Niche overlap and diet breadth in bumblebees; are rare species more specialized in their choice of flowers?

The ecology of all bumblebees (Bombus spp.) is similar, yet some species have declined greatly while others remain abundant. We examine whether abundance is related to diet breadth. The floral visits of bumblebees were examined on Salisbury Plain, UK. All of the species examined gathered pollen mostly from Fabaceae. All species gathered nectar from a broader range of flowers than they did pollen, and longer-tongued bees had a narrower diet breadth when collecting nectar. B. hortorum (the species with the longest tongue) specialized on Trifolium pratense. As predicted, abundant species had a broader diet than rare species. Species with similar-length tongues visiting similar flowers. However, interspecific competition did not appear to be important since species with similar tongue lengths and high niche overlap co-occurred at high abundance. We suggest that the rare species may be those with short colony cycles, in which dependence on high quality food to rear larvae quickly forces specialization.