Causes of rarity in bumblebees

Abstract

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.

Introduction

Many bumblebee species have suffered declines in recent decades, both in Europe and in North America (Peters, 1972; Williams, 1982, Williams, 1986; Rasmont, 1995; Kosior, 1995; Buchmann and Nabhan, 1996; Westrich, 1996; Westrich et al., 1998). Of the 25 species known from the UK, three species are extinct and several more are now confined to a handful of sites and have uncertain futures. The consensus is that declines in numbers of bumblebees are linked to the intensification of farming practices (Williams, 1986; Osborne and Corbet, 1994; Goulson, 2003).

The plight of our bumblebee fauna deserves particular attention because loss of bee species will almost certainly have repercussions for other wildlife. A large number of wild plants are pollinated predominantly or exclusively by bumblebees, sometimes by particular species of bumblebee (Corbet et al., 1991; Osborne et al., 1991). Thus it seems probable that reductions in the abundance and species richness of bumblebees may lead to widespread changes in plant communities (Corbet et al., 1991). These changes will have further knock-on effects for associated herbivores and other animals dependent on plant resources.

Although many bumblebee species have become more scarce, a small subset of species (six in the UK) remain abundant and ubiquitous. At present we have little indication as to why bumblebee species differ so widely in their abundance and susceptibility to environmental change. Morphologically, all species are remarkably similar, the only obvious differences being variation in size and tongue length. They have broadly similar annual life cycles (although a few species are partially bivoltine), and all depend exclusively on nectar and pollen for food. Most species do not have precise habitat requirements, so far as is known (Williams, 1986).

For practical reasons, most studies of bumblebee ecology and behaviour focus on the common species, notably Bombus terrestris, Bombus lucorum, Bombus lapidarius, Bombus pratorum, Bombus pascuorum and Bombus hortorum in much of Europe. For most of the approximately 40 remaining European bumblebee species, we have very little information on forage use, or indeed on any other aspects of their ecology. A recent study in southern UK suggested that rare bumblebees may have more specialized diets, while those species that remain abundant are generalists (Goulson and Darvill, 2004). Thus studies of the forage used by common species may be of little help in developing appropriate conservation strategies for the rare ones. Ecological studies of rare and declining species are urgently needed if appropriate conservation measures are to be deployed. In particular, we need to improve our understanding of the causes of rarity in bumblebees.

Here we quantify and compare forage use by a broad range of UK bumblebee species to test whether rarity is correlated with dietary specialization. We quantify the tongue length of these species, since tongue length is likely to constrain foraging behaviour. We also review data on latitudinal range, phenology, and UK distribution, in an attempt to understand what factors determine bumblebee abundance and susceptibility to environmental change.