Web-location by linyphiid spiders: prey-specific aggregation and foraging strategies

1. Web-location cannot be designed simply to optimize foraging success, but must be a compromise between competing factors, including microhabitat parameters (physical structure, microclimate) and predator avoidance. 2. We tested the hypothesis that, despite these compromises, linyphiid spiders managed to locate their webs within microsites with enhanced prey resources. 3. We tested further the 'equilibrium' hypothesis that spiders, within a community of species, occupy different, relatively narrow niche axes and despite being generalists do not simply aggregate to the same resources. 4. In the first study of its kind, prey resources at web-sites were compared with those at matched non-web-sites with the same microhabitat properties. This was carried out for two subfamilies of Linyphiidae, one of which (Linyphiinae) is web-dependent and locates its web just above the ground, and the other (Erigoninae) is less web-dependent, locating its web on the ground and often hunting away from its web. We hypothesized that these subfamilies would locate their webs at microsites with enhanced densities of the prey most likely to be captured by their contrasting foraging strategies. We used spider weights as a surrogate for nutritional state, and compared spiders in webs with those without webs. 5. We found that even within a relatively uniform crop of wheat, spiders located their webs at microsites with greater prey resources. The most numerous prey, Collembola, were at significantly greater density at web-sites of both subfamilies of spider. However, there were significantly more Collembola at web-sites of the surface-hunting Erigoninae. By contrast, significantly more non-Collembola prey, especially aphids and Thysanoptera, were found at web-sites of the Linyphiinae, whose aerial webs might be expected to intercept such prey more efficiently. 6. These differences were also present at matched non-web-sites, suggesting that microsite selection by the spiders was determined by the spectra of prey present. Comparison of the weights of web-owning and non-web-owning Tenuiphantes tenuis (Linyphiinae) demonstrated the nutritional benefits of web-owning. 7. The results suggested that the spiders used a combination of web-location strategies at the microhabitat level, foraging behaviour and (known) high-intensity intraspecific competition, to exploit prey-rich microsites efficiently within fields in a dynamic manner.