Paleotropical bat-pollinated plants avoid interspecific pollen transfer despite sharing pollinators
Pollination syndromes, or suites of floral characteristics attractive to specific groups of flower-visiting animals, promote intraspecific pollen movement by encouraging visitation fidelity of certain pollinator species and deterring the visits of other animal species. Yet flower species which share the same pollination syndrome also potentially share the same pollinators, which introduces the risk of interspecific pollen transfer and reduced plant reproductive success. For example, many plants exhibiting the bat pollination syndrome in southern Thailand are visited by Eonycteris spelaea, the most common nectar bat in the area. We hypothesized that these bat-pollinated plants reduce interspecific pollen movement through (a) non-overlapping flowering phenologies and (b) differential placement of pollen on the pollinator’s body. We tested our hypotheses with four plant species pollinated by E. spelaea. To compare flowering phenologies, we observed ten individuals of each plant species every two weeks for 12 months, noting when they were in bloom. To examine if interspecific pollen transfer is reduced due to differences in pollen placement on the pollinator, we conducted an experimental manipulation to compare pollen transfer by E. spelaea between conspecific flowers versus between heterospecific flowers, and then quantified pollen from four different locations on the bat’s body.
We found no overlap in the flowering phenologies of Ceiba pentandra and Durio zibethinus (two “big bang” plant species with short, highly synchronous flowering periods). In contrast, Musa acuminata and Parkia speciosa exhibit “steady state” phenologies; consequently, their long flowering seasons overlapped with each other and with the flowering seasons of other bat-pollinated plant species. While an overlap in flowering phenologies would seem to indicate movement of pollen between flowers of different species, our experiment with E. spelaea revealed that these bats transfer significantly more pollen between conspecific flowers than between heterospecific flowers. Furthermore, we found that the flowers of different plant species place pollen on different areas of E. spelaea, which accounts for the low levels of interspecific pollen movement. The combination of non-overlapping flowering phenologies between some plant species, and differences in pollen placement between others, ensures that no two plant species are ever in direct competition, despite sharing a common pollinator. These mechanisms promote the coexistence of plant species with similar niches. This study also reveals how nectarivores with broad diets can still be effective pollinators.