Landscape context of sheetweb spider (Araneae: Linyphiidae) abundance in cereal fields
Sheetweb spiders (Araneae: Linyphiidae, subfamily Linyphiinae) exemplify a highly mobile group of farmland arthropods with very variable abundances in crops within and between years. Despite their dominance in crops during summer, they overwinter predominantly in perennial non-crop habitats, and their densities in crops during spring should therefore depend on the amount of favourable overwintering habitat in the surrounding landscape. We tested the effect of landscape composition on sheetweb spider abundance with special attention to the range of their aerial dispersal and weather dependence. Location
The study was carried out in 18 non-overlapping landscape sectors of 3 km radius around the city of Göttingen (Germany), forming a gradient from structurally simple, with up to 85% arable land, to structurally complex, with high percentages of grassland and other non-crop habitats. Methods
Sheetweb spider abundances in winter wheat fields were sampled during May and June 2001–2003 with a distance method. They were related to landscape composition at 11 spatial scales between 95 and 3000 m radius around the study sites. Results
In 2001 and 2003, spider abundances were enhanced by high percentages of non-crop habitats in 1–3 km circumference (e.g. from 18 to 130 m−2 in late May 2001), and multiplied during consecutive sampling periods (e.g. from on average 36 to 131 m−2 between mid-May and late June 2001). Spider abundances were constantly low and unrelated to the landscape context in 2002. In that year, immigration appeared to be inhibited by factors connected to exceptionally high amounts of rain during May. Main conclusions
Sheetweb spiders responded to landscape composition up to several kilometres away and the effects varied between years, demonstrating the need to consider large space and time-scales to understand their population dynamics. Semi-natural habitats should be preserved to enhance these important natural enemies of crop pests in agricultural landscapes.