Mathematical modeling of the dynamic role of nectar microbes in mediating plant-pollinator interactions

Background/Question/Methods

Mutualistic interactions between two species can be facilitated or undermined by third-party species, whose impact not only depends on their intrinsic traits, but also on the population ratio of the species involved in the interactions. Furthermore, the role of third-party species in mediating the mutualistic interactions may change when other species coexist and compete for the niche. For example, species of yeasts (e.g., Metschnikowia reukaufii) and bacteria (e.g., Gluconobacter sp.) are commonly found in the nectar of flowers. Recent empirical work indicates that the microbes differentially affect nectar chemistry, pollinator preference for nectar, and resulting seed set by plants, and as a result, yeasts and bacteria may benefit or inhibit the plant-pollinator mutualism, respectively. In addition, empirical results also demonstrate competitive interactions between yeasts and bacteria in nectar. Here we take a mathematical approach integrating a biological market model and a SIR model to analyze the interactions between plants, pollinators and nectar microbes. We assume that pollinators modify their foraging behavior in response to the microbes and flower density, that yeasts are transmitted between flowers via pollinators, and that the microbial species engage in competitive interactions within floral nectar. Under these assumptions, we use the model to investigate how the impact of nectar microbes on plants and pollinators depends on three variables: the attractiveness of yeast-containing nectar to the pollinators, the flower density per pollinator, and the dispersal rate of nectar bacteria.

Results/Conclusions

Without nectar bacteria, yeasts that enhance the attractiveness of nectar may increase (or decrease) the host plant fitness when the flower density per pollinator is high (or low), but pollinators do not benefit from their existence. In the presence of bacteria, yeasts may benefit the host plant even without directly enhancing the attractiveness of nectar as they competitively exclude bacteria that dispel pollinators. When bacteria are present, pollinators also benefit from the existence of yeasts as they reduce the frequency of bacteria-degraded nectar, and the effect is particularly strong when flower density per pollinator is low or when dispersal rate of bacteria is high. Our results indicate the dynamic role of nectar microbes in mediating plant-pollinator interactions, which depends on the population densities of flowering plants and pollinators as well as competition within the microbial community.