The response of animal pollinators to the spatial distribution of plants is crucial to understanding how plant-pollinator interactions contribute to plant reproductive processes. For plant species that aggregate in patches, attributes of the population, such as the size and distribution of patches and the density of plants, may affect pollinator visitation to a greater degree than characteristics of individual flowers. We examine how distribution and characteristics of patches of the self-incompatible vernal pool species, Lasthenia californica (California goldfields) impact pollinator movement using pollen load size and pollen germination rates as proxies to infer visitation patterns. Plant density and the size and distribution of patches were estimated from aerial images captured during drone surveys of an upland prairie ecosystem in Southern Oregon.
We found that denser patches receive the highest deposition of compatible pollen on stigmas. Large patches have an accumulation of low quality pollen and reduced pollen germination success, most likely due to pollinators moving genetically-related pollen within patches. Patterns of pollinator behavior inferred from the quantity and quality of pollen deposited on stigmas are consistent with the optimal foraging theory. Pollinators optimize foraging by preferentially moving within patches that are larger or are in close proximity to larger patches. Patches established in close proximity to larger patches are found to receive a greater pollen load. Patches distributed at a greater distance from neighboring patches experience a reduced pollen load due to decreased local pollinator activity. Our results suggest that individual patch characteristics do not significantly affect overall pollinator visitation rates, however the distribution of nearby patches contributes to pollinator behavior.