Abstract
Plant-pollinator systems can exist along the gradient from extreme specialization to extreme generalization. Theoretical work predicts that high pollinator reliability may lead to specialized pollination systems, while pollinator unpredictability may promote generalized pollination systems. However, empirical tests of these predictions are constrained by the availability of accurate field observations, particularly in some groups of plants, such as trees pollinated by nocturnal animals. Plant species that face variable pollinator availability within their distributional range represent an ideal study system to address the effect of pollination predictability on the specialization level of pollination systems. This is the case of Ceiba pentandra, a tropical tree with chiropterophilic flowers. Most previous studies have described the pollination system of this tree species as mainly or exclusively bat-pollinated. Here we studied the pollination system of C. pentandra in the Yucatan Peninsula, a region where no species of specialized nectarivorous bats occur. We assessed quantity (visitation rate) and quality pollination components (pollen deposition, fruit/seed set) for diurnal and nocturnal visitors in two different locations and years. We expected to find a generalized pollination system due to the absence of specialized pollinators. As predicted, we found five functional groups of nocturnal and diurnal pollinators. Diurnal and nocturnal pollinators contributed to a similar extent to quantity (visitation rate) and quality pollination components (fruit & seed set). However, the contribution of diurnal and nocturnal pollinators also varied either spatially or temporally. We conclude that the pollination system of C. pentandra in the Yucatan Peninsula is highly generalized and that temporal and/or spatial unpredictability in the effectiveness of its pollinators may explain this finding.