Miller-Struttman et al. 2015. Functional mismatch in a bumble bee pollination mutualism under climate change (Abstract). Science 349: 1541-1544.
Alpine plants with long tubed flowers are pollinated by long-tongued bees. In this paper, Dr Candace Galen and her collaborators use a combination of historical and contemporary studies of bee and plant populations in the Rocky Mountains, Colorado, USA to show that as climate warming occurs, alpine plants tend to flower less abundantly at lower altitudes and so resources become limited for bumble bees. Alpine bumble bees start foraging on whatever is available and expand their food resources to include species that bear flowers with shorter tubes; over the past 40 years, they have evolved shorter tongues.
Flowering decreases at higher temperatures (lower elevations)
The authors measured flower density and found evidence that there are large regions at the medium altitudes where flowers tend to be less abundant (red region in map) compared to the higher altitudes (blue). This, they suggest, is due to the higher frequency of warmer summers since 1985 compared to the years 1960 to 1985. Decrease in flower density is associated with increases of >3.25C.
Change in flower abundance at landscape and local scales along a 400-m altitudinal gradient on Pennsylvania Mountain. (A) Map showing areas where Flower Density (PFD) decreased (1.95 km2), is stable (1.29 km2), and increased (0.10 km2). Unshaded (excluded) areas contain cliff, talus, mining disturbance, and subalpine forest. (B) PFD (mean >± SE) for plots in krummholz (KRUM); tundra slopes (SLOPE); wet meadow (SWALE), false summit (FSUMMIT); and summit (SUMMIT) habitats (N = 6 species; F4,385 = 5.55, P = 0.0002). Asterisks indicate significant differences at P < 0.05. (C) Total flower production (in millions) is the product of total surface area for (A) each habitat (table S5) (15) and (B) mean PFD.”
With reduced flowering, food becomes scarce, and bumble bees expand their diet–they also feed on shorter tubed flowers. Over time, adaptive evolution occurs and results in shorter tongued bumble bees.
The authors studied two species of Bombus, bumble bees. They found evidence that tongue lengths had reduced over the past 40 years, corresponding to increased foraging on shorter tubed flowers at the lower altitudes.
“Change in tongue length for B. balteatus and B. sylvicola on Mount Evans, Niwot Ridge, and Pennsylvania Mountain. (A) B. balteatus. (B) B. sylvicola. Bars represent least squares means >± SE. (15). Asterisks denote significant differences (P < 0.05) between means. Dagger denotes a trend (P < 0.06).”
These results may be generalized. Bee communities contain short-tongued and long-tongued forms. Taking entire communities in the areas studied, the authors show using empirical studies and modelling that, compared to the past, bee communities today tend to contain more shorter-tongued species (3A, B), bees tend to have shorter tongues (3C, D), and tend to visit more species with short-tubed flowers (3E, F).
“Changing bumble bee community composition, bumble bee tongue length distributions, and tube depth distributions of visited flowers over time. (A and B) Bumble bee community composition. (C and D) Bumble bee tongue length. (E and F) Flower tube depth distribution. Bombus species abundance in alpine communities is indicated by the proportion of total foragers (15). Species are ordered by increasing tongue length [in (A), species’ names follow (18)]. Bimodality of the density functions (15) indicates that bumble bee communities contain two predominant phenotypes, short-tongued and long-tongued [(C) and (D)]. (E) and (F) show the tube depth density functions for flowers visited by, respectively, B. balteatus and B. sylvicola in the Front Range [Mount Evans and Niwot Ridge (15)]. For tongue length [(C) and (D)] and tube depth [(E) and (F)], representative density functions for simulated communities (15) are shown.”