Costs of aggregation: shadow competition in a sit-and-wait predator

Shadow competition, when sedentary foragers closer to a source of food redu ce its availability to those further away, is predicted to increase with th e size and density of a group. We tested the occurrence of shadow competiti on and examined its consequences for a burrowing spider Seothyra henscheli (Eresidae) in the Namib Desert. Differences between individual spiders occu rring inside or on the periphery of clusters compared to solitary spiders w ere examined in a natural population, by experimental manipulation of densi ties and by computer simulation of the experimental manipulation, Spiders i n the population grew more slowly in clusters than did solitary spiders and this was confirmed by the experiment. The experiment showed that spiders g rew more rapidly on the periphery of a cluster than inside it, but that sur vival showed the opposite trend. The largest effect was in the highest dens ity, where all spiders maintained active webs throughout the experiment, in dicating a state of hunger. Modeling indicated that such effects may be exp lained by the way ants, the principal prey of the spiders, reach spider web s at different locations within the patches of different densities. Modelin g confirmed that shadow competition adequately explains the patterns of for aging, growth and survival of sedentary foragers such as these spiders. and is likely to have wider implications for other sit-and-wait predators.