Going from great lengths: Bumble bee tongue lengths and floral tubes of alpine host plants show concerted declines

Background/Question/Methods

As species respond to climate change, the composition of mutualistic partners in communities is shifting. By altering the distribution of important interaction traits in one trophic level, changes in community structure may influence the evolution of traits in another. Plant-pollinator interactions are mediated by pollinator tongue length and floral tube length, with shifts in one often driving the evolution of the other. Here, we explore two potential underlying mechanisms leading to changes in bumble bee tongue length distributions over time in Rocky Mountain alpine communities: changes in species distributions (immigration hypothesis) and selection (evolution hypothesis). The immigration hypothesis states that an influx of bumble bee species with shorter tongues will cause a shift in alpine bumble bee communities to shorter-tongued phenotypes. The evolution hypothesis predicts that decreases in floral tube length due to changes in flower production of alpine species will select for a reduction in tongue length of resident bumble bees. To discriminate between the two scenarios, we sampled tongue length, tube length and species composition in three Colorado alpine sites using methods of past surveys during the 1960s-70s. We then tested for temporal shifts in community composition and phenotypic trait distributions of bees and plants using mixture models.

Results/Conclusions

In the past, alpine bumble bee communities were dominated by two species, a short-tongued and a long-tongued species, whereas subalpine communities were more diverse and included species of intermediate tongue lengths. Since the 1960s and 1970s, subalpine bumble bees have migrated into the alpine, and now co-occur with resident alpine bumble bees. The influx of short-tongued bumble bees over time has caused the current guild of alpine bumble bees to exhibit a shorter tongue length distribution, in accord with the immigration hypothesis.  However, the distribution of floral tube lengths in plants comprising bumble bee resources has also shifted towards shorter-tubed flowers in the alpine. Tongue length of the long-tongued alpine resident species Bombus balteatus has decreased over time by 19%, in accord with the evolution hypothesis. This result suggests that selection for foraging efficiency has favored the evolution of shorter-tongued bees. While previous studies indicate that long-tongued bumblebees are declining worldwide, ours is the first to show that changes in tongue length distributions are also driven by evolutionary change.