Plant communities in organic and conventional agriculture - comparing local, landscape and regional effects

Abstract

During the last decades increasing agricultural intensification in Europe has led to a decrease in species richness and a change in species composition of arable weeds. The drivers of these diversity losses operate at various spatial scales ranging from the local field and farm scale up to the landscape and regional scale. The aim of this study was to examine the effects of landscape complexity (i.e. simple vs. complex landscapes) and local management (i.e. organic vs. conventional farming) on arable weed communities with particular focus on species richness, temporal and spatial species turnover (i.e. β-diversity), and community structure.The study was conducted in three agricultural regions in Germany. To examine landscape effects, in each region circular landscape sectors were selected along a gradient of landscape complexity ranging from structurally simple landscapes, with a high percentage of arable land, to structurally complex landscapes with a high percentage of semi-natural habitats. In the centre of each landscape, arable weed species were recorded within either one focal conventional field or within a pair of one organic and one conventional field.Local species richness of arable weeds was strongly enhanced by landscape complexity in conventional fields, and to a lesser extent in organic fields. This landscape effect is likely due to a larger species pool in complex landscapes since they provide many disturbed habitats for arable weeds such as fallows, road verges and field margins, thereby enhancing the probability of propagule immigration into local fields. A three-year study revealed that inter-annual variation in arable weed communities appeared to be mediated by landscape effects, as β-diversity (i.e. temporal species turnover) increased with increasing landscape complexity, and community similarity among years was high in simple but low in complex landscapes. The high temporal heterogeneity in arable weed communities appeared to be related to the plant s niche breadths in response to differing weather conditions since species occurring predominately in complex landscapes were generally less frequent than! those in simple landscapes. Rarer species may have smaller niche breadths and increased sensitivity to changing conditions.Organic fields had a greater species richness and a higher spatial species turnover (i.e. β-diversity) than conventional fields. An evaluation of the relative contribution of β-diversity at three spatial scales (i.e. plot, field, region) to total observed species richness revealed that β-diversity at the field and regional scale explained most of the overall species richness, indicating considerable differences in community composition among fields and regions due to environmental heterogeneity. In addition to the general positive effect of organic farming on plant diversity, insect pollinated plants benefited disproportionately from organic farming, which appeared to be related to higher pollinator densities in organic fields, whereas in conventional fields non-insect pollinated plants dominate presumably due to a limitation of pollinators.These results show that both complex landscapes and organic farming are of major importance for community patterns, enhancing species richness and temporal and spatial species turnover (i.e. β-diversity) of arable weeds. Hence, maintenance and enhancement of arable weed diversity requires a multi-scale perspective, incorporating both local and regional aspects.