POLLEN REMOVAL BY BUMBLE BEES AND ITS IMPLICATIONS FOR POLLEN DISPERSAL

Measured pollen removal from Aconitum delphinifolium, Aralia hispida, Lupinus sericeus, Mertensia paniculata, Pedicularis bracteosa and P. contorta. Much interspecific variation in pollen removal depends on the likelihood of contact between pollinator and anthers. Bumble bees Bombus spp. removed a median of 50-80% of the available pollen during 1st visits to species with exposed anthers. Only 19% was removed during initial manipulation of lupine flowers, which present pollen indirectly. Within species, removal increased with visit duration; it did not additionally depend on the number of visits involved. Removal typically changed in direct proportion to differences in pollen availability. Pollen size also affected removal, but it is unclear that this effect is direct. In most cases, removal was unaffected by species or caste of bee involved. Succeeding visits to a flower removed either a fixed or, more commonly, a declining proportion of the pollen remaining in a flower. A model of pollen dispersal indicates that a constant removal proportion would maximize dispersal when the number of visits a flower receives varies little. Diminishing proportional removal would allow plants to maintain pollen dispersal when the frequency of pollinator visits is uncertain. -from Author