A new study in the journal Science shows that two alpine bumblebee species have responded to a decline in flowering due to warming temperatures by evolving shorter tongues.
The forest bumblebee (Bombus sylvicola) is one of two bumblebee species in the central Rocky Mountains that has responded to a decline in flowering in alpine habitats by evolving a shorter tongue. Image credit: Nicole Miller-Struttman.
“We are not saying climate change isn’t a problem for bumblebees – it is a major problem. However, our findings indicate that some bumblebees may be able to adapt if provided adequate habitat, and are largely shielded from environmental pollutants, such as pesticides,” said senior author Prof. Candace Galen of the University of Missouri.
Many co-evolved species have precisely matched traits; for example, long-tongued bumblebees are well adapted for obtaining nectar from deep flowers with long corolla tubes.
Recent studies suggest long-tongued bumblebees are declining in number. To better understand why, Prof. Galen and co-authors studied several high-altitude sites in Colorado where two species of long-tongued alpine bumblebees – the golden-belted bumblebee (Bombus balteatus) and the forest bumblebee (Bombus sylvicola) – live.
Using bumblebee specimens collected between 1966 and 1980 at three alpine sites of the central Rocky Mountains, the scientists measured changes in tongue length. They then resurveyed bumblebee species in the same alpine locations between 2012 and 2014 and measured their tongue lengths.
They found that for populations of two species on three mountains, tongue length had decreased by 24% over the 40-year period.
“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,” Prof. Galen said.
“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.”
Next, using archived specimens and field surveys of bumblebees and host plants, the team examined possible mechanisms for this change.
“It was not a result of decreasing body size, competition from invaders, or co-evolution with flowers in the area,” they explained.
Instead, it is a result of warming summers, which reduced numbers of the deep flowers these species preferred, forcing the insects to be general foragers capable of feeding across remaining flowers, including many shallow flowers.
“Co-occurring flowers have not become shallower, nor are small-flowered plants more prolific. We argue that declining floral resources because of warmer summers have favored generalist foraging, leading to a mismatch between shorter-tongued bees and the longer-tubed plants they once pollinated,” the scientists said.
“The finding of rapid adaptation is a glimmer of hope for bumblebees, whose populations worldwide are declining,” Prof. Galen said.
“It suggests that the findings we can manage locally, like pesticides, habitat destruction and planting companion plants, can actually make a difference because these factors can buy pollinators time for natural selection and evolution, thus allowing the species to keep pace with the things that we can’t manage locally.”
“They seem to be giving bumblebees running shoes in this race against climate change.”
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Nicole E. Miller-Struttmann et al. 2015. Functional mismatch in a bumble bee pollination mutualism under climate change. Science, vol. 349, no. 6255, pp. 1541-1544; doi: 10.1126/science.aab0868
