The dispersal of pollen is a highly uncertain process, resulting in highly variable pollen loads among stigmas between plants and even among stigmas of the same plant. Causes of high variation include: differences in pollinator visitation rates, differences in pollen dispersal properties among pollinators, and chance events when a pollinator visits a flower (e.g. anther-pollinator-stigma positioning). Understanding variation has important implications as highly variable pollen import may reduce seed set. The latter two causes of variation were quantified for a butterfy and a bumble bee species foraging on Brassica napus, and the consequences for seed production were investigated using a simulation model.

Results/Conclusions

Data revealed differences among pollinator species but not among individuals of the same species. Although butterflies and bumble bees transferred a similar proportion of pollen from their body to a stigma each flower visit, butterfly deposition was much more variable. The simulation model revealed signifiant reductions in seed set across a range of realistic parameter values. We refer to the situations where seed set is reduced under variable pollen import, but would not occur if import were constant, as variance limitation. We discuss evolutionary issues raised by this largely unrecognised form of seed limitation, including: selection for pollinator specialists versus generalists, sexual selection, and optimal pollen:ovule ratios.