Shadow competition, when sedentary foragers closer to a source of food reduce its availability to those further away, is predicted to increase with the size and density of a group. We tested the occurrence of shadow competition and examined its consequences for a burrowing spider Seothyra henscheli (Eresidae) in the Namib Desert. Differences between individual spiders occurring inside or on the periphery of clusters compared to solitary spiders were examined in a natural population, by experimental manipulation of densities and by computer simulation of the experimental manipulation. Spiders in the population grew more slowly in clusters than did solitary spiders and this was confirmed by the experiment. The experiment showed that spiders grew more rapidly on the periphery of a cluster than inside it, but that survival showed the opposite trend. The largest effect was in the highest density, where all spiders maintained active webs throughout the experiment, indicating a state of hunger. Modeling indicated that such effects may be explained by the way ants, the principal prey of the spiders, reach spider webs at different locations within the patches of different densities. Modeling confirmed that shadow competition adequately explains the patterns of foraging, growth and survival of sedentary foragers such as these spiders, and is likely to have wider implications for other sit-and-wait predators.