Several studies have documented low levels of insect diversity, abundance and activity in alpine ecosystems around the world1,7. It has been hypothesized that these factors may limit pollination of alpine plants8,9. We have measured pollination of alpine and foothill populations of Campanula rotundifolia by calculating a corrected visitation rate that incorporated information on pollinator abundance, visitation rates, pollen deposition and duration of stigma receptivity. Although pollinator visitation rates were significantly lower in alpine populations, corrected visitation rates showed that more effective pollination, combined with a longer period of stigma receptivity, compensated for this, resulting in comparable levels of pollination in populations from high and low elevations.
We conducted pollination studies from 1994 to 1996 at four low sites (1,800-2,000 m) and five high sites (3,300-3,500 m) in the Rocky Mountains. As in other studies1,2,5,6, we found that low-elevation populations were visited by a more diverse community of pollinators than the high-elevation populations. Pollinator visitation rates were significantly higher in foothill than in alpine populations in all three years of the study (P < 0.01 Kruskal-Wallis test)(Fig. 1a). However, visitation rates for specific pollinators differed between elevations. In the foothills, medium-sized solitary bees (Duforea maura, Andrena sp. and Colletes sp.) exhibited the highest visitation rate (0.0109 visits per flower per min), whereas bumblebees (Bombus sp.) had the highest visitation rate in the alpine populations (0.0017; P <0.05, Kruskal-Wallis test) (Fig. 1b). Medium-sized solitary bees were also most abundant in the foothills (51%), in contrast to the alpine populations in which bumblebees were the most common visitor (60%) (Fig. 1c). As in other studies7,10, we also found that bumblebees deposited significantly more pollen per visit than any other type of pollinator (ANOVA, P < 0.001).
The duration of stigma receptivity also differed significantly between alpine and foothill populations. In 1993, alpine flowers were receptive for 2.31 days, whereas foothill flowers were receptive for 1.46 days (t=−3.14, P < 0.001). We obtained similar results in 1995 (3.00 days as opposed to 2.21 days, t =4.07, P <0.0005). We used these data to calculate a corrected visitation rate for alpine and foothill populations (Fig. 1d). All foothill visitation rates were multiplied by 2/3 to correct for the shorter duration of stigmatic receptivity at low elevations. That quantity was then multiplied by a measure of relative pollen deposition for each pollinator category and its relative abundance at each elevation. In the foothills, the uncorrected visitation rate for all insect types combined was 0.0168 in contrast to a corrected rate of 0.0019. The uncorrected visitation rate in the alpine populations was 0.0023, and the corrected rate was 0.0011.
Although pollinator diversity and activity in populations of C. rotundifolia decrease with increasing elevation, our results show that pollination in alpine and foothill populations is more comparable than would be expected from visitation rates and insect diversity alone. This is in contrast to the widespread assumption that restricted insect activity at high elevations limits pollination of alpine plants relative to that at lower elevations. The most important factors contributing to these results are the dominant role of bumblebees as pollinators in alpine populations and the longer duration of stigmatic receptivity in high-elevation plants. The latter point is particularly relevant to recent research that has investigated increased floral longevity as an adaptation to low pollinator visitation rates11.
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