Mutualisms provide benefits to interacting species, but they also involve costs. If costs come to exceed benefits as population density or the frequency of encounters between species increases, the interaction will no longer be mutualistic. Thus curves that represent benefits and costs as functions of interaction frequency are important tools for predicting when a mutualism will tip over into antagonism. Currently, most of what we know about benefit and cost curves in pollination mutualisms comes from highly specialized pollinating seed consumer mutualisms, such as the yucca moth/yucca interaction. There, benefits are predicted to saturate as the number of visits to a flower increases (because the amount of pollen needed to fertilize all the flower’s ovules is finite), but costs should continue to increase (because pollinator offspring consume developing seeds), leading to a peak in net seed production at an intermediate number of visits. But for most plant-pollinator mutualisms, costs of floral visits (which include reward production costs, damage caused by visitors, removal of previously deposited pollen, pollen competition, and infection by pathogens and nectary-inhabiting yeasts) are more diverse than simple consumption of seeds; how such costs scale with interaction frequency remains largely unknown.

Results/Conclusions

We present several reasonable benefit and cost curves that are appropriate for typical pollinator-plant interactions, and we show how they can result in a wide diversity of relationships between net benefit (benefit minus cost) and interaction frequency. We then present net benefit curves for three typical pollination mutualisms from two continents, which we measured by allowing varying numbers of insect visits per flower. Interestingly, in two of these cases, net benefits peaked at an intermediate number of visits (as expected for pollinating seed consumer mutualisms). In the third case, net benefits continued to increase with increasing number of visits, but we found evidence of a significant cost of visits by nectar robbers. We discuss the implications of the shape of the net benefit curve for the ecology and evolution of plant-pollinator mutualisms, as well as the challenges that lie ahead for disentangling the underlying benefit and cost curves for typical pollination mutualisms.