Farming system and landscape characteristics differentially affect two dominant taxa of predatory arthropods

Despite the role generalist predators may play in biological regulation, the influence of landscape composition in shaping their assemblages remains little studied, especially when landscape interacts with local factors. In this study, we investigated the effects of farming systems along gradients in landscape elements on the structure and composition of carabid and spider assemblages. Twenty pairs of organic vs. conventional spatially-matched fields were sampled in 2013 along increasing percentage covers of organic farming and semi-natural habitats in the landscape. A total of 24241 spiders and 27767 carabids belonging to 120 and 75 species respectively were collected by pitfall traps. Farming systems locally had a strong influence on the community structure (activity-density and species richness) and composition for both spiders and carabids. Structure of spider assemblages was mostly affected by local and landscape factors, whereas that of carabids was more driven by landscape variables and the interaction of the two levels. Spider and carabid community compositions were mostly determined by field farming systems and wood percentage around the field. Our study underlines the importance of landscape context in shaping assemblages of predatory arthropods, and suggests that mechanisms behind the distribution of individual species strongly differ between spiders and carabids.