Effects of amount of semi-natural habitats and crop mosaic heterogeneity on generalist predator communities (Araenae and Carabidae) and biological control potential

Carabid beetles and spiders are important generalist predators which may generate an effective control of pest populations. However, to maintain or promote them, it is necessary to better understand the factors that may influence predator communities and predation rates. Previous work has demonstrated that agricultural landscapes which contain significant areas of semi-natural habitats support higher biodiversity and better ecosystem services. But little is known on the effects of the heterogeneity of the cropped area itself. In this study, we investigated the relative and interactive effects of crop heterogeneity (crop composition and configuration) and amount of semi-natural habitats. Within the European FarmLand project (www.farmland-biodiversity.org), we worked in six agricultural regions spanning across western Europe, and differing in terms of landscape complexity : Göttingen (Germany), Armorique, Plaine et Val de Sèvre, Coteaux de Gascogne and Camargue (France), and Lleida (Catalonia, Spain). In each region, carabids and spiders were sampled using pitfall traps along independent gradients in crop composition (crop diversity) and crop configuration (crop mean size), and biological control potential was estimated by recording the removal rate of experimentally added aphids. Agricultural landscapes with more heterogeneous croplands have higher biodiversity and biological control potential, but these effects are modulated by the regional context, and in particular on the amount of semi-natural habitats.