FLOWERING PHENOLOGY AND DENSITY-DEPENDENT POLLINATION SUCCESS IN CEPHAELIS ELATA (RUBIACEAE) (COSTA RICA)

In cloud forest near Monteverde, Costa Rica, the self-incompatible, distylous treelet, Cephaelis elata (Rubiaceae), is pollinated by the hummingbird, Lampornis calolaema. I investigated the importance of two potentially conflicting relationships affecting pollination service to C. elata flowers: (1) positive density-dependence in pollinator visitation, and (2) negative influences of large floral displays and pollinator territoriality on pollen transfer between plants of different floral morphs. I measured effects of floral display size, flower density, and nearest-mate distance on pollen receipt (numbers of stylar pollen tubes) and pollen donation (measured with powdered dye) at two sites during two 7-mo flowering seasons.; Lampornis calolaema males defended feeding territories composed of rich patches of flowers of C. elata and other short-corolla species; females foraged mainly at dispersed flowers. Due to the small size of most C. elata floral displays (median = 3 flowers/plant), individual territories usually contained many plants. Consequently, compatible pollen transfer within territories was high, and pollination success of C. elata flowers within territories was often greater than pollination success outside territories.; At each site, the density and dispersion of flowers influenced pollination service to flowers. There was, however, considerable seasonal variation in the strength and, in some cases, even the direction of the relationships examined. (1) Hummingbird visit rates to flowers were often highest during seasonal flowering peaks and at plants with many flowers. (2) Pollen receipt, and occasionally pollen donation, were greatest during flowering peaks. (3) The amount of pollen received by flowers was negatively correlated with the distance to the nearest compatible plant. (4) Presumably due to the spatial segregation of morphs and limited pollen carryover (measured in the lab with captive L. calolaema), flowers in dense patches frequently received fewer compatible pollen grains than isolated flowers. These results suggest pollination service may be highest at plants that do not produce large numbers of flowers per day, that spread out flowering over time, yet still flower in phase with the population.