Moths vs. bees: Linking temporal variation in pollinator community structure to reproductive dynamics and pollen movement in an annual plant
Patterns of pollen movement are central to the ecology and evolution of plant populations, as they have immediate consequences for individual fitness, mating system dynamics and population connectivity. Understanding the processes that shape these patterns, however, is complex, particularly for animal-pollinated species, as pollen dispersal is shaped largely by pollinator foraging behavior. For example, many bees are thought to be central-place foragers, whereas many moths, birds and bats are thought to have broader foraging ranges. Here we examine how functionally distinct pollinator guilds influence fitness, mating system dynamics and pollen dispersal in the annual, self-incompatible grassland herb Oenothera harringtonii. We first conducted a selective exclusion experiment to determine whether nocturnal pollinators (hawkmoths) facilitate greater maternal fitness than diurnal pollinators (solitary bees). We then sampled offspring arrays with genetic markers to determine whether mating system parameters differ between nocturnal and diurnal pollination environments. Finally, we reconstructed individual pollination events through paternity analysis to determine whether nocturnal pollinators move pollen over greater distances than diurnal pollinators.
Pollinator exclusion had significant effects on maternal fitness and mating system dynamics. Flowers limited to visits from diurnal pollinators (bees) were characterized by reduced maternal fitness (smaller fruits) and fewer effective pollen donors relative to both open-pollinated and day-excluded (hawkmoth pollinated) flowers. Further, inbreeding was alleviated when flowers were limited to visits from nocturnal pollinators (hawkmoths). While the average distance of pollen movement varied between exclusion treatments, sporadic long-distance (1-2 km) pollination events were detected in all treatments. Collectively, these findings suggest that nocturnal pollinators are more effective agents of pollen movement and gene flow in O. harringtonii, and are consistent with expectations based on its biology and the foraging behavior of its pollinators. This study highlights the dynamic nature of pollen dispersal and demonstrates how selective exclusion experiments can be paired with molecular techniques to gain more nuanced insights into plant reproductive ecology and patterns of gene flow.