Floral isolation and pollination in two hummingbird-pollinated plants: the roles of exploitation barriers and pollinator competition

Abstract

Differences in feeding performance and aggressive abilities between species and sexes of hummingbirds are often associated with the partitioning of their food sources, but whether such partitioning results in floral isolation (reproductive isolation at the stage of pollination) has received little attention. We examined components of floral isolation and pollinator effectiveness of Heliconia caribaea and H. bihai on the island of Dominica, West Indies. The short flowers of H. caribaea match the short bills of male Anthracothorax jugularis, its primary pollinator, whereas the long flowers of H. bihai match the long bills of female A. jugularis, its primary pollinator. In pollination experiments, both sexes of A. jugularis were equally effective at pollinating the short flowers of H. caribaea, which they preferred to H. bihai, whereas females were more effective at pollinating the long flowers of H. bihai. Moreover, an average difference in length of 12 mm between H. caribaea and H. bihai flowers did not prevent heterospecific pollen transfer, and both sexes transported pollen between the two plant species. In field studies using powdered dyes as pollen analogs, however, heterospecific pollen transfer was minimal, with only 2 of 168 flowers receiving dye from the other species. The length of H. bihai flowers acted as an exploitation barrier to male A. jugularis, which were unable to completely remove nectar from 88% of the flowers they visited. In contrast, interference competition combined with high floral fidelity through traplining prevented female A. jugularis from transferring pollen between the two Heliconia species. A combination of exploitation barriers, interference and exploitative competition, and pollinator preferences maintains floral isolation between these heliconias, and may have contributed to the evolution of this hummingbird-plant system.