On the distribution of bumble bees (Hymenoptera, Apidae) with particular regard to patterns within the British Isles
Williams, P. H. (1985)
PhD thesis, Department of Applied Biology. Cambridge: University of Cambridge. 180 pp.
If ecology can be considered as the study of the distribution and abundance of organisms, then the mechanisms governing the distributions of bumble bees are not only of practical importance to pollination and conservation interests, but also of primary theoretical concern. In this dissertation the distributions of bumble bees are examined at successively finer scales and patterns described from analyses of surveys.
The cladistic method is used to classify sixty species by characters of the male genitalia selected for their functional significance. The phylogenetic interpretation of the composite character state tree is considered in conjunction with present patterns in distribution between continents for the evaluation of possible histories of spread and speciation.
Regional patterns of distribution within England, Wales and Scotland in the Bumblebee Distribution Maps Scheme data are described using a classificatory approach. Three major biogeogarphic elements and four biogeographic regions are defined. Large reductions in the distributional ranges recorded after 1960 are found for two of the elements, which have resulted in the emergence of a new Central Impoverished Region encompassing twenty-three vice-counties in central England.
Local patterns of distribution are described from Kent within the Southern Region, the sole region wherein all three elements retain a strong representation after 1960. Those species that occur only in particular regions of Britain after 1960 are also very locally restricted within this region and less abundant where present. They are further associated with certain open habitats, characterised by coastal and old meadow vegetation. This may be indicative of a relative lack of disturbance. A general mechanism is proposed, founded here on considerations of the economics of energy, to explain a species' pattern of distribution, in which fragmentation at the margins of the distribution occurs in response to a mosaic of effectively only two types of habitat presenting differing fundamental levels of limiting resources.
Coexistence is examined within a small area of Dungeness in Kent, where there is a particularly large assemblage of species. Flower choice by worker bumble bees and partitioning of nectar resources is compared with that at Shoreham, which is a more typical, species-poor site in north Kent. The possible causes and effects of differences in the flora utilised by bumble bees are discussed. The marginal mosaic model is used to predict an alternative form of resource-dependent 'structure' in local species assemblages that is amenable to testing.
Finally, the apparent decline of the British bumble bees nearer the edges of their distributions is discussed with regard to factors including climate and changes in land use.