| Abstract: | Characters involved in pollinator attraction are likely maintained by selection. Plants that invest more in floral displays and/or rewards are expected to attract more pollinators than those that do not. A large number of plants, however, are severely pollen-limited yet either produce small rewards or none at all. The orchid, Comparettia falcata, is a pollinator-dependent, self-compatible epiphyte distributed throughout the Greater Antilles, Central and South America. In Puerto Rico where it is pollinated by the hummingbird Chlorostilbon maugaeus, C. falcata presents a smaller nectar reward than most other plants pollinated by the same species. To determine whether or not selection would favor the production of higher nectar levels, we enhanced the quantity of nectar offered by flowers in a Puerto Rican population for two flowering seasons. We monitored visitation frequencies, pollen movement, and reproductive success at three sites with different canopy coverages. Daily censuses of hummingbirds provided estimates of relative pollinator abundance. A multiway contingency test employing Wal?s statistic showed no overall differences in reproductive success between plants with enhanced rewards and unmanipulated controls. Site differences, however, were clear. Plants of the mid- and high-light sites had greater success than those of the low-light site, and the differences were usually at least two-fold. There was a significant site-treatment interaction in reproductive success that could be attributed to the overall trend whereby controls of both mid- and high-light sites did better than the nectar-enhanced plants. Most of the observed pollinations (85%) with stained pollinia resulted in self-pollinations that did not differ among treatments. Seed crops from self- and cross-pollinations revealed no differences in the number of viable seeds. Because we found little evidence of selection for increasing nectar reward via inbreeding depression or male and female reproductive success, and previous studies have indicated that meager natural levels of reward are better than none at all, we suspect that reward production in C. falcata may be driven by a combination of pollination-limitation and resource constraints. |
