The β‐diversity of arable weed communities on organic and conventional cereal farms in two contrasting regions

Abstract Questions Do diversity components (α, β and γ) differ across contrasting regions? What is the relative contribution of organic and conventional farming practices to the structuring of arable weed species diversity and different functional groups (legumes, grasses and broad‐leaves)? To what extent do arable weed communities differ between regions and farming systems (organic vs conventional)? Location Twenty‐six farms in total in northeast S pain ( C atalonia) and north G ermany ( L ower S axony). Methods We examined the weed flora in paired organic and conventional farms at each locality and assessed diversity components by additive partitioning of species richness (α, β and γ). Results The weed species composition differed greatly between the two regions. Only 18 of 135 arable weed species were shared. The α‐, β‐ and γ‐diversity of all functional groups was significantly higher under organic farming practices for both regions, indicating an increasing homogenization of local communities by agricultural intensification within each region. β‐diversity contributed most to the total observed species richness in both regions (69.8% and 35.8% on organic and conventional farms, respectively, in C atalonia; and 62.4% and 53.0% on organic and conventional farms, respectively, in L ower S axony). These results demonstrate the great importance of environmental heterogeneity and of farm‐specific differences in agricultural practices for the richness of arable weed species. Conclusions Regardless of the substantial differences in arable weed community composition across regions, our study emphasizes the great importance of organic farming for arable weed species richness. Understanding the patterns and causes of the dissimilarity of local communities appears to be a key factor for species conservation and the development of effective European‐wide agri‐environmental schemes at landscape and regional scales. This approach is in contrast to current nature conservation practices that are restricted primarily to local (field‐ and farm‐scale) implementation.