Despite isolation from most direct anthropogenic disturbance, remote alpine habitats face consequences of warming summer temperatures due to climate change. In the high elevation terrain of the Colorado Rocky Mountains, total abundance of flower resources has decreased, forcing local bumblebee populations to forage on dwindling resources. Over the last 30 years, bumblebees have developed wider diet breadth and shorter tongue lengths to maximize foraging efficiency in a more sparsely flowering landscape. In this study, we investigate how these shifts in bee behavior and functional traits are influencing the floral morphology and pollination success of a long-tubed alpine wildflower, Polemonium viscosum. To assess whether shorter-tongued bumblebees are selecting for shorter flowers, we tested for a relationship between flower size with pollination success for six populations across three mountains in the Colorado Rocky Mountains. We predicted that selection would favor shallower flowers in the sweet-scented morph, which is primarily pollinated by bumblebee queens, but would be weaker in the skunky-scented morph, which is pollinated by both flies and bumblebees. We then tested for a temporal change in flower size and reproductive success between these contemporary populations (2017) and data collected by Dr. Candace Galen in 1985 in the same populations using the same methods.
Results/Conclusions
Selection did not favor shallower flowers, contrary to our prediction. Flower size did not affect pollination success for flowers of the sweet-scented morph collected in 2017. This finding contrasts with the selection for larger corollas observed in 1985, suggesting that selection for deeper flowers may be weakening but has not reversed direction. For the skunky morph, flower size had no effect on the quantity of conspecific pollen, but did significantly affect pollen purity with larger flowers receiving less heterospecific pollen. This relationship was also observed in 1985, indicating that pollination success has remained consistent through time. Surprisingly, flowers of both morphs received nearly twice as much pollen in 2017 relative to those observed in 1985. In fact, these populations are no longer pollen limited. While increased pollen loads may weaken selection for floral traits by bumblebees, we observed no differences in the average flower size of each population when compared to the 1985 observations. Our results indicate that current bumblebee populations are not favoring shallower flowers in this alpine forb. As flower density has declined in the alpine, bumblebee foragers may have improved efficiency, moving more pollen to the few available flowers, and weakening selection for larger flowers.