Causes of rarity in bumblebees

Many bumblebee (Bombus) species are thought to have declined in abundance in the last 50 years, while a small number of species remain abundant. Here we examine which factors render some British bumblebee species more susceptible to environmental change than others. We present forage data on workers of 15 bumblebee species gathered from 172 one hour searches conducted at sites in southern UK, the Hebrides (western Scotland) and in New Zealand (to which UK bumblebees were introduced). We also review data on distribution, phenology and tongue length of these species. Overall, Fabaceae appear to be the major pollen source for most bumblebee species. In particular, long-tongued, late emerging species such as Bombus ruderatus, Bombus humilis and Bombus subterraneus specialize heavily in gathering pollen from Fabaceae, and this group of bumblebee species have all declined. Some of them are also at the edge of their geographic range in the UK, which may have rendered them more sensitive to environmental change. The decline of many bumblebee species is probably attributable largely to the loss of unimproved flower-rich grasslands, a habitat rich in Fabaceae. The bumblebee species that remain abundant are mostly short-tongued species that emerge early in the season and have less specialized diets; these species are very common in suburban gardens where they are able to exploit the broad range of floral resources. A third group of bumblebees are strongly associated with Ericaceae in moorland and heathland habitats, and have probably always had restricted distributions. A small number of species are not so easily categorised. Bombus soroeensis and B. ruderarius are not dietary specialists, nor are they close to the limit of their geographic range, but nevertheless they have declined. Much of the ecology of rare bumblebee species remains poorly understood and in need of further study.