growth rate, indicating that though a shortage of food limits growth, immature spiders are not competing for food. Hence, for immature L. marginata, food supply acts as a density—independent limiting factor. For mature ♀ ♀, supplementing the food supply improved survival and increased fecundity. Spider density had a negative effect upon both survival and fecundity. There were no statistically significant interactions between food supply and density, probably because not enough food was added. It appears that mature L. marginata compete for food; hence, for them, food supply acts as a density—dependent limiting factor.

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