Biodiversity in Agricultural Landscapes: Landscape and Scale-Dependent Effects of Organic Farming
Sammanfattning: Declines in the distribution and abundance of many farmland species during recent decades have been attributed to agricultural intensification. Agri-environment schemes are commonly used in Europe to reduce this loss of farmland biodiversity, but their effectiveness may depend on several external factors. In this thesis, I have used one such scheme, organic faming, as a landscape scale experiment to test if its effect on the species richness and abundance of butterflies, bumble bees and plants differs depending on the surrounding landscape context or on which spatial scale organic farming is applied. In the first studies, including pairs of organic and conventional farms located in either heterogeneous or homogeneous landscapes, I found that the species richness and abundance of both butterflies and bumble bees were enhanced by organic farming compared to conventional farming, partly related to higher abundance of flowers on organically managed farmland. However, the enhancing effect depended on the landscape context, such that the species richness and abundance were only significantly higher on organic farms in intensively farmed homogeneous landscapes, but not on the ones in less intensively farmed heterogeneous landscapes. Bumble bee species were classified into three groups based on their colony sizes, reflecting differences in their landscape perception. The abundances of bumble bees with small and large colonies were positively associated with organic farming, while the ones with medium sized colonies instead were affected by landscape heterogeneity, which may indicate differences between groups in sensitivity to resources fragmentation. For the scale-dependent effect of organic farming, I found that local butterfly and plant species richness and butterfly abundance were enhanced by organic farming at a local scale. In addition, my results showed that, besides the local effect, the amount of organic farming in the surrounding landscape can have an additive (species richness) or interactive effect (abundance). Local species richness was positively affected by a large proportion of organic farming at the landscape scale, while the local farming practice was of larger importance for local butterfly abundance in conventionally managed landscapes compared to organically managed ones. In conclusion, I show that the effectiveness of agri-environment schemes to promote biodiversity may depend on the landscape heterogeneity and on how large proportion of a landscape that are subjected to the scheme. These results consequently imply that to increase their efficiency it may be important to consider the spatial arrangement of schemes and their allocation at a landscape scale.
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