Bumble bees turn generalist to survive climate change on high peaks
by Allen Best
Pennsylvania Mountain rises to 13,006 feet in the Mosquito Range of Colorado. From the summit, you can look down on the old mining towns of Leadville and Fairplay as well as a mining operation on a nearby mountain.
The above-treeline slopes of Pennsylvania Mountain, however, have remained pristine, distant from pesticides and other human influences.
Or so researchers studying wildflowers and bumble bees thought. Then they noticed something that surprised them. The bumble bees above timberline were different than they were in the past. Historically, alpine bumble bees comprised 95 to 99 percent of bumble bees on the above-treeline slopes of Pennsylvania Mountain as well as two other sites in Colorado’s Front Range. Now they share the habitat with lowland species, but even more surprisingly, the tongues of the alpine species are shorter than in the past.
These bumble bees were part of what scientists call an ecological partnership, or a mutualism. For long-tongued bees, with tongues up to half the length of their bodies, they were able to efficiently forage and pollinate the long-tubed wildflowers of Indian paintbrush, monkshood, and other alpine species. Both partners benefited from this specialization. The flowers get pollinated and the bees get nectar.
To find out why tongues of bees had shrunk, they pursued several hypotheses through field research on the three mountains during the summers from 2008 through 2014.
They have concluded that the high-altitude bees have been adjusting to warming temperatures. In their research summary, published in the Sept. 25 issue of the journal Science, they point to a decline in wildflowers as the base cause. With fewer of the already more-rare long-tubed wildflowers available, the bumble bees adapted to the broader menu of more abundant shorter-tubed wildflowers. To adapt, the bees developed shorter tongues over the space of 40 years. The bees reproduce annually.
Scientists not involved with the study described it as important. “Very powerful,” said Koos Biesmeijer, an ecologist at the Naturalis Biodiversity Center in Leiden, the Netherlands. That the changes occurred in just 40 years “is a really significant finding,” he told Science. He said this suggests that bee populations can adapt to effects created by warming temperatures.
Jennifer C. Geib, a biologist at Appalachian State University in Boone, N.C., and one of the study authors, told Mountain Town News that she had been traveling to Colorado to study the plants on Pennsylvania Mountain since she was a graduate student in 2003. Her study was about how the abundance of bumblebee pollinators benefitted the plants.
In 2012, she, her former adviser Dr. Candace Galen, and Dr. Nicole Miller-Struttmann of SUNY-Old Westbury, formed a collaboration to compare alpine plant-pollinator interactions in modern times to those of the past. That’s when the anomaly was discovered. The short-tongued bumble bees were much higher on the slopes than was expected.
Geib says the current study would have been impossible had it not been for researchers in the 1960s and 1970s who had taken measurements, providing baselines for comparison.
One important comparison is temperature change. The paper reports that summer minimum temperatures have increased about two degrees Celsius (3.6 degrees Fahrenheit) on Pennsylvania Mountain since 1960. But some plants like it cooler. The changes have not favored them. From 1960 to 1985, temperatures associated with reduced flowering occurred 12 percent of the years on Pennsylvania Mountain, but 48 percent of the years since 1985.
Warming summer minimum temperatures in the last 56 years have also been recorded on two other mountains in Colorado’s Front Range, Niwot Ridge and Mt. Evans, which were used in the study from 2012 to 2014.
Alpine flowers don’t grow as well when nighttime temperatures stay above 37.85 degrees Fahrenheit (3.25 degrees Celsius).
This loss of wildflowers wasn’t universal on the mountain. Toward the summit, flowers still did well. By definition, however, there’s less land near mountain summits. On Pennsylvania Mountain, total food resources for alpine bumble bees have fallen 60 percent since the 1970s.
Bumble bees adapted by developing tongues that are on average nearly 25 percent shorter. As bees reproduce every year, this adaptation has occurred over the span of 40 generations. With fewer of the long-tubed wildflowers to draw nectar from, they improved their odds by having a broader menu.
Geib routinely arrives in Colorado in mid-June, hiking every morning above treeline to study bumble bees and the wildflowers until thunder clouds chase her down the slopes in the afternoons. She stays two months.
The effect of this new tongue length cannot be registered quickly on the wildflowers because they have long lives, 50 and even 100 years.
Does this mean that the wildflowers above treeline will forevermore be more scarce? Not necessarily, says Geib. Climate change models predict increasing warmth, decreasing soil moisture, and decreased snowpack.
“But if this doesn’t hold true, then the plants should be able to recover. If that happens, then instead of this being a change, it would just be a hiccup.”
Photos courtesy of the Mountain Area Land Trust.