Oil flowers for oil bees? pollinator performance confirms partially the pollination syndrome predicted

Abstract Oil-producing flowers rely on pollination by oil bees, resulting in a narrow pollination niche. However, some observations have pointed to the high frequency of pollen bee visits widening the spectrum of potential pollinators leading to a more generalist system. In this way, we investigate the role of oil and pollen bees as effective pollinators to better describe the realized pollination niche of these species. We combined visitation frequency data, flowering phenology, pollen deposition on the stigmas and reproductive rates for three oil-producing flowers species in Pampean grasslands from Southern Brazil. Herbertia pulchella and Cypella herbertii strongly overlapped in their flowering peaks with higher pollination performance of oil bees, while Kelissa brasiliensis has its flowering peak temporally separated of them and both bee groups showed similar performances. All of them showed a moderate pollination limitation with fewer values of natural fruit set compared to hand pollinated flowers fruit set. Our results emphasizes the significance of the pollination efficiency difference among pollinator groups contributing to a better understanding of the realized pollination niches of plants. Additionally, we observed that oil bees play a fundamental role for two of the three plant species studied, as predicted by Vogel’s pollination syndrome for this particular system with an extremely specialized floral resource. In some conditions, pollen bees may contribute to minimising the variation of fruit set. These findings could aid in understanding the evolution of floral traits in oil flowers of the Iridaceae in a broader context.