Teasing out the functional groups of oil-collecting bees in the light of the pollination of Nierembergia flowers

Pollination interactions between plants bearing oil-flowers and oil-collecting bees are regarded as specialized, and within the major group of bees, oil-collecting bees are considered a special functional group of pollinators, yet they encompass a great diversity, not only at taxonomic level but also in morphological and behavioral aspects. Since the functional group of oil-collecting bees is functionally diverse, we examined whether these bees, despite their gross commonalities, may be exerting divergence within and among oil-flowered species or, alternatively, promoting convergence to some general oil-flower morphology that can be pollinated by a functional range of oil-bees. To answer this question, we recorded pollinator assemblages of 18 taxa of Nierembergia, as well as the foraging behavior, visitation frequency, and morphometric characterization of all oil-collecting bees species that pollinate the flowers of those taxa, by field observations and sampling. We found that the pollinator assemblages of the species and varieties of Nierembergia largely differed among taxa. 14 species of oil-collecting bees, belonging to the genera Chalepogenus, Centris, Tapinotaspis, Caenonomada, Tetrapedia, and Arhysoceble, were recorded as pollinators of Nierembergia flowers. These pollinator species varied in multivariate body size, rewards-gathering behavior, and way they carry out pollination on flowers. According to our results, Nierembergia flowers are pollinated by five functional groups of oil-collecting bees that presumably promote divergence within and among Nierembergia taxa. Floral resource-harvesting behavior of oil-collecting bees species that pollinate Nierembergia flowers would be a key feature in the definition of functional groups and in the diversification of flower features in Nierembergia.