UM’s Ray Callaway in a dense cluster of knapweed
UM ecologist promotes concepts of
plant community interdependence
By Deborah Richie
High on the wind-scoured ridges of Lolo Peak south of Missoula, cushion plants survive blizzards and frigid temperatures. You can find them on peaks around the world. The species may be different, but many support a community of plants nestled within the cushion mats. The best way to investigate the miniscule neighborhood is to lie down and look closely – hence the nickname “belly plants.”
For UM Division of Biology Professor Ragan (Ray) Callaway, cushion plants exemplify a brilliant ecological process in nature that ecologists call “facilitation.” That term refers to how one plant facilitates the survival of another or perhaps several that all live together in a community. Callaway, who has taught at UM for the past 18 years, is one of the world’s experts on plant facilitation.
“We are finding that stress tolerant, tough, alpine cushion plants are fundamental drivers of biodiversity in alpine systems,” Callaway says.
A cushion plant escapes brutal winds by growing low to the ground, and its dense, leathery leaves raise temperatures at the leafy surface. The cushion structure and warmth benefits fragile alpine flowers that grow within its microclimate. In addition to competing with one another, these alpine communities prosper via cooperative strategies.
Callaway is part of a 10-member international team studying alpine ecosystems in 80 sites around the world, including the Alps, the Caucasus, the Andes, the Brooks Range in Alaska, New Zealand’s Southern Alps, the Pyrenees and the Rocky Mountains. For the past 15 years, the “Alpine Pals” have studied more than 60 species of cushion plants and some 2,000 species that share the rugged life of mountain summits.
“Cushion plants have evolved from more than 50 different plant lines,” says Callaway. “They’re beautiful examples of convergent evolution.”
In other words, very different kinds of plants have evolved into a cushion shape to colonize some of the harshest places on Earth. In Montana the moss campion cushion is in the carnation family, while in Latin America cushion plants belong to aster and carrot families. There’s even a cactus cushion.
Callaway is known for taking a biogeographic approach to understanding how plants behave. He’s trekked to Central Europe to research knapweed in its native habitat. There, he found it growing individually or in very small patches, nothing at all like the swathes threatening native plant diversity in the western U.S.
“For invasive plants, you have to know how they function in their native environment to understand why they explode here,” says Callaway. “For cushion plants, we can look at what part of the evolutionary tree they came from and how that affects the way they increase diversity.”
Whether studying cushion plants or invasive weeds, Callaway’s work demonstrates a key concept that has recently transformed plant ecology. Only a decade ago most ecologists accepted the premise that plants are individualistic in behavior. Callaway was one of the first modern scientists to challenge that accepted view. In field experiments, he found that plants often depended on one another. Later, teaming up with a wide range of collaborators, he documented numerous examples of facilitation, a clear indication of interdependence among plant species.
Interdependence has big implications for conservation, according to Callaway. It means that the loss of a seemingly insignificant species may harm other species in that community.
In the plant ecology world, competition is often cited as the primary, or most important, interaction. What Callaway and collaborators have discovered is that in harsh environments, such as mountaintops or the upper limits of intertidal zones, plants thrive by helping one another out. They serve as positive facilitators.
For example, near timberline in the Northern Rockies, whitebark pine is the dominant tree. At the highest elevations, subalpine fir grows close to whitebarks that protect them from icy storms. The death of the pine results in poor growth for the fir. However, as you descend from treeline, the strategy changes to competition between whitebarks and subalpine firs.
Callaway assembled enough compelling data to write a groundbreaking 415-page textbook, “Positive Interactions and Interdependence in Plant Communities,” which was published in 2007. He lays out his well-documented case in the first few pages:
“In this book I argue that plant communities are not simply suites of species that happen to be dispersed to and adapted to the same biotic conditions at a given place. I argue that many if not most plant communities have fascinating interdependent characteristics, with some species creating conditions that are crucial for the occurrence and abundance of other species.”
That big-picture thinking has earned Callaway high standing among scientists worldwide, says John Maron, UM professor and director of the Organismal Biology and Ecology program (OBE) that includes Callaway’s lab.
“The bottom line by any metric is that Ray is having a huge influence on the field,” Maron says, noting Callaway’s litany of published work, international talks, citations and papers reviewed. Callaway’s research has appeared in many premier scientific publications.
Callaway’s name appears most recently with three co-authors in the June 2011 edition of Science. Their article, “Terrestrial Ecosystem Responses to Species Gains and Losses,” weaves together existing research to make the case that losing a species or adding a species has big impacts on ecosystems across the globe and both should be studied simultaneously.
While Maron showers praise on Callaway for his out-of-the-box thinking and his productivity as a published professor, he adds a playful caveat to his compliments. It happens that Maron is more of a population ecologist, while Callaway is a trained community ecologist.
“Our offices are next door, and we go to the gym and we talk about our subjects all the time,” Maron says with a laugh. “We disagree about as much as we agree on, but we are good friends. Disagreeing is tremendously valuable because it forces you to look from a different perspective.”
Maron is proud of the OBE division he has directed for the past two years and the spirit of cooperation, engagement and creativity that dominates their lively faculty gatherings. Callaway’s position of seniority often puts him in a leadership role, yet he doesn’t take himself too seriously, Maron says.
“It’s fun to be an underdog,” he says of comparing the OBE division to the most prestigious institutions, “but now we are totally on other scientists’ radar, and we hear them say, ‘you’re an awesome group.’”
Scientists from around the globe regularly visit the faculty housed in the modest old Natural Sciences Building by the University greenhouse. They particularly want to spend time with Callaway.
“He’s such a collaborator, and they want to soak up some of the magic,” Maron says.
On a late summer day among the drying grasslands of Mount Jumbo, I join Callaway for a chance to see the all-too-familiar spotted knapweed from a scientist’s viewpoint. He breaks off a piece and shows where grasshoppers have stripped the stem, a minor setback to the hardy weed that’s easily identified by its thistle-like, purple-pink flower.
We take a closer look at the plant’s deeply lobed leaves. He tells me the leaves might contain fungi that help knapweed spread even more quickly than expected. Some knapweed carry the fungi and some don’t, according to research by one of Callaway’s graduate students, Erik Aschehoug.
To successfully control knapweed takes a holistic understanding of all its weapons — from fungi power to deadly chemicals in its roots that makes the soil toxic for native plants like bunchgrasses, Callaway says.
Back in its homeland, knapweed has long associations with its neighboring plants and the microbes in the soil. There, knapweed’s array of weaponry has little impact, because over eons its fellow organisms evolved ways to defend themselves, Callaway says. In our country, knapweed dates only to the early 1900s, when it sneaked into the U.S. from Eurasia among alfalfa and clover seeds.
Given the multipronged strategies for invasion, Callaway doubts that introducing predators from Eurasia will succeed, and those come with their own risk as well. Instead, he advocates preserving as much plant diversity as possible. Knapweed has a harder time gaining a foothold among healthy ecosystems, he says.
Chatting with Callaway in the field and in his office, I noticed he’d much rather talk about the work of his collaborators and graduate students than his own research.
He tells me that graduate student Marnie Rout recently discovered the main mechanism for invasion by one of the world’s worst exotic grasses. Called Johnson grass, this plant has proved to be bad news in Texas, drastically reducing native diversity. Rout found that the invader is enslaving a suite of bacteria in the soil to fix nitrogen and other services just for itself, and to the detriment of the native grasses. Rout’s research required a series of molecular, biochemical and experimental tests that Callaway says he can only admire.
“I take the shovel-and-hand-clipper approach to ecology,” he says. “I do simple experiments, like removing neighboring plants to see what happens.”
Accompanying his simple approach is an insatiable curiosity and a search for patterns and meaning.
“My whole career has been like that,” he says. “I see a pattern in the field and get interested.”
He says he stumbled upon his doctoral research at the University of California, Santa Barbara, by noticing unusual patterns among the blue oaks on the hillsides. He wondered why some of the trees sheltered myriad plants, while other oaks had nothing growing under them. He found that the oaks that sent their taproots straight down into the groundwater left plenty of space near the surface for grasses, flowers, and shrubs to flourish in the welcome shade. In contrast, the blue oaks that struggled to find water sent out a dense network of roots close to the surface. The roots overrode the facilitating (positive) effect of the shade and prevented understory plants from gaining a toehold.
After earning his Ph.D. in 1990, Callaway continued his exploration of plant interdependence and facilitation. He takes the same method of inquiry today as he did more than 20 years ago.
“I head down the trail and ask goofy questions about nature,” Callaway says.
His colleague Maron views him slightly differently. “One of the marvelous things about Ray is he is very much an idea guy. While ecologists often debate nitty-gritty details about how to do science, the field is still motivated by ideas, and Ray has had his share of good ones.”
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Callaway samples cushion plants in New Zealand’s fog-shrouded high country.
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