Antagonists and their effects in a hummingbird—plant interaction: Field experiments

Abstract

In the plant—hummingbird interaction both the rewards offered by plants and the pollen transport effected by hummingbirds are usually exploited by antagonists such as herbivores and nectar robbers, which do not provide a benefit to either mutualistic partner. These antagonists can directly and indirectly affect plant fitness by forcing hummingbirds to modify their foraging behaviour. The ecological significance of antagonists for the plant—hummingbird interaction is largely unknown. The goal of this work was to investigate the combined effects of herbivores and nectar robbers on the interaction between the distylous herb Bouvardia ternifolia and its hummingbird visitors. By simulating different intensities of herbivory and nectar theft, we studied (1) changes in the foraging behaviour of visiting hummingbirds, (2) differences in the use of floral morphs, (3) changes in nectar characteristics, and (4) their effects on plant fitness. On plants of both morphs, higher levels of leaf defoliation and nectar removal were related to a decrease in the number of visited flowers, an increase in the time-to-arrival of hummingbirds, and a reduction in nectar availability and sugar production. The pollination experiments showed that seed production on both floral morphs was significantly reduced by herbivory and nectar removal. These results suggest that herbivores and nectar robbers can directly affect hummingbird foraging patterns and reduce the fitness of the host plants.