Site selection behavior of spider communities: Temporal and altitudinal variation in web placement

Web-building spiders are an important component of arthropod communities in agroecosystems, potentially impacting prey populations and contributing to valuable levels of pest regulation. Integral to their effectiveness as predators is the web, the primary tool of prey acquisition for most species. The placement and architecture of a web can reflect a number of variables, including prey availability, competition with neighboring heterospecific and conspecific spiders, and vegetational structure. Unraveling the factors that influence web placement can help elucidate the roles of different species in community dynamics, including their effects on populations of other natural enemies and of economically important pests. Spider webs were extensively surveyed and measured in an alfalfa agroecosystem to understand the mechanisms of web-construction and identify co-existing and competitive interactions between species. Measurements revealed highly significant stratification of the four most common species (three linyphiids and one tetragnathid), with the following trends in terms of web height: Glenognatha foxi > Grammonota inornata > Tennesseellum formicum=Erigone autumnalis; and web surface area: Glenognatha foxi > Grammonota inornata=Tennesseellum formicum > Erigone autumnalis. These results suggest a partitioning of resources between at least two groups: the higher placement of Glengnatha foxi (Tetragnathidae) webs was suitable for capturing airborne prey, such as Diptera, while the linyphiid webs were located at a lower stratum optimal for capturing ground-based prey such as Collembola. The lack of significant response in web parameters to seasonal progression or vegetational structure suggested stable feeding and interaction patterns as habitat complexity gradients varied throughout the season.