Floral size variation in Campanula rotundifolia (Campanulaceae) along altitudinal gradients: patterns and possible selective mechanisms

We investigated patterns of flower-size variation along altitudinal gradients in the bee-pollinated perennial Campanula rotundifolia (Campanulaceae) by examining 22 Norwegian populations at altitudes between 240 and 1100 m a.s.l. We explored potential mechanisms for the underlying pattern by quantifying pollinator-faunal composition, pollinator-visitation rates and pollen limitation of seed set in subsets of the study populations. Despite a decrease in plant size, several measures of flower size increased with elevation. Bumble bees were the main pollinators at both alpine and lowland sites in the study area. However, species composition of the pollinator fauna differed, and pollinators were larger in higher-elevation than in lower-elevation sites. Pollinator visitation rates were lower at higher-elevations than at lower elevations. Pollen limitation of seed set did not vary significantly with altitude. Our results are consistent with differences in bumble-bee size and visitation rates as causal mechanisms for the relatively larger flowers at higher elevations, in three non-mutually exclusive ways: 1) Larger flowers reflect selection for increased attractiveness where pollinators are rare. 2) Larger and fewer flowers represent a risk avoidance strategy where the probability of pollination is low on any given day. 3) Flower size variation reflects selection to improve the fit of pollinators with fertile structures by matching flower size to pollinator size across sites.