Contemporary pollen flow as a multiscale process: Evidence from the insect-pollinated herb, Pulsatilla vulgaris

1. Understanding the drivers and spatial scale of gene flow is essential for the management of species living in fragmented landscapes. In plants, contemporary pollen flow is typically modelled as a single spatial process, with pollen flow declining exponentially within a short distance of mother plants. However, growing evidence suggests that many species do not conform to these patterns, often showing an excess of long-distance dispersal events or sometimes even multimodality in dispersal kernels. This suggests that a single function might be insufficient to capture the true complexity of pollination, which in reality is often achieved by multiple pollinators that vary in their foraging ranges and interactions with the landscape. 2. We reconstructed realized pollen flow and assessed pollen immigration for seven populations of the insect-pollinated herb Pulsatilla vulgaris. We quantified the effects of distance, floral resources and landscape composition over multiple spatial scales and tested the hypotheses that within-population pollen flow is related to resources and landscape context measured locally, and that among-population pollen flow is related to features measured at larger spatial scales. 3. We found that pollen flow within populations was more likely to occur amongst near neighbours, but that among-population pollen flow was random with respect to source populations. We further found that local floral density could explain patterns of within-population pollination distances and population-level selfing rates, whereas pollen immigration rates were best explained by the proportion of forest within a radius of 500 m around focal populations. 4. Synthesis. Together, our results suggest that within- and among-population contemporary pollen flow may be governed by different underlying processes, possibly related to differences in the foraging range and habitat use of bee species that contribute to pollination at different scales. This highlights the critical need for researchers to take a more pollinator-eyed view of contemporary pollen flow in plants by (1) recognizing that within- and among-population gene flow by pollen may depend on different sets of pollinators that respond to features at different spatial scales (2) considering additional factors that may alter attractiveness, detectability and accessibility of plants to pollinators beyond the effects of distance.