The effects of landscape complexity on arable weed species diversity in organic and conventional farming

Summary There is growing concern about declining species diversity in agro‐ecosystems caused by agricultural intensification at the field and landscape scales. Species diversity of arable weeds is classically related to local abiotic factors and resource conditions. It is believed to be enhanced by organic farming but the surrounding landscape may also be important. This study assessed the ruderal vegetation, seed bank and seed rain in 24 winter wheat fields to examine the relative importance of organic vs. conventional farming and landscape complexity for weed species diversity. Diversity was partitioned into its additive components: alpha, beta and gamma diversity. Percentage arable land in a circular landscape sector of 1‐km radius around each study site was used as an indicator of landscape complexity. Weed species diversity in the vegetation, seed rain and seed bank was higher in organic than in conventional fields. Increasing landscape complexity enhanced species diversity more strongly in the vegetation of conventional than organic fields, to the extent that diversity was similar in both farming systems when the landscape was complex. Species diversity of the seed bank was increased by landscape complexity irrespective of farming system. Overall diversity was largely determined by the high heterogeneity between and within the fields (beta diversity). Only in very few cases could higher weed species diversity in complex landscapes and/or organic farming be related to species dependence on landscape or farming system. Synthesis and applications. Local weed species diversity was influenced by both landscape complexity and farming system. Species diversity under organic farming systems was clearly higher in simple landscapes. Conventional vegetation reached similar diversity levels when the surrounding landscape was complex through the presence of refugia for weed populations. Consequently, agri‐environment schemes designed to preserve and enhance biodiversity should not only consider the management of single fields but also of the surrounding landscape.