Environment-Dependent Pollen Limitation and Selection on Floral Traits in an Alpine Species

Abstract

Phenotypic selection on flower size in animal-pollinated plants can only occur when fitness is limited by pollinator visitation and when there is a relationship between flower size and pollinator attraction. The purpose of this study was (a) to examine how abiotic environmental conditions affect pollen limitation on female reproductive success and pollinator flower visitation and (b) to reveal whether phenotypic selection on floral traits in an alpine flowering species varied across an abiotic environmental gradient. In alpine populations of the perennial herb Ranunculus acris, I found that female reproductive success in a low-altitude population was significantly increased by experimental pollen addition, while plants at a higher altitude population did not respond to this treatment, presumably because low temperatures limited their seed production. At the low altitude, pollinators (muscoid flies) preferred to visit relatively large flowers, while no sign of flower discrimination occurred at the high altitude. Phenotypic selection regression analysis found that selection on flower size traits was more intense and frequent at the low-altitude compared to the high-altitude population. This result is consistent with the findings of stronger pollen limitation on seed set and the existence of pollinator discrimination on flower size at the low altitude compared to the high altitude. Pollinator visitation rates to flowers were nearly three times higher at the low altitude, compared to the high altitude. Thus, this study shows that the commonly assumed pathway from low pollinator availability to strong pollen limitation is not always correct but supports the view that selection on floral traits is positively related to the degree of pollen limitation. In R. acris environmental factors interact with pollen availability in determining the reproductive success of plants, and this interrupts the pathway from low pollinator availability via significant pollen limitation to selection on floral traits.