Woodlands Facilitate Reproductive Behaviour and Niche Partitioning in Farmland Bumblebee Communities

Open and closed woodland likely dominated Europe's landscapes during the evolution of its contemporary pollinator diversity, contributing distinct floral and non-floral resources and cooler and darker microclimates to otherwise treeless environments. As such, we would expect present-day, generalist pollinator species, such as farmland bumblebees, to exploit woodland resources to differing degrees, and for evolutionary adaptations, such as species' light sensitivity, to accord with relative use of shaded understories. We would also expect exploitation to differ between the pollen-collecting and reproductive castes. However, a lack of temperate woodland sampling has meant these predictions remain largely untested. To address this knowledge gap, we sampled bumblebee activity (activity-density) with blue vane traps in shaded woodland understories and in sun-exposed field margins and woodland canopies from May to July, at 12 sites across agricultural landscapes in Norfolk, England. We then examined relative levels of activity in the understory according to bumblebee species and their light sensitivity, and bumblebee caste. We found that levels of activity in the understory were highly species-specific and greatest for Bombus hortorum L. and B. pratorum L. Across species, activity levels in the two sun-exposed habitats (relative to the understory) were highly positively correlated with each other, and the most light-sensitive species were more active in the understory. Additionally, the reproductive castes were more active in the understory than workers, especially when temperatures reached heatwave thresholds. These results suggest that woodlands (1) support bumblebee reproductive behaviour, (2) have played a key role in facilitating niche differentiation, mediated by the provision of shade and (3) could be important for maintaining bumblebee diversity in agricultural landscapes-and under a warming climate-with attendant benefits for pollination services.