Concrete field margins and insecticide use erode and restructure spider assemblages in subtropical smallholder landscapes

Agricultural intensification is a major driver of farmland biodiversity loss. In Chinese smallholder systems, intensification involves increased insecticide use and farmland edge sealing, i.e. the transformation of vegetated field margins into concrete borders. However, the effects of these stressors on important natural enemies of crop pests like spiders remain unknown. We investigated how insecticide use and edge sealing affect spider assemblages along a gradient of landscape-scale semi-natural habitat cover across 55 fields comprising three main crops (maize, rice and tobacco) in a southern Chinese smallholder farming system. Farmland edge sealing reduced spider richness and functional diversity across all crops, while increasing insecticide use decreased spider densities. The stressors imposed different trait-based filters on spider assemblages: Under edge sealing, spider assemblages became on average smaller with fewer active hunters, while assemblages became on average larger with fewer passive hunters and good dispersers under increased insecticide use. Increasing landscape semi-natural habitat cover benefitted the functional diversity of spiders, indicating that offsetting the functional homogenization caused by edge sealing would require a 29.2% increase within 400 m. We demonstrate that farmland edge sealing and increased insecticide use are detrimental to spider numbers and the composition of spider assemblages, which may ultimately threaten natural pest regulation. Mitigating species losses caused by edge sealing with semi-natural habitats is unfeasible as it requires the dedication of large areas. We thus see a pressing need to reduce insecticide use and halt field margin conversion to conserve the functionality of farmland spider assemblages.