Living on a mountain is hard for bees and flowers. It's cold. There's extreme weather. And new research has found it's getting even harder for both flowers and bees to make a living in alpine evironments lately. Scientists compared over 40 years of mountain bumblebee and flower records on three Colorado mountains, and found major decreases in both bees and flowers. But they also found clear evidence of rapid evolution by the bees, suggesting it's not time to give up on mountain bumble bees just yet.
Entomologists and botanists get teased about traveling the world, meeting interesting insects and plants, and then killing them. But it's a morbid habit that pays off; it creates a long-term, stable record of the biological past. Museum collections may look like a creepy charnel house to outsiders, full of corpses, pins, and mothballs. Our libraries of dead things become a book of evolutionary change for future scientists to read.
Preserving organisms from taxonomic or ecological studies lets us travel back in time. "People are always interested in having their data looked at and reanalyzed in a different way, a way that they hadn't thought about previously. That is one of the great things about having open access data," said Dr. Nicole Miller-Struttmann, lead author on the new bumble bee study.
To investigate how flowers and bumble bees changed, a team of scientists dug through over 40 years of records. They tracked down thousands of bumble bee specimens collected on mountains in Colorado between 1966 and 1980, and compared them to bumblebees collected in the same areas between 2012 and 2014. They also used herbarium specimens of flowers collected during similar time frames, and surveyed flowers in the field.
Plants on mountains often have very narrow temperature tolerances; too much heat can reduce flowering. On one of the mountains in the study, between 1960 to 1985 only 12 percent of the years were hot enough to reduce flowering. Since 1985, 48 percent percent of years were too hot for flowers that bumblebees typically forage on.
Since 1970, the total number of flowers available for bees on the mountain study sites declined by 60 percent overall. What did that mean for bees?
Over 95 percent of bees in the study regions between 1966 and 1980 were just two species of "long-tongued" bees. These bees specialize in flowers with a narrow, elongated tubular shape. Their long tongue means they are able to reach the nectar hidden at the bottom of a flower, and can muscle out their shorter-tongued relatives. This is an an example of coevolution, where two species reciprocally affect each other over evolutionary time.
Bees collected from 2012 to 2014 were different, though. The long-tongued species of bumble bees declined by 24 percent. At the same time, warming temperatures and changes in flowering plants allowed some lower altitude bees to live at higher mountain elevations. The entire community of bumble bees changed. Long-tongued bumble bees responded to the scarcity of flowers by becoming less selective; the range of plants they foraged on changed significantly, and included flowers with no long nectar tubes.
The scientists wondered if the bees physically changed too, and measured body length and tongue length on their historic and modern bee specimens. How do you measure a bee's tongue? Miller-Struttmann explains: "They tuck their tongue back into their body, so they sort of fold it back up along their chin, I guess you could say. We had to rehydrate historic specimens, and then fold the tongue out, and then measure it under a microscope with calipers."
What no one expected was that the tongues of long-tongued bees would get shorter. A lot shorter. "A 24 percent decrease in tongue length is really dramatic," says Miller-Struttmann. "That was in 40 years, in 40 generations, I should say, because these bumblebees only have one generation a year. That's a pretty short period of time to see such a dramatic shift." Bumble bee bodies also got slightly smaller, but not as much as the tongues shrank. The research team did not find changes in the depth of the flowers bumble bees were visiting. The bees' shape changed, but the flowers didn't.
Building and maneuvering a big tongue takes energy, and bees with shorter tongues may have done better at diverting that energy into more babies. In the short term, the bumble bees seem to be hanging on. But what about longer term?
Right now bumblebees and plants they historically fed on are mismatched physiologically. The bees may not be as good a pollinator for those plants, which could cause further declines in flowers. In the long term, perhaps they will also evolve, but they're much longer-lived species. Their generation time is decades, not yearly. Change will be slower—or may not happen at all.
Dr. David Inouye has researched flowers and alpine bees at the Rocky Mountain Biological Laboratory for decades. He said "this study is a great example of the value of archiving data... an example of a change in bumble bees that is unexpected, and would not have been discovered without access to historical data. We have evidence from elsewhere in the Rocky Mountains that bumble bee queens of eight species have moved up 230m in altitude over about the same time span, and these kinds of changes in bumble bee communities will have interesting consequences over both ecological and evolutionary time scales."
This study also highlights a common problem for mountain or other remote refuges—as the climate warms, the places where plants and animals thrive move slowly away from the areas we've designated for their conservation. By increasing areas set aside for nature, or making sure we have connections between isolated nature refuges, we can try to help bees and plants adapt to our new warmer world.
Miller-Struttmann, et al. 2015. Functional mismatch in a bumble bee pollination mutualism under climate change. http://www.sciencemag.org/lookup/doi/10.1126/science.aab0868
Nicole E. Miller-Struttmann & Candace Galen. 2014. High-altitude multi-taskers: bumble bee food plant use broadens along an altitudinal productivity gradient, Oecologia: 176(4) 1033
