A nectar reward: Is more better?

Characters involved in pollinaror attraction are likely maintained by selec tion. Plants that invest more in flora displays and/or rewards are expected to attract more pollinators than those that do nor. A large number of plan ts, however, are severely pollen-limited yet either produce small rewards o r none at all. The orchid, Comparettia falcata, is a pollinator-dependent, self-compatible epiphyte distributed throughout the Greater Antilles, Centr al and South America. In Puerto Rico where it is pollinated by the hummingb ird Chlorostilbon maugaeus, C. falcata presents a smaller nectar reward tha n most other plants pollinated by the same species. To determine whether or nor selection would favor the production of higher nectar levels, we enhan ced the quantity of nectar offered by flowers in a Puerto Rican population for two flowering seasons. We monitored visitation frequencies, pollen move ment, and reproductive success at three sites with different canopy coverag es. Daily censuses of hummingbirds provided estimates of relative pollinato r abundance. A multiway contingency test employing Wald'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 sire, and the differences were usually at lease two-fold. The re was a significant site-treatment interaction in reproductive success tha t could be attributed to the overall trend whereby controls of both mid- an d high-light sites did better than the nectar-enhanced plants. Most of the observed pollinations (85%) with stained pollinia resulted in self-pollinat ions that did nor differ among treatments. Seed crops from self and cross-p ollinations revealed no differences in the number of viable seeds. Because we found little evidence of selection for increasing nectar reward via inbr eeding depression or male and female reproductive success, and previous stu dies have indicated that meager natural levels of reward are better than no ne at all, we suspect thar reward production in C. falcata may be driven by a combination of pollination-limitation and resource constraints.