| dc.description.abstracteng | Agricultural intensification and the loss of suitable habitats are drivers of insect declines. Besides the promotion of semi-natural habitats (SNH), the utilization of diversified crop rotations including flowering crops is an option to sustain pollinators. As flowering and nitrogen fixing crops, grain legumes provide several environmental benefits but are underrepresented in the EU. Since 2013 they are promoted within the greening reform of the Common Agricultural Policy (CAP). On-field biodiversity measures are often criticized, though their possible benefits for biodiversity are widely unknown. My PhD thesis aims at investigating the landscape-scale effect of conventional faba bean (Vicia faba L.) cultivation on wild bee communities, their resource utilization and their pollination services. Experiments were conducted in 1km x 1km study landscapes in Germany. For the first three chapters a paired study design was applied with one landscape of a pair comprising at least one faba bean field and the respective control landscape without grain legumes.
In the first chapter we studied effects of faba bean cultivation on functional groups of wild bees. We conducted pollinator surveys at field margins of different crops in 30 study landscapes. Bumblebee densities were more than twice as high in faba bean compared to control landscapes, while non-Bombus wild bees were only driven by the amount of SNH. These results indicate that a combination of on- and off-field greening measures is necessary to conserve farmland biodiversity and we recommend that the CAP should furthermore promote both. In the second chapter we investigated the effect of two mass-flowering crops (MFC) on the functional trait composition of bee communities in SNH. Faba bean cultivation promoted bumblebees, especially long-tongued species, social bees and bees foraging on Fabaceae. High oilseed rape (OSR) covers changed wild bee community composition in favor for solitary bees. Local flower cover of SNH was the main driver of non-Bombus wild bees and wild bee species richness. Thus, different MFC promote specific functional bee groups adapted to the crop`s flower morphology. Concluding, the cultivation of functionally diverse crops, combined with a high local flower cover of SNH needs to be targeted by management practices to sustain diverse pollinator communities. In the third chapter we studied landscape and faba bean cultivation effects on the colony development and resource utilization of bumblebees (Bombus terrestris L.). The colonies` reproductive success was not influenced by landscape composition but enhanced by landscape diversity. Faba bean cultivation and pollen diversity interacted and resulted in more young queens with increasing pollen diversity in landscapes without faba bean. In addition, colonies that collected a higher pollen diversity had a reduced A. sociella depredation. Increased parasitism had a cascading negative effect on the reproductive success of bumblebees by limiting colony growth. Our study shows that high landscape diversity and diverse pollen diets can enhance the reproductive success of bumblebees. A diverse diet even mitigated depredation by wax moths. To sustain vital bumblebee populations and their pollination services, diverse and floral rich habitat types should be conserved or restored in agricultural landscapes. In the fourth chapter we investigated, how bee densities in OSR fields are affected by past and current MFC cultivation and how insect pollination affects OSR yield components. Bee densities were positively affected by past MFC covers and negatively affected by current OSR covers. Pollinator exclusion decreased seed number per pod and increased seed weight. Insect pollination interacted with the plants` pod number in shaping yields, while compensating for low pod numbers. In the fifth chapter we analyzed the effect of landscape composition on the densities of different functional bee groups and their foraging behavior in faba bean fields in 11 different landscapes. Moreover, the effect of insect pollination and of landscape composition was tested on faba bean yield components. Landscape composition affected the bees` foraging behavior in the crop. Increasing covers of faba bean and of semi-natural habitats in a landscape had a positive effect on bee densities and on faba bean yield, while oilseed rape cover had a negative effect. Our study emphasizes the importance of considering landscape management in order to maximize flowering crop yields.
In conclusion, Vicia faba cultivation is an effective measure to promote wild bees, which are needed for high crop yields. Though, since benefits for pollinator groups depend on the crops` flower morphology, functional trait composition of wild bee communities can be changed in landscapes with a dominating mass-flowering crop species. Thus, landscape and local farm management should aim at increasing farmland plant diversity. The combination of a high cover and quality of semi-natural habitats as well as of diversified farming practices targeting a high spatial and temporal crop diversity is essential to conserve diverse pollinator communities and to sustain high crop yields. | de |