summer comes late and passes quickly in the high Rockies, but its fleeting window of warmth brings with it a stunningly beautiful eruption of green grasses and flowering plants. Among the most striking stakeholders in this world above the trees is Polemonium viscosum, the alpine sky pilot.
At less than six inches tall, the sky pilot wouldn’t attract much attention in more plant-friendly climes. But on the skree-strewn tundra above 12,000 feet, the sky pilots’ comely blossoms tower over the low grasses clinging to thin, windswept soils; its musty scent beckoning pollinating fauna that will allow the plant to propagate during this brief respite from the snows.
For more than 35 years, MU’s Candi Galen, a professor of biological sciences, has observed the sky pilot and its summertime visitors. For her, the alpine tundra helps throw the bond between flowering plant and pollinator into clear relief, just as the absence of trees reveals the contours of the mountainsides. The result has yielded trailblazing insights into how flowering plants use scent and other stratagems to attract friends and repel enemies — discoveries that could help conservationists looking for ways to save plant genetic diversity in the face of climate change.
Galen’s field “laboratory” is located high on the slopes of Pennsylvania Mountain, a 13,000 foot, twin-peaked rise in the Pike National Forest. Its accessible location within the Mosquito Range and the relative ease of its ascent would seem to recommend the mountain as a prime destination for hikers. But the Pennsylvania peaks, as one mountaineering website put it, are “oft-overlooked.” Although a passionate hiker herself, this suits Galen.
“I’ve been working at this site since the late 1970s,” she says, adding that her first journey up the mountain involved working as an undergraduate assistant on a community-wide pollination study that one of her current graduate students is now following up on.
“It just ended up being a great place to study these sky pilots. They’re abundant up there. Also, relative to other places in the Rockies, Pennsylvania Mountain is not that steep, so you can get to higher elevations. It’s also relatively isolated and relatively protected. You can set up experiments there and come back 40 years later and find those same experimental plots.”
Like all plants, sky pilots are sessile — fixed in place — so they must rely on third parties to transport their pollen to potential mates. At lower elevations, any number of flies and bees are available to get the job done. But high on the alpine tundra, the options are much more limited.
Galen says, in fact, that the sky pilot mostly relies on a single pollinator, a bumblebee: Bombus balteatus. And because of the plants’ flowering time, even these bumblebees are in short supply. “Sky pilots are an earlier flowering plant, so they flower before the worker bees come out,” Galen says, leaving only queen bees available for the task.
Above 12,000 feet, “early” is mid-June. For the eager B. balteatus, the sky pilot’s purple blooms are a welcome sight, one of the few available sources of food at this time of year. Thus the bees sip nectar, the pilots are pollinated, and all’s right on Pennsylvania Mountain.
Biologists call this beneficial relationship between pollinators and plant a “mutualism” — a concept at the heart of understanding a plant’s ecological and evolutionary responses to its environment.
“Mutualisms are ecological relationships between organisms of different species where both partners benefit,” says Galen. “They’re sort of an ecological version of 'I’ll scratch your back if you scratch mine’ except across species.”
“My lab works on mutualisms both above and below ground,” she adds. “For about 30 years, I’ve been working with colleagues on plant-pollinator mutualisms —relationships where plants receive pollen transport as a benefit from attracting animal pollinators, and pollinators receive food and energetic rewards. The other mutualisms we work on are below ground: plant-mycorrhizal relationships. Mycorrhizal fungi provide plants with nutrients and water, among other benefits, in return for which they receive energetic resources, i.e., carbohydrates.”
Above ground, Galen has been examining how the alpine sky pilots’ evolution has been shaped by its relationship with bumblebees. She is specifically interested in how natural selection for bumblebees’ pollinating services has shaped the plant’s floral display — that is, the size, color, shape, nectar production, etc., of its flowers.
Her most recent work seeks to shed light on the sometimes vexing question of exactly what role fragrance plays in the pollination process. On the surface, this particular mutualism seems straightforward. Sky pilots use fragrance to advertise that nectar is available, a sort of biological calling card that says, as Galen puts it, “Hey, here’s the food (nectar or pollen). Come and try some.” But this classic view of fragrance tells only part of the story. And maybe not even the most interesting part.
Recent studies have teased out a much more nuanced understanding of flower power. Using observational techniques in the field and advanced tools in the laboratory, scientists have shown how particular fragrance compounds attract some animals while, at the same time, they repel others.
“It all depends on who is perceiving the fragrance,” says Galen.
In sky pilots, things get further complicated. Sky pilots’ flowers and fragrances do more than signal a reward to potential pollinators. They attract ants, creatures that not only fail to accomplish pollination, but also cause damage that can lead to sterility. Nonetheless, natural selection for bumblebee pollination services remains strong. To get to the bottom of what’s responsible, chemically speaking, for this curious phenomenon, Galen recently teamed up with Robert Raguso, a biochemist and professor at Cornell University.
“Alpine sky pilots have a very complicated fragrance that has maybe three or four volatile constituents in it, but it turns out that one of those volatiles, 2-phenylethanol, accounts for 99 percent of the fragrance total. This fact makes it relatively easy for us to manipulate the fragrance and then ask how the different insect partners sky pilots interact with respond to those manipulations,” explains Galen.
It also helps that 2-phenylethanol is not at all extraordinary. “It’s what makes roses smell like roses. It is one of the most common fragrance components out there in the flower world,” says Galen. “You can pull it right off the shelf. It’s not toxic. It’s a relatively benign sort of thing to go squirting around into plants in their natural habitats.” Beyond these practicalities, what was of more interest to Galen and her colleagues was the wide variation in the concentration of 2-phenylethanol among flowers.
“We were really interested in asking, 'How does 2-phenylethanol concentration affect the nature of the interaction that these alpine sky pilots have with their pollinators and with ants who act as parasites on that pollination relationship?’” says Galen.
The traditional view of fragrances, like most floral traits, is that they are attractive and that they are there to entice pollinators to visit the plants. Thus, the researchers expected that bumblebees would prefer flowers emitting higher concentrations of 2-phenylethanol.
“Because 2-phenylethanol is absorbed into the nectar, we thought, this is sort of like a calling card, like those people at [grocery stores] handing out those trays of samples, 'try some of this and then you’ll want to buy some because it’s so tasty,’” says Galen.
The researchers sought to confirm this by conducting a simple “choice test” among blooming sky pilots. Some flowers were selected for “phenotypic engineering” — in this case, an injection of a smell-boosting mix of sugar and 2-phenylethanol. The treated flowers were then enclosed in Reynolds oven bags so the researchers, using gas chromatography mass spectrometry, could verify their enhanced fragrance levels. Once completed, Galen and Raguso “unbagged” the extra-smelly flowers and let nature take its slightly modified course.
When the bumblebees arrived, members of the research team were also there to carefully record whether bees preferred 2-phenylethanol-enhanced blossoms to those left untreated. The investigators also noted bumblebees' flower preference, their total time spent probing enhanced and control flowers, and how many visits the bees made to both types of flowers.
Ants got a similar test. The researchers placed flowers supplemented with sugar and varying levels of 2-phenylethanol randomly around nests of ants, and then recorded how keen the ants were on visiting them. For these field tests, they measured ant response by comparing damage levels among both treated and untreated flowers.
Parallel to the fieldwork, the team also conducted “choice tests” back in the laboratory. Bumblebees were given a choice of artificial nectar made with or without 2-phenylethanol. “Sort of like the cafeteria,” says Galen. “What item do you want?” They tested ants’ enthusiasm for the differing concentrations of 2-phenylethanol using fragrance-infused petri dishes.
After three summers of experiments and observations, Galen and Raguso at last had an answer. It was not what they expected.
It turns out that both bees and ants avoided the 2-phenylethanol enriched flowers. Galen says it looked as if, rather than signaling “come check me out,” the flowers with greater concentrations of 2-phenylethanol seemed to be primarily signaling “stay away.”
“What we found out was that higher concentrations of 2-phenylethanol decrease the parasitism dramatically. But it comes at a cost. That same [stay away] message that is there for ants is also being perceived by bumblebees, which flowers depend on for pollination,” says Galen.
Bumblebees, however, are less put off by flowers with intermediate concentrations of the compound, (although they do not stay very long). Based on these findings, Galen surmised that what 2-phenylethanol is doing for pollinators is something akin to saying, “Come, have some nectar, and then leave.”
Think of it this way: Say you love desserts with peppermint, particularly the Thin Mint cookies you purchase every year from your local scout troop. If, inexplicably, some unscrupulous scouts decided to double the level of mint per cookie, you’d likely find this once-irresistible treat far less appetizing and would limit your intake accordingly.
“It’s a sanction,” says Galen. “It’s like saying, 'Don’t eat all that. Leave some for someone else.’ These nectar rewards are pretty valuable to insect pollinators. If you put out this valuable reward and don’t protect it in any way, then one pollinator may come along and take the whole thing. Then you have pollen from only that one pollinator, but you have other flowers that are opening.”
By encouraging pollinators to keep moving, in other words, the 2-phenylethanol may be ensuring a diversity of genetic material from a variety of individual bumblebees.
The take-home message, says Galen, is that it is not only important who is perceiving the fragrance. “We also should look at fragrances as we do color, size, or any other flower trait, as a quantitative trait. It’s not so much what you produce, per se, as how much you produce that really makes the difference.”
For sky pilots, hardy flowers thriving in hostile conditions, perfecting the delicate balance between attracting bumblebees and discouraging ants isn’t the only challenge. Climate change also has become a big part of the survival equation.
Galen’s years on Pennsylvania Mountain uniquely position her to observe these changes.
In the Rockies, she says, climatologists have documented that, while daily high temperatures have stayed fairly consistent, average low temperatures are creeping upwards. One effect of this is that treelines are moving higher. Galen and her students are already working to find out what else might change.
“The question of how that influences flowering communities is interesting to me, but not so much as the question of 'what happens then?’ Are we squeezing plants out entirely? Or are there places of refuge in the alpine? Can we identify places where the treeline is not moving up as fast, the sort of slopes or aspects or orientations or topography that is associated with cooler conditions?”
James Franklin, one of her graduate students, is surveying plots Galen helped survey over three-and-a-half decades ago. His findings may begin to provide answers.
“With climate change, temperatures are changing but also precipitation is changing, especially in terms of overwinter precipitation,” says Franklin. Gesturing to a Google Earth aerial shot of the mountain, he points to the reduced snow pack. “In a more normal year, the snow would be covering this whole area, and it’s lower even than this right now. On the other side of it, the temperatures are warmer. Under these conditions, it’s going to be quick flowering and over, even in alpine habitats.”
“As it turns out, you have very different species growing under different topographical conditions in the Rockies — a gentle slope versus a steep slope, for example. What I’m trying to do is to identify mountain areas that are suitable for plants like sky pilots that bumblebees depend on, and then ask how those environments are distributed across the landscape,” says Franklin.
He adds that this is scientific ground that’s been covered before. “Thirty five years ago, [Galen worked on a project that] did the exact same study. So I know what the distributions were for these plant species back then. I can compare that data to the present and see what’s changed,” Franklin says.
Already it’s obvious things are changing for Bombus balteatus. And not for the better. Queen bumblebees now find themselves facing competitors from lower elevations. “We have seen a [different] bumblebee species for two years in a row now,” says Galen. “Last year, it was the most abundant bumblebee queen that we saw above the timberline; this year, it is one of the more abundant ones. If you go back to the records from 40 years ago, it was never described as an alpine species; it was never found above the plains. It’s a greatbig bumblebee, so it could be a pretty good competitor. We don’t know.”
As climate change unfolds along the Continental Divide and elsewhere, it may well be the next generation of scientists who discover how plants and bumblebees — and all of us — will ultimately be affected. Galen wants to be a part of that process.
In 2011, she was awarded a $1 million “GK12” grant from the National Science Foundation to fund a three-year program that places MU graduate students into fourth and fifth grade public school classrooms. The “Show Me Nature: From Elements to Ecosystems,” program aims to help graduate students learn to explain their work to a science-challenged public, while also inspiring more young people to pursue scientific careers. For Galen, the program is a natural. “More mutualisms. GK12 is sort of an extension of my ecology stuff. I’m interested in mutualisms between species, and GK12 is partnerships. So, it’s really the same kind of idea.”
Having two young kids in the Columbia Public School system also helped. “I was talking to teachers and administrators and being made aware of the challenges that were coming up on their end,” Galen says, referring specifically to the loss of the science specialists at the elementary schools. “At the same time, I had graduate students, colleagues, myself, struggling with talking to family members about the research and science we do.”
John Nies, 35, teaches math, science, social studies and literacy at Columbia’s Grant Elementary School. When Galen asked if anyone would like to do their GK12 orientation at 12,000 feet, Nies didn’t hesitate: “I said, 'I could do that.’” So this past June, Nies headed out to Pennsylvania Mountain, where he met up with Franklin and Galen.
“I got a sense of what they do everyday,” Nies says. “We hiked up the mountain and saw the plots where James is doing his study. He gave us a background on the geology and geography of the mountain and the biology of the plants and animals and how they inter-relate up there. I got to do some of his work, using his materials and his methods. So I really got into what his study is about and what his scientific process is.”
Franklin, in turn, is spending the academic year in Nies’ fifth-grade classroom helping develop the science curriculum and learning from Nies how to better explain his research.
“As a group,” Galen says, “we scientists have become more aware of the fact that, if we can do a good job explaining science to the public, then the public is more inclined to do what we hope, [which is] support our ability to do science.”