A gradient of pollination specialization in three species of Bolivian Centropogon.

PREMISE

Closely related plant species with overlapping ranges often experience competition for pollination services. Such competition can select for divergence in floral traits that attract pollinators or determine pollen placement. While most species in Centropogon (Campanulaceae: Lobelioideae) have flowers that suggest adaptation to bat or hummingbird pollination, actual pollinators are rarely documented, and a few species have a mix of traits from both pollination syndromes. We studied the pollination biology of a "mixed-syndrome" species and its co-occurring congeners to examine the relationship between floral traits and visitation patterns for Centropogon.

METHODS

Fieldwork at two sites in Bolivian cloud forests involved filming floral visitors, quantifying pollen transfer, and measuring floral traits. Stamen exsertion, which determines pollen placement, was measured from herbarium specimens across the geographic range of these species to test for character displacement.

RESULTS

Results show a generalization gradient, from primarily bat pollination in white-flowered Centropogon incanus, to bat pollination with secondary hummingbird pollination in the cream-flowered C. brittonianus, to equal reliance on both pollinators in the red-flowered, mixed-syndrome C. mandonis. Pollen transfer between these species is further reduced by differences in stamen exsertion that are accentuated in zones of sympatry, a pattern consistent with character displacement.

CONCLUSIONS

Our results demonstrate that key differences in floral color and shape mediate a gradient of specialization in Bolivian Centropogon. Interspecific pollen transfer is further reduced by potential character displacement of a key trait. Broadly, our results have implications for understanding the hyper-diversity of Andean cloud forests, in which multiple species of the same genus frequently co-occur.