Floral abundance and bee density affect species-specific foraging patterns of alpine bumble bees

Abstract

In response to the qualitative and quantitative changes in floral resources, bumble bees flexibly forage multiple plant species throughout the growing season. During the short summer in an alpine ecosystem, the activity of worker bees is maximized in the middle of the season, when the competition for floral resources may be intense. We predicted that the foraging patterns of bumble bees are affected by both relative floral abundance and interactions between bumble bee species. We recorded the floral abundance of individual plant species and the foraging frequency of bumble bees in an alpine meadow during the major flowering period over 3 years. Two bumble bee species were common during the major flowering period. Although they tended to visit abundant floral species, the shorter-tongued species (Bombus hypocrita) showed a more diverse and flexible floral choice than the longer-tongued species (Bombus beaticola). The degree of floral use overlap between two bumble bee species tended to decrease when the foraging density of the longer-tongued species was high. These results indicated that multiple bumble bee species are able to coexist when certain bee species can flexibly change targeting flowers in response to the temporal variations in flowering species and the density of competing bee species. The extent of foraging flexibility is related to the morphological traits of bee species and availability of floral resources.