Old World fruit and nectar bats: Comparing the interactions of bat-pollinated plants with facultative versus obligate pollinators

Background/Question/Methods

Many plant species rely on animals for pollination. Consequently, the floral design, reproductive success, and genetic diversity of these plants are influenced by pollinator foraging behavior, which transfers pollen (gametes) between conspecific individuals. Concurrently, host plants offer nectar rewards that influence pollinator behavior; natural selection favors the most capable foragers. My research explores both sides of the plant-pollinator mutualism by examining bat species of the family Pteropodidae and their host plants. This family is ideal for studying pollination since species are readily classified as nectar bats, which feed almost exclusively on floral resources (nectar and pollen), or fruit bats, which primarily eat fruit but opportunistically consume floral resources. Thus, I can compare the interactions of bat-pollinated plants with obligate pollinators (nectar bats) versus facultative pollinators (fruit bats). I netted a total of 417 nectar and fruit bats across four national parks in Thailand and quantified pollen loads (number of pollen grains of each plant species) carried by each; this measure predicts potential pollinator impact on plant reproductive success and gene flow. To assess how host plants differentially influence the behaviors of obligate versus facultative pollinators, I analyzed the time and location of bat captures to compare temporal and spatial differences in foraging behavior.

Results/Conclusions

As expected, nectar bats carry significantly more pollen than fruit bats (Kruskal-Wallis, p<0.001). This finding supports my prediction that, compared to fruit bats, nectar bats confer greater potential reproductive success and increased gene flow through pollination to their host plants. Nectar bats also visit bat-pollinated flowers significantly earlier in the night than fruit bats (Kolmogorov-Smirnov, p<0.001), however, nectar and fruit bat presence did not vary between habitat types. The temporal differences in foraging behavior are likely explained by relative dependence on host plant resources: nectar bats should forage early, before floral resources are depleted; fruit bats do not face this pressure, since fruit abundance is relatively constant over the course of a night. My results demonstrate that plant-pollinator mutualisms are dynamic two-way interactions, with tighter associations producing stronger selective pressures on both plants and pollinators. How pollinators influence plant reproductive success is of particular interest given that many economically-important crops are animal-pollinated. Additionally, studying pollinator-mediated gene flow of plants is increasingly important as habitat degradation and loss transform once-contiguous native landscapes into isolated patches.