Agri-environment schemes enhance pollinator richness and abundance but bumblebee reproduction depends on field size.
Abstract
Pollinators have experienced a dramatic decrease world-wide due to agricultural intensification. In many countries, agri-environment schemes (AES) have been introduced to counteract this current trend. However, until now, the relative importance of each AES for biodiversity and ecosystem services is still little understood and might change depending on landscape context. Complex landscape-experiments are required to fill this knowledge gap, enabling the implementation of sustainable intensification of food production. In our study, we compared the effectiveness of the two most popular AES in Germany, organic farming and flower strips, in supporting pollinators and flower resources. We selected nine landscapes along a gradient of increasing field size, (configurational heterogeneity), each with a triplet of winter wheat fields: one organic, one conventional with flower strip and one conventional without flower strip as a control. We surveyed insect-pollinated plants and pollinators (bumblebees, solitary bees and hoverflies). Additionally, we placed bumblebee colonies in the field edges to monitor their growth (colony weight gain) and reproduction (queen production). Flower strips stood out with the highest abundance and richness of pollinators. In contrast, bumblebee colony growth and plant richness benefited equally from organic and flower strip schemes. At the landscape scale, smaller fields had a positive effect on plant richness and bumblebee reproduction in flower strips. By contrast, bumblebee colonies in organic agriculture benefited most from large fields, as large organic fields provided much more flower resources than the narrow flower strips. Synthesis and applications. Our results showed that both local and landscape management shaped pollinator communities and their reproduction. Overall, organic farming and flower strips appeared to be effective tools to mitigate flower shortage in conventional cereal fields, with organic farming supporting the highest flowering plant cover per field. Flower strips enhanced local pollinator richness most, but increased bumblebee reproduction only when the surrounding landscapes had small fields with long field borders. Therefore, our results reveal that European Union policies need to take into account that the effectiveness of agri-environment schemes depends on the structure of the surrounding landscape.