Illegitimate flower visitors and pollinators are associated with different flower morphology: Evidence from five species of Penstemon (Plantaginaceae)

Abstract

The role of pollinators as drivers of morphological diversification has been extensively documented. However, how non-pollinators, such as pollen thieves and nectar robbers, affect flower morphology has received little attention. In this study, we used five species of Penstemon to investigate the relationship between pollinators and non-pollinators, and the flower morphology of the plants they visit. The results suggest that smaller flowers are more likely to be predominantly visited by small and very small pollen-collecting bees, and that larger flowers are more likely to be visited by large bees. Very small and small pollen-collecting bees rarely contacted the stigma and can thus be characterized as illegitimate visitors. However, in the absence of a more efficient pollen vector, these visitors can act as occasional pollinators. On the other hand, nectar robbing by wasps was not correlated with flower size but was associated with premature wilting. Pollinator assemblages varied geographically, but the presence and frequency of non-pollinator visitors did not, and instead they were species-specific. Finally, our results suggest that non-pollinator visitors have a detrimental effect on flower appearance from the pollinators' perspective.