Herrera et al. (2010) studied the yeast communities established in the nectar of Helleborus foetidus, which is pollinated by bumblebees. Yeasts colonize H. foetidus floral nectar following probing of the nectaries by foraging bumble-bees carrying inocula on their mouthparts. Thus, the potential composition of yeast communities in the nectar can be estimated by examining the species composition of yeast inocula that are "‘travelling’" on the mouthparts of foraging bumblebees (i.e. the potential species pool). Herrera et al. compared this potential species pool with the actual yeast communities sampled from virgin nectar probed by bees. They found that only a small fraction of the potential species pool actually became established in nectar and that these species tended to be closely related. They suggested that this pattern is likely the result of H. foetidus nectar representing a harsh environment for most yeasts, such that only a few phylogenetically related nectar specialists physiologically endowed to tolerate a combination of high osmotic pressure and fungicidal compounds are able to develop.
Herrera et al. (2010) found that metabolic heat produced by nectarivorous (nectar-eating) yeasts alters the within-flower thermal environment of winter-blooming Helleborus foetidus. Flowers with yeast-containing nectaries had warmer interiors than flowers with "clean" nectar, and the magnitude of warming depended on the density of yeast populations in nectar. Although the authors did not directly evaluate the ecological consequences of yeast-induced floral warming of H. foetidus flowers in this study, they note that considerable circumstantial evidence suggests that some consequences are to be expected. Regardless of whether of it is produced by active endothermy or passive solar heating, previous studies have shown that floral warming can enhance plant reproduction through mechanisms that include increased pollinator visitation, pollen germination, pollen tube growth, fertilization success, fruit development, and seed size (Herrera et al. 2010 and references therein). Herrera et al. suggest that if the warming by yeasts of the interior of H. foetidus flowers raises the temperature of the gynoecium (female sex organs), then yeast warming could have beneficial effects on the maternal component of reproductive success, as found in other species. Warmth alone can act as a metabolic reward for pollinators and this effect is most likely under the cool conditions characteristic of the flowering season of H. foetidus (Herrera et al. 2010).
