An altitudinal cline in UV floral pattern corresponds with a behavioral change of a generalist pollinator assemblage

Abstract

Spatial variation in pollinator communities or behaviors can underlie floral diversification. Floral traits in the UV spectrum are common and mediate plant-pollinator interactions, but the role of pollinators in driving or maintaining their geographic variation has not been fully explored. We identify an altitudinal cline of increasing relative size of the UV bullseye pattern in Argentina anserina (Rosaceae) flowers, and assess whether pollination context contributes to clinal variation. At four sites that varied in altitude, we document the pollinator assemblage, and pollinator preference and visitation behavior. We then determine how pollinator visits affected pollen receipt and export. Finally, we describe how the functional relationship between UV floral phenotype and pollen receipt changed with altitude. Floral UV bullseye size increased with altitude, which corresponded with a change from a hymenopteran- to a dipteran-dominated pollinator assemblage. While dipteran and hymenopteran preferences for bullseye size were similar, flowers with large bullseyes received more foraging visits than those with small bullseyes at higher altitude. The reverse was observed at the lower altitudes; pollinators approached large-bullseye flowers often but rarely foraged. These differences are expected to affect fitness because foraging visits increased pollen export and receipt. Indeed, when natural variation in bullseye size was considered, it had a stronger effect on pollen receipt than other traits (flower size, display, or color). Plants with larger bullseyes tended to receive more pollen at the highest-altitude site, while those with smaller ones received more pollen at the lowest-altitude site. Results suggest that altitudinal changes in preference and behavior of the overall pollinator assemblage, but not differential preferences of pollinator taxonomic groups, could contribute to clinal variation in a UV floral trait for a generalist-pollinated plant.