In the midst of a widespread decline in bees, particularly in the United States, a few bumblebees are finding a way to cope: shorter tongues. In just 40 years, the tongues of two bumblebee species living high up in Colorado’s Rocky Mountains have shrunk by almost 25% in their average length, according to a new study. A warming world has spurred these changes, researchers conclude, because the total number of flowers has declined in this region—and the shorter tongue enables the bees to suck nectar from more kinds of flowers.
"It’s one of the best examples of the effect of climate that I’ve seen," says Sydney Cameron, an entomologist at the University of Illinois, Urbana-Champaign, who was not involved with the work. But the bees' successful adaptation may be the silver lining of a very dark cloud. A variety of so-called long-tubed flowers, including penstemon, Indian paintbrush, clover, wild indigo, monkshood, bluebell, snapdragon, larkspur, and foxglove, require long-tongued bumblebees for pollination. "The reality is that long-tube flowers will disappear," Cameron warns. And then, "you are losing biodiversity on a major scale."
Many of the bumblebees that first arose sport tongues about half the length of their bodies, having evolved special relationships with particular long-tubed flowers. Matching tongue length to flower depth makes foraging and pollination more efficient, and many such matches have evolved through time.
But globally, long-tongued bumblebees are declining, says Cameron, as are many bees, in part because of climate change, pesticide use, and habitat loss. There were also indications that in some species, long tongues were shrinking.
To find out what was happening in the alpine regions of the Rockies, Nicole Miller-Struttmann tapped historical specimens of bees collected from three Colorado peaks. This evolutionary ecologist, from the State University of New York at Old Westbury, and her colleagues measured tongue lengths of two species of bumblebees, Bombus balteatus and B. sylvicola, collected between 1966 and 1980 and again between 2012 and 2014. The comparison of the two periods revealed the 24% shrinkage. "If there had not been that historical record, we would have completely missed that these bees might have been evolving," notes Jeremy Kerr, an ecologist at the University of Ottawa in Canada, who was not involved with the work.
Then they set out to figure out what drove that evolution. One possibility is that the bees are just getting smaller. But measurements of overall size and different body parts ruled this out. Miller-Struttmann and her colleagues then compared other decades-old data about plants visited by the bees with recent work on bee visits, and discovered that these two species had acquired broader tastes than their recent ancestors, taking nectar from many more kinds of flowers than before. This clue made them look more carefully at what was happening to the flowers over the past 40 years.
Plant surveys from the 1970s and from just a few years ago revealed that flower density on the mountain slopes has dropped more than 70%, the team reports today in Science. Climate change is to blame, Miller-Struttmann says. Other work has shown that alpine flowers don't grow as well when summer minimum temperatures exceed 3.25°C—the soil doesn't cool off and dries up as a result. Nights were too warm just 12% of the years between 1960 to 1985, but since then there have been hot years 48% of the time, the research team notes.
The warming has had a cascading effect. It reduced the number of all flowers. For bumblebees specializing in just a few species, that meant there were too few flowers to go around. So bees with shorter tongues, which are better able to make use of the broader diversity of short-tubed flowers, did better and gradually came to dominate the peaks, the researchers suggest.
It's very hard to figure out what is happening to species as the climate warms, says Koos Biesmeijer, an ecologist at the Naturalis Biodiversity Center in Leiden, the Netherlands. So this study is "very powerful." And that these changes occurred in just 40 years—or about 40 generations for bees, "is a really significant finding," Kerr adds. If other bumblebees are likewise adjusting, "there’s a prospect that bee populations that are facing climate change can evolve to some degree to not suffer the negative impacts."
But even if these bumblebees do okay, the flowers they used to specialize in might not. With shorter tongues the bees are not as efficient, and now that they visit many kinds of flowers, the pollen they transfer to the long-tubed flowers may not always be the right type. "This is not trivial," Cameron says.
But, such a negative effect on the flowers has yet to be documented, Kerr cautions. Miller-Struttman agrees but says that is something she plans to look at next. Also, it's not clear whether these results hold true for other bumblebees elsewhere in the world. "That's the question going forward," Kerr notes.