Land use at different spatial scales alters the functional role of web-building spiders in arthropod food webs

Web-building spiders are abundant generalist predators in agroecosystems that primarily forage on insects including economically important pests. Local management and landscape composition influence spider and prey communities and thereby their trophic interactions and functional role in arthropod food webs. We compared predator-prey interactions between organically managed cereal fields and sown flower-rich fields, both supported by agri-environmental schemes. The surrounding landscape of twelve study sites differed in the percentage of arable crops within a radius of 500 m around each site. We analyzed 1036 hand-collected web-building spiders with 5270 prey items from webs and 6777 potentially available prey items sampled by fenced suction sampling. Thysanoptera significantly dominated prey composition of web-building spiders in cereal fields located in landscapes with low percentages of arable crops, while Nematocera dominated prey composition in sown flower-rich fields. The captured prey numbers per spider web, irrespective of taxonomic identity, increased with the availability of potential prey, independent of habitat type or landscape composition. We did not find any effect on the compositions of web-building spiders and potential prey. Our results suggest that spider webs act as traps for prey that depend on prey density. However, this simple interpretation is only valid for the overall prey quantity, while capture success of single prey taxa may be habitat-specific and depend on landscape features. The impact of land use at different spatial scales on the functional role of web-building spiders should caution us towards density-based estimates of predation processes, e.g. when assessing the impact of agri-environmental schemes on arthropod food webs.