Are there fitness advantages in being a rewardless orchid? Reward supplementation experiments with Barlia robertiana

The Orchidaceae characteristically contain a very large number of species t hat attract pollinators but do not offer them any form of reward in return for visitation. Such a strategy is highly unusual in the plant kingdom. We conducted experiments in order to manipulate the reward strategy of the rew ardless bumble-bee-pollinated orchid Barlia robertiana by adding sucrose so lution to inflorescences. We found that supplementation decreased the proba bility of a pollinator removing pollinia by approximately ten times. Despit e pollinators visiting many more flowers per inflorescence on supplemented plants, right times fewer pollinia were removed from supplemented infloresc ences during each visit. Pollinia deposition patterns were not significantl y affected by supplementation and no geitonogamous deposition was recorded. In populations where inflorescences were supplemented for 20 days, pollini a removal was reduced by over half for supplemented inflorescences, whereas fruit set was unmodified by supplementation. We conclude that rewardlessne ss would increase total seed paternity, but not change either total seed ma ternity or the probability that offspring were outcrossed in this species. To the authors' knowledge this is the first time that there has been an une quivocal experimental demonstration of an evolutionary advantage the reward lessness in the Orchidaceae.