Specialization, generalization, and pollination syndromes: the role of pollinator competition

The relative importance of specialization and generalization has prompted a lively debate in studies of plant-pollinator associations and pollination syndromes. However, most work focuses on floral and pollinator morphology as determinants of interactions, with less consideration of how pollinator competition may determine interactions with plants. We propose that pollinator competition also plays a role in explaining patterns of plant-pollinator specialization, generalization, and pollination syndromes, especially when explanations based solely on floral and plant morphology fail to predict plant-pollinator interactions. We begin with a brief discussion of theoretical works on pollinator competition, focusing on competitive asymmetries in foraging efficiency between pollinators and how such asymmetries can lead to resource partitioning (specialization), reproductive isolation, and floral divergence in plants (including pollination syndromes). We then review works on how pollinator foraging costs relate to morphology of flowers and pollinators and provide examples on how expectations based on morphologically determined foraging costs differ from how bumble bee, hummingbird, and bat pollinators forage when under competition in the field. Next, we illustrate how pollinator competition influences plant reproductive isolation in a specialized plant-hummingbird association when plant traits fail to do so. We conclude with examples of how pollinator competition can be evaluated in field studies, from experimental manipulations of plants and pollinators to “before and after” studies on the effects of natural disturbance events on plant-pollinator associations. Studies taking pollinator competition into account may provide novel insights to plant-pollinator associations, especially when morphology fails to predict interactions.