It’s complicated: Subtle characteristics of communities affect the nature of plant-pollinator relationships

Background/Question/Methods

The nature of interspecific relationships can be context-dependent, and relationships between plants and floral visitors are no exception. Floral visitors can increase plant fitness by acting as pollinators, or may lower plant fitness by acting as nectar or pollen thieves. The same insect may act as a mutualist in some situations and a parasite in others; the role of these “conditional parasites” may depend on the context, including the plant’s reproductive biology and the composition of the pollinator community. Evidence from theoretical work indicates that conditional parasitism can occur. However, we do not understand the circumstances that will promote this variation in relationships in nature, and which aspects of plant and visitor biology will be most important. 

To investigate the potential factors influencing conditional parasitism, we constructed a set of simulation pollen depletion models that track two floral visitors visiting a population of dichogamous plants through time. Floral visitors vary in the number of pollen grains that they remove and deposit, and models vary in the rules governing the relationships between pollen availability, removal, and deposition. In addition, we simulate aspects of plant reproductive biology and visitor foraging behavior. 

Results/Conclusions

These models allow us to explore various aspects of the interactions between plants and floral visitors, identify those aspects that have the most potential to influence these relationships, and generate hypotheses about the conditions that may cause conditional parasitism in real populations.

Conditional parasitism occurred more readily when visitation rates were high, one visitor removed high quantities of pollen, the relationship between pollen availability and overall pollen deposition was direct and immediate, and the one visitor exhibited a bias towards male-phase flowers. Conditional parasitism occurred less readily when visitation rates were low, both visitors had low removal rates, the relationship between pollen availability and overall deposition was indirect, and when plants dispensed pollen over time. Surprisingly, pollen deposition (the proportion of pollen removed that was subsequently deposited) had little effect on the context at which conditional parasitism occurs. Moreover, when the relationship between pollen removal and deposition is decelerating rather than proportional, conditional parasitism occurs less readily.   

These models will act as a tool for future empirical work, which will allow us to determine the prevalence and importance of conditional parasitism in plant-pollinator interactions in nature.