Species Structure of North European Bumblebee Communities

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Abstract
The structure of North European bumblebee communities was studied on regional and local scales. As the bumblebee proboscis has a potentially important role in resource partitioning, the structure of bumblebee communities was analyzed in terms of proboscis length relationships. The regional numbers of bumblebee species in 100-km squares in northwestern Europe were frequently between 11 and 15. No obvious south to north gradient in species richness was found. The slightly lower regional numbers of species in Finland are probably due to the lack of alpine habitats with associated species. England [UK] and Ireland have fewer species than equally sized areas in Scandinavia. Most of the bumblebee species (25/29) in northern Europe have proboscises shorter than 12 mm. Short-tongued species tend to be more widely distributed than those with long tongues. Bombus hortorum, a long-tongued species, deviates clearly from this trend by being more widely distributed than the other species with a long proboscis, all of which are either habitat (alpine) or food specialists. Local communities consist of a number of coexisting species, most of which differ only slightly in terms of proboscis length. Only a few species (1-3 per community) having long proboscises and these species are seldom abundant. When species in the realized communities were compared in terms of proboscis length distances, no statistically significant differences were revealed from distances expected on the basis of random sampling from the local species pool. The same was the case also when only the abundant species were taken into account. Thus, propositions advocating proboscis length differences and food competition as the single assembly rule of bumblebee communities should be supported by good evidence. Other conceivable factors include competition for nesting sites, nest predation, and other mechanisms that shape the present-day local communities and regional species pools.