Mixed effects of landscape complexity and farming practice on weed seed removal

Agricultural intensification negatively affects farmland biodiversity, and thereby, ecosystem services such as predation of weed seeds. Landscape complexity (large and diverse areas of semi-natural habitats) and organic farming (without pesticides and mineral fertilizers) can counteract this negative trend, but little is known about their relative importance. We experimentally studied seed removal of four economically relevant weed species by excluding (i) vertebrates, (ii) vertebrates plus large invertebrates and (iii) neither vertebrates nor invertebrates (control) at the field edge and in the interior of 22 organically or conventionally managed winter wheat fields, located across a gradient of landscape complexity (41–94% arable land). Arable land (%) was used as a simple predictor of landscape complexity, as it is closely related to landscape parameters such as habitat type diversity. Seed predation and removal were mainly influenced by large invertebrates, seed species identity, with Cirsium arvense being more affected than Poa trivialis, Apera spica-venti and Galium aparine and landscape complexity in interaction with farming practice, as it increased in conventional fields, but decreased in organic fields as landscape complexity decreased. Seed predation and removal did not consistently correspond to seed predator densities, as small mammal abundances decreased in conventional fields, but increased in organic fields as landscape complexity decreased and carabid activity densities were higher in organic compared to conventional fields. Slug activity density showed same response to landscape complexity and farming practice like seed removal and appeared to additionally influence the movement of seeds. In conclusion, our assessment of interacting local and landscape effect adds to the current knowledge of seed predation and removal. Farming practice and landscape complexity affect seed predator species differentially, thereby resulting in complex pattern of seed predation and removal. The service of weed seed predation is provided by small mammals and invertebrates and the disservice of seed dispersal and movement is provided by slugs.