Bee foraging and pollination : Consequences of spatial and temporal variation in flower resources

Sammanfattning: Wild bees foraging in contemporary agricultural landscapes are, because of agricultural intensification, faced with the challenges of reduced flower-rich habitats, as well as a changed spatio-temporal distribution of flower resources. As a result of this and other stressors such as pesticide exposure, widespread declines of bees have been reported. However, all bees are not equally affected. To be able to mitigate bee declines and promote the important pollination services they provide to both crops and wild plants, it is crucial to understand why bees with different functional traits respond differently to varying resources in space and time, and how competitive interactions between bees modify these responses. Furthermore, it is important to study needs for crop pollination and pollinator deficiencies, especially in small-holder farming communities in the global south where the dependence on pollinated crops is great and the process of agricultural intensification often intense. In this thesis, I use replicated large scale landscape designs and experiments to investigate effects of varying flower resources and competition on bee foraging behaviour and fitness, and the effects of varying bee abundances on the pollination and pollen limitation of a crop. I found that multiple resources are important to bee fitness, not only forbs in seminatural habitats, but also crops and woody plant species, in particular highlighting the importance of trees as pollen resources for bees. The mentioned resources are both spatially and temporally spread-out, illustrating the importance of resource complementation in both space and time. I further found that bumblebees collected diversities of pollen during single foraging trips, and that although individual bumblebees show some degree of temporary preferences, these are temporary and change over time. This likely facilitates bee persistence in agricultural landscapes where resources may change swiftly. I found that it was possible to detect competition effects from honeybees by observing the foraging behaviour in bumblebees, which opens up new ways to study competition pressures on wild bees. Finally, I found that traits related to nesting rather than traits related to foraging determine bee communities in smallholder agricultural landscapes compared to adjacent forests, which suggests that bee communities in these agricultural landscapes may benefit from the enhancement of appropriate nesting habitats. This could benefit many crops, for example chili crops, which I show require pollinators for sufficient fruit set, although there was no evidence for pollinator deficiency in the area studied. Altogether, my thesis contributes increased knowledge on resource needs and behaviour in bees in contemporary agricultural temperate and tropical landscapes, which may inform more efficient policy actions for bee conservation.