Functional significance of interspecific variation in Burmeistera flower morphology:: Evidence from nectar bat captures in Ecuador

What causes flowers to diverge? While a plant's primary pollinator should strongly influence floral phenotype, selective pressures may also be exerted by other flower visitors or competition with other plants for pollination. Species of the primarily bat-pollinated genus Burmeistera (Campanulaceae) frequently cooccur, with up to four species in a given site, and broadly overlap in flowering phenology, typically flowering throughout the year. The genus displays extensive interspecific variation in floral morphology in the degree that the reproductive parts (anthers and stigma) are exserted outside of the corolla, and species can be roughly classified as either long or short-exserted. I tested two hypotheses regarding the functional significance of such variation: (1) exsertion lengths correspond to pollination by bat species of different sizes; and (2) variation serves to partition pollinator's bodies spatially and thus reduces interspecific pollen transfer. I captured bats in Ecuador to evaluate the identity and location of the Burmeistera pollen they were carrying. Results show that exsertion does not correspond to specialization on different pollinators; different bat species carried pollen of both flower types just as frequently. In support of the second hypothesis, pollen from flowers of different exsertion lengths was found to occur on different regions of bats' heads. This may serve to reduce competition for pollination among coexisting Burmeistera.