Pollen-rewarding plants face a delicate balancing act: they need to attract pollen-collecting visitors while also preventing consumptive emasculation (the consumption of pollen by their pollinators). A recent study by Heiling and colleagues, published in the American Journal of Botany, asks how pollen-rewarding plants solve this problem. The researchers have developed a model to determine the optimal pollen package size that balances these conflicting constraints.
Pollen packages refer to the quantity of pollen available to pollinators in a single visit. Offering a smaller pollen package may discourage visitors from grooming themselves. A plant would want a carrier to avoid grooming because grooming would lead to the loss of pollen and less arriving on the next plant. But get the package too small, and the plant is less attractive to pollen-collecting visitors. What package size best balances these two constraints?
Heiling and colleagues developed a mathematical model to examine the combined effects of pollinators’ grooming behaviours and package size preferences on the optimal package size, the package size that maximizes pollen donation. Using this model, they also investigated Darwin’s conjecture that selection should favour increased pollen production in pollen-rewarding plants. In their article, Heiling and colleagues write:
In The Origin of Species, Darwin (1859, p. 90) argued that when floral visitors view pollen as a reward, “[plant] individuals which produced more and more pollen, and had larger anthers, would be selected”, even when most of the pollen is consumed by the visitors. But is it inevitably true that pollen-rewarding plants that produce more pollen will have greater male fitness? Darwin drew an analogy between nectar and pollen as rewards, suggesting that greater production of either would increase a plant’s attractiveness to visitors seeking that reward, and thus increase male success. As pollen is the only reward presented by strictly pollen-rewarding species, Darwin’s conjecture might suggest that strictly pollen-rewarding plants should produce more pollen than their relatives that produce both nectar and pollen. There is some empirical evidence that this is indeed the case (Simpson and Neff, 1983; Golubov et al., 1999; Etcheverry et al., 2012). However, greater production of pollen per plant will also alter the pollen standing crop in the plant population. Greater pollen availability could potentially alter pollinator visitation rates and the preferences of pollen-collecting floral visitors, which in turn may influence whether increasing pollen production actually does increase male fitness.Heiling et al. 2023
The study found that when package size preferences are weak, minimizing package size reduces grooming losses and should be favoured. This finding agrees with previous theoretical studies but ignores the opinions of the pollinators. When there are preferences for larger pollen packages, plants should produce larger packages despite the increased risk of grooming loss. This is because the loss associated with the non-removal of smaller packages becomes even greater.
As Darwin suggested, the total pollen donation does increase with production. However, the study also found that if floral visitation declines or package size preference increases with overall pollen availability, the fraction of pollen donated may decline as per-plant pollen production increases. This means that increasing production may result in diminishing returns.
Heiling and colleagues’ research provides valuable insights into the evolutionary strategies of pollen-rewarding plants. Their findings suggest that plants can balance the conflicting constraints on pollen donation by producing intermediate-sized pollen packages. This ensures that the plants remain attractive to pollinators while minimizing the amount of pollen that goes to waste.
Furthermore, the study implies that strictly pollen-rewarding plants may have responded to past selection pressures by producing more pollen in total. However, diminishing returns may limit the strength of that selection. As plants increase pollen production, the proportion of pollen that gets donated might decrease, which could lead to a plateau in the benefits of producing more pollen.
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Heiling, J.M., Irwin, R.E. and Morris, W.F. (2023) “Conflicting constraints on male mating success shape reward size in pollen-rewarding plants,” American Journal of Botany, p. e16158. Available at: https://doi.org/10.1002/ajb2.16158.