Resource partitioning and per-flower foraging efficiency in two bumble bee species

Two sympatric bumble bee species partition nectar resources in a subalpine Colorado meadow: the larger bee with the longer proboscis, Bombus appositus, preferentially visits the species with the longer nectar spur, Delphinium barbeyi, while the smaller B. flavifrons prefers the shorter-spurred Aconitum columbianum. We tested whether per-flower foraging efficiency (nectar obtained per second spent in the flower) differs for the two bee species on the two plants, and whether efficiency differences can explain the observed visitation differences. In our exclosure, as in the field, the longer-tongued B. appositus foraged almost exclusively on Delphinium. Bombus appositus gained significantly more nectar per second on Delphinium flowers than on Aconitum. Although the smaller B. flavifrons visited Aconitum more frequently, there was no significant difference in nectar gained per second for these bees on Delphinium and Aconitum in the exclosure, where all flowers had full nectar. We hypothesize that the shorter-tongued bees may be displaced from the longer-spurred flowers by exploitative (consumptive) competition in the field, and therefore invest in learning how co forage efficiently from the more complex Aconitum flowers. Resource partitioning between these bee species can be explained, at least in part by efficiency differences related to morphology.