The pollination biology of Burmeistera (Campanulaceae): specialization and syndromes

The floral traits of plants with specialized pollination systems both facilitate the primary pollinator and restrict other potential pollinators. To explore interactions between pollinators and floral traits of the genus Burmeistera, I filmed floral visitors and measured pollen deposition for 10 species in six cloud forest sites throughout northern Ecuador. Nine species were primarily bat-pollinated (84-100% of pollen transfer); another (B. rubrosepala) was exclusively hummingbird-pollinated. According to a principal components analysis of 11 floral measurements, flowers of B. rubrosepala were morphologically distinct. Floral traits of all species closely matched traditional ornithophilous and chiropterophilous pollination syndromes; flowers of B. rubrosepala were bright red, lacked odor, opened in the afternoon, and had narrow corolla apertures and flexible pedicels, which positioned them below the foliage. Flowers of the bat-pollinated species were dull-colored, emitted odor, opened in the evening, and had wide apertures and rigid pedicels, which positioned them beyond the foliage. Aperture width appeared most critical to restricting pollination; hummingbirds visited wide flowers without contacting the reproductive parts, and bats did not visit the narrow flowers of B. rubrosepala. Aperture width may impose an adaptive trade-off that favors the high degree of specialization in the genus. Other floral measurements were highly variable amongst bat-pollinated species, including stigma exsertion, calyx lobe morphology, and pedicel length. Because multiple species of Burmeistera often coexist, such morphological diversity may reduce pollen competition by encouraging pollinator fidelity and/or spatially partitioning pollinator's bodies.