A new study shows that two alpine bumblebee species have responded to a decline in flowering due to warming temperatures by evolving shorter tongues. The results suggest that some bumblebee species may be able to adapt to environmental challenges caused by climate change.

“We are not saying climate change isn’t a problem for bumblebees. It is a major problem. However, these findings indicate that some bumblebees may be able to adapt if provided adequate habitat and are largely shielded from other sources of anthropogenic harm, like pesticide use and habitat destruction,” said University of Missouri biologist Candace Galen, co-author of the study, which appears in the latest issue of Science.

Rising temperatures in alpine habitats worldwide have resulted in declines in flowering in these areas. During the past decade, scientists have also reported dramatic declines in populations of several bumblebee species, including some endemic to alpine regions.

Since bumblebees have co-evolved with the floral resources they need for nutrition, Galen and her colleagues were curious about how resident bumblebees were faring in the alpine regions of the Rocky Mountains. They looked specifically at changes in tongue length among resident bumblebees because this trait is intimately tied to the bee-flower mutualism.

“Bumblebees with longer tongues specialize on flowers with deep, long tubes, while short-tongued bumblebees, in contrast, are generalists and tend to alight on a variety of flowers,” explained Galen.

Galen and her colleagues measured the tongue lengths of alpine bumblebee species collected between 1966 and 1980 at three alpine sites of the central Rocky Mountains and archived at the University of Colorado and other museums. They resurveyed bumblebee species in the same alpine locations between 2012 and 2014 and measured their tongue lengths.

The scientists also tracked bee visits to flowers of six plant species preferred by these bumblebees species historically, and monitored flowering of these plants over time, comparing their data with earlier surveys conducted between 1977 and 1980.

They found that the relative abundances of bumblebees with longer tongues in the three alpine locations declined over the 40-year period. The mean tongue lengths of two high alpine bumblebee species, Bombus balteatus and B. sylvicola, had decreased by 24%.

“A morphological change of 24% over this amount of time is dramatic, especially when we take into account that this change has occurred over just 40 generations,” said Galen. “Most evolutionary change occurs on a timescale of a few hundreds, thousands, or millions of years. Forty years is a timescale that happens in a human lifetime.”

When they looked at the flowers visited by the alpine bumblebee species, they found that the bees’ favorite flowers had not shifted to a shallower form, but were less prolific. Overall, they found that the total food resources available to alpine bumblebees had fallen by 60% since the 1970s.

The scientists plugged their data into a mathematical model to calculate the relative advantage of being a specialist versus a generalist in a simple environment with a long-tubed flower and a short-tubed one. They found that when deep flowers are in the minority and the total flower density drops, the advantage of being a generalist increases sharply.

“Basically, it shows you can afford to be a specialist if there is enough of the plant you are specializing on. As you start losing resources, then that’s going to have a disproportionate impact on that specialist whose flowers are not really common to begin with,” said MU biologist Ricardo Holdo, who conducted the modeling work as part of the study. “This is one of the real strengths of the study: it shows that the change in the flower population is actually exerting a selective pressure on the pollinator.”

Galen and her collaborators are now working on experiments to see what this change in bumblebees means for long-flowered plants that relied on them for pollination.

As part of the study, the researchers ruled out that the change in tongue size was linked to an influx of sub-alpine bumblebees, decreasing body size, or a change in flower sizes.

The finding of rapid adaptation, said Galen, is “a glimmer of hope” for bumblebees, whose populations worldwide are declining.

“It suggests that the things we can manage locally, like pesticides and habitat destruction and planting companion plants, can actually make a difference because they can buy pollinators time for natural selection and evolution to allow them to keep pace with some of the things that we can’t manage locally,” said Galen. “They seem to be giving bumblebees ‘running shoes’ in this race against climate change.”

The corresponding and lead author of the study, titled “Functional mismatch in a bumblebee pollination mutualism under climate change,” is Nicole Miller-Struttmann, who completed the research as a postdoctoral fellow at the University of Missouri and is now at SUNY College at Old Westbury, New York. In addition to Galen and Holdo, other coauthors include Jennifer Geib, James D. Franklin, Peter G. Kevan, Diane Ebert-May, Austin M. Lynn, Jessica Kettenbach, and Elizabeth Hedrick.

A grant from the National Science Foundation supported this research.

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Written by: Melody Kroll

Sep 30, 2015: Stories about this research appeared in 313 international, national and online outlets, including The Daily News, BBC News, Wired, The Washington Post, Christian Science Monitor, The Atlantic, Associated Press, and Smithsonian Magazine.

Related research strengths:

Ecology, Evolution

