Pollinator community and function: in oilseed rape fields and in drought-stressed grassland.

This thesis explores (i) how pollinator community characteristics relate to pollination service, and (ii) how pollinator communities will be affected by climate change-induced drought. Chapter 1 gives an overview of the evidence for a relationship between biodiversity and ecosystem services (BES), with particular reference to pollination services. The major threats to pollinators are discussed, including how these affect the characteristics of pollinator communities and ultimately, pollination services. Chapter 2 explores how pollinator community characteristics affect the pollination service to winter-sown oilseed rape, an economically important crop. A modelling approach was used to estimate the pollination service that is provided by different flower visitors and to explore the mechanisms that are driving the pollination service in this system. Overall, the contribution of bees and non-bees to pollination service was estimated to be similar. Functional group richness had a positive effect on estimated pollen deposition, providing evidence for a BES relationship due to community structuring. However, the abundance of common species was the primary driver of pollination service. Chapter 3 explores how pollinators are likely to be affected by drought events, which are predicted to increase in frequency and intensity due to climate change. In a field experiment in calcareous grassland, rooves were placed over plant communities to simulate a single period of drought. Subsequent effects on floral resources and flower visitor activity were measured. Flowers in the drought treatment were less likely to contain nectar and racemes had fewer flowers overall. At the community scale, there were substantially fewer flowers in the drought treatment, suggesting that drought events will cause periods of floral resource scarcity. In both case studies, changes to pollinator communities, by loss of biodiversity or due to effects of drought, have the potential to reduce pollination of both crop and wild plants