“What’s the buzz with bilberries?” – Bilberry pollinator community structure and function in relation to forest type and habitat structure

Abstract: Over the last century, Swedish forests have undergone rapid land-use change due to intensification of management practices for timber production and the introduction of monocultures, which have resulted in considerable habitat loss and negative impacts to biodiversity. Insects form a significant component of forest biodiversity, yet forest-dwelling pollinators remain understudied, and little is known about how management strategies affect forest insect pollinator communities. The European bilberry (Vaccinium myrtillus), which forms a dominant component of Swedish forests' understory, is highly dependent on insect pollinators and is sensitive to habitat changes due to forest management. Here, I investigated how the structure and function of bilberry pollinator community differed between forest types (natural vs. managed forest) as well as in response to habitat structure variables (stand basal area, deadwood volume and floral density). Firstly, I recorded the abundance and diversity of the main bilberry floral visitors (nine bee species and four hoverfly morphospecies) in each forest type throughout the bilberry flowering period. Secondly, I conducted a pollination experiment to compare the pollinator effectiveness (i.e., pollen deposition per single visit), of the most common floral visitor taxa, and quantify bilberry pollination dependency (i.e., probability of fruit set with or without insect pollination). I found that bumblebees were the most important pollinators in this system due to their high pollination effectiveness and abundance, but solitary bees, honeybees and hoverflies were also effective pollinators on a per visit basis. Deadwood availability and stem basal area were important determinants of bee community structure. The hoverfly community composition differed between forest types, which was likely driven by the higher floral densities in the managed forests. Furthermore, I found that bilberry was strongly pollen-limited; with only 40% of open-pollinated flowers developing into fruit. This study demonstrates that natural and managed forests can host similar, diverse floral visitor communities, which contribute significantly to bilberry pollination, and therefore fruit development. Given the ecological and cultural importance of bilberry within boreal forests, forest management should consider the importance of mature forests as valuable habitats for insect pollinators, for example by keeping stands that differ in age and deadwood availability.