Individual and multilevel selection on nectar traits in a plant-pollinator mutualism

In consumer-resource species interactions, consumers can play a role in shaping the evolution of their own resources by acting as an agent of selection on the consumed species' traits. Many consumers forage on groups or patches of their resources and may change their foraging behavior within and among groups with different trait compositions. If a consumer's behavioral changes affect the fitness of their resource species, this creates the potential for consumers to impose multilevel selection on the traits of the resource species. Here, we examine how pollinator foraging behavior may create individual and multilevel selection on nectar resources in the beetle-pollinated understory wildflower, Amianthium muscaetoxicum (Fly Poison). We measured nectar traits, pollinator behavior, and reproductive outcomes of individual plants, small plant neighborhoods, and whole plant patches and examined phenotypic selection at each level. We found that nectar volume at the patch level shaped patterns of pollinator visitation, but phenotypic selection on nectar traits was most consistent with beetles engaging with plants as discrete individuals, not as groups. However, we did detect a neighbor contrast component of multilevel selection: the direction of selection on nectar volume was more negative or flat in patches with lower mean nectar production and more positive in patches with higher mean nectar production. Overall, we found strong support that selection favors individual plants with high-quantity, high-quality nectar resources across the forest.