Restoration and management of wildflower-rich machair for the conservation of bumblebees

Abstract

Over the last half century, the widespread decline of bumblebees across the agricultural landscapes of Western Europe and North America has been well documented. This decline has undoubtedly been driven to a large extent by the intensification of agriculture, which has fragmented landscapes and removed large areas of suitable foraging habitat, nesting and hibernation sites. Consequently, some of the rarest Bombus species now persist only in isolated pockets of semi-natural habitat, which have been subjected to little agricultural intensification. Of the 25 Bombus species native to the UK, three species have gone extinct in recent decades and several others are severely threatened. Remaining populations of the UK’s rarest bumblebee, Bombus distinguendus, have become strongly associated with florally-rich machair grassland habitats found only in the North and West of Scotland and Western Ireland. Machair, a unique habitat that forms on soils rich in shell sand, has been maintained by rotational agricultural practices implemented by crofters. However, recent changes in crofting practices, which include the intensive grazing of machair in some areas, or conversely the abandonment of machair management all together in others, have resulted in sections of machair that have become degraded and consequently exhibit low floral abundance and species diversity. This has significant implications for species such as B. distinguendus, which have for the most part come to rely of the florally-rich swards of machair grassland. This thesis aimed to develop a greater understanding of how machair grassland habitats are utilised by foraging bumblebees, including B. distinguendus, and in turn examined the potential for restoring degraded areas of machair via a variety of methods. The research presented here examines the influence of current crofting practices on the abundance of bumblebees and their forage plant species and combines this information with a detailed exploration of the machair seed bank and potential machair restoration treatments. The specific foraging requirements of B. distinguendus were found to be similar to those of other long-tongued bumblebee species and the provision of plants from the Fabaceae family was found to be of particular importance. Current crofting practices implemented in the North and West of Scotland were, on the whole, found support low numbers of foraging bumblebees. Similarly, existing habitat management schemes, designed to provide early cover for corncrakes and foraging resources for bumblebees, were found to be largely ineffective in attracting B. distinguendus, when compared with florally-rich machair habitat. In addition, this research suggests that the existing machair seed bank is unlikely to provide a sufficient resource for reinstating florally-rich habitat to degraded areas of machair. However, this thesis has demonstrated that it is possible to implement seed mixes on machair which can reinstate species typical of machair plant communities and which also attract high numbers of foraging bumblebees. The findings of these habitat assessments and restoration trials are examined in full in the following chapters and implications for the future management of wildflower-rich machair are discussed throughout.