Does specialization explain rarity and decline among British bumblebees? A response to Goulson et al.

Recent studies of British bumblebees have proposed a seemingly simple explanation for the decline in some species: that greater dietary specialization among the rarer species has put them at greater risk. However, comparisons of dietary specialization require: (1) that bees have access to the same dietary options among which to make their choices; (2) that the differing relative breadths of dietary choices made are not obscured by the differing sample sizes among bee species. Using one of the few suitable data sets, I find no evidence for a relationship between, on the one hand, rarity or declines in British bumblebees and, on the other, their dietary breadths, the strengths of their dietary preferences, or their proboscis lengths (which influence dietary choices). In contrast, there is support for a relationship between rarity or declines within Britain and the sizes of species' European ranges, particularly when these measures are adjusted to represent their ranges near sea level. Adjusted range sizes may reflect overall niche breadth and perhaps climatic and habitat specialization. This is not to say that climate change is the driving factor for declines or that changing food-plant availability is unimportant, but that climatic and habitat specialization may be a better indicator of risk of decline, which deserves further study.