Land-use at different spatial scales affects prey composition of web-building spiders
Understanding the factors driving predator-prey interactions in agricultural systems is necessary for the development of management regimes that reduce or even eliminate the need for pesticide applications by promoting biological pest control. Web-building spiders play a crucial role in this context, since they are abundant generalist predators in agro-ecosystems that primarily forage on insects including economically important crop pests. Both, local management and landscape composition affect spider communities, the availability of prey and, as a consequence, may alter trophic interactions. We investigated predator-prey interactions in two agricultural habitat types: six organically managed cereal fields and six flowering fields. The surrounding landscape of study sites differed in the percentage of arable crops within a radius of 500 m around each site. We hand-collected 1072 web-building spiders with 5409 prey items in their webs at three sampling dates between May and July 2013. In addition, we quantified 33599 potential prey items by suction sampling.
The composition of eight common prey taxa in spider webs differed significantly according to habitat type and landscape composition. Thysanoptera prey was more often observed in spider webs in cereal fields, while Nematocera dominated prey compositions in flowering fields and their numbers decreased with increasing percentages of arable crops. The per capita quantity of prey increased with the abundance of available prey, independent of habitat type or landscape composition. The community composition of web-building spiders and the composition of potentially available prey were not affected by habitat type or percentage of arable crops. Referring to common prey taxa in agricultural landscapes, our results suggest that the quantity of prey items captured in spider webs depends on prey availability. In contrast, the type of captured prey taxa is at least partly determined by habitat and landscape features. These results indicate an opportunity for developing local management practices and regional land-use schemes that aim at high predation rates on selected, economically important prey taxa in the future.