Below- and aboveground farmland biodiversity in relation to local and regional management

Abstract

The expansion of global croplands, plantations and pastures in the last decades, which led to a dramatic change in the spatial structuring of agricultural landscapes is one of the main drivers of biodiversity loss. Intensified agriculture, e.g. increased use of agrochemicals, on a local scale and simplification of landscapes on a regional scale were accompanied by a dramatic loss of biodiversity in European agricultural landscapes. However, the relative importance of local and regional drivers differs between taxa and habitats.The first part of the thesis deals with effects of agricultural intensification on both above- and belowground communities. We analysed the diversity of vascular plants, carabid beetles and birds in agricultural landscapes at the scale of plots in cereal fields (n= 1350), farms (n= 270), and European regions (n= 9). We partitioned diversity into its additive components α, β, and γ, and assessed the relative contribution of β-diversity to total species richness at each spatial scale. Agricultural intensification negatively affected the species richness of plants and birds, but not carabid beetles, at all spatial scales. Local agricultural intensification was closely related to β-diversity on larger scales up to the farm and region level, and thereby an indicator of farm and region wide biodiversity losses. β-diversity at the scale of farms (13 - 21%) and regions (68% - 80%) accounted for the major part of the total species richness for all three taxa. For plants, relative importance of α-diversity decreased with agricultural intensification, while relative importance of β-diversity on the farm scale increased with agricultural intensification for carabids and birds. Hence, agricultural intensification does not necessarily homogenize local communities, presumably due to the heterogeneity of farming practices. We sampled diversity and abundance of earthworms and collembolans, soil respiration rate and microbial biomass in 12 pairs of organically and conventionally managed winter wheat fields in landscapes differing in structural complexity to investigate interacting effects of agricultural intensification on local and landscape scales on belowground biota. Results show that only the in-field abundance of collembolans was slightly enhanced under organic farming, in contrast, soil respiration rate was higher under conventional management. Landscape simplification increased microbial biomass carbon in organically managed fields and decreased it in conventionally managed fields. As the same pattern was found for earthworm species richness, earthworms may have enhanced microbial biomass. In general, organic farming appears to be more efficient in conserving aboveground than belowground diversity.In the second part of the thesis we studied the relative importance of spatial and environmental factors for the community composition of belowground biota. The relative importance of environmental or spatial processes determines whether communities are mainly formed by species-environment interactions or dispersal events. In soils as very patchy, complex and poorly connected habitats, dispersal might shape the community composition more strongly than in less complex and better connected habitats. We used a variance partitioning approach to investigate the relative importance of environmental conditions and spatial distances for soil bacterial, collembolan, carabid beetle and earthworm community composition in agricultural fields and old set-aside fallows, comparing samples from a very small scale of 40 m with samples from a regional scale of up to 13 km. We found that environmental conditions are important drivers of soil bacterial community composition. In addition, the spatial distribution of our sampling sites also influenced bacterial community composition, indicating a certain degree of provincialism, probably caused by dispersal limitation even on very small scales. The macroorganism taxa studied showed different patterns depending on their average body size. The larger macroorganisms did not show any spatial signals on the local scales. In contrast, their spatial signal is most prominent on the regional scale, while smaller macroorganisms, i.e. collembola, showed a spatial signal on the local but not on the regional scale. Differences between bacteria and macroorganisms are likely to be due to different dispersal modes for bacteria and macroorganisms in soils.We conclude that agricultural intensification had negative effects on most of the studied aboveground taxa, with detrimental effects acting over multiple spatial scales. In contrast, organic farming appears to be more efficient in conserving aboveground than belowground diversity, which could be due to the enormous functional redundancy of the belowground community and therefore high resilience and resistance to anthropogenic disturbances. However, landscape context played a significant role in interaction with local management, emphasizing the importance of agri-environmental schemes designed for local and regional scales alike. The relative importance of environmental and spatial factors for soil communities appeared to depend on the dispersal mode of the taxa considered.