Pollen‐Foraging Dynamics of Subalpine Bumblebees (Bombus Latr.)

Abstract

Abstract A 23‐year study on Mount Rainier of nectar‐foraging hummingbirds and pollen‐foraging bumblebees (Bom‐bus Latr.) indicated a constant relative frequency of Bombus species in the area and a wide occurrence of insect species on flowers plus a wide range of flower species pollinated by a single insect. Early‐blooming plants had a high queen/worker pollinator ratio, while workers predominated on later‐blooming plants. No altitudinal difference in distribution of Bombus species occurred. Analysis of 955 corbicular pollen loads from 1158 bumblebees collected yielded 49.5% monolecty for queens and 34.5% for workers with a wider range for queens than for workers. Colors of corollas photographed in visible and ultraviolet light corresponded to the visual spectra of animal pollinators. In the stable plant‐pollinator community investigated, pollinating insect activity appears phenologically controlled by floral anthesis and behaviorally related to floral form and function. Bumblebee tongue length, however. Is not a valid determinant for foraging niche separation except in Castilleja with a deep corolla tube accommodating long tongues of hummingbirds and bumblebees. It is concluded that in a biotic community well‐coordinated with the physical environment, limited flower constancy of pollen‐foraging pollinators appears to contribute a necessary degree of adaptive versatility through sharing of insect and floral resources. Flower constancy is primarily a function of the circumstance in which a pollinator operates and not simply a characteristic of the pollinator itself.