Photo ©2010 Amy Eckert

BY Greg Breining

When Macalester Biology Professor Mark Davis expounds publicly on exotic species—creatures from dandelions to carp that come from somewhere else—he eventually mentions LLT, his shorthand for Learn to Love Them. Exotic species are here to stay, he says. No way to get rid of them. No way to prevent more from showing up in our forests, lakes, and farmland. So get used to it. And relax. With important exceptions, most aren’t as bad as people think.

“When I first say that, people in the audience almost get physically sick,” says Davis. “It’s amazing how extensive the indoctrination has been—nonnative species are bad. We’ve got to get rid of them. Boy, if you want nature to stop, you’re going to be miserable.”

There’s a reason he’s meeting resistance. Davis, the DeWitt Wallace Professor of Biology and department chair, is challenging recent trends in the field of “invasion biology.” Once a study of the way species colonize new environments, the discipline has adopted an orthodoxy that states that exotics—also called nonnatives or aliens—are dangerously invasive and stand ready to “degrade” native ecosystems.

Davis and a small cohort of iconoclasts propound a different view: Exotic species are part of nature’s rough and tumble, and there’s little point in ripping out buckthorn or throwing Asian carp on the bank to die. In his new book, Invasion Biology (Oxford University Press, 2009) Davis explores various theories on the function of exotic species in ecosystems and argues for a more objective view of a phenomenon that will only accelerate with increased global trade and travel.

Among Davis’s more unorthodox or controversial assertions:

Exotic species act a lot like natives. In fact, says Davis, a scientist who didn’t know the history of individual organisms would have difficulty in distinguishing natives from nonnatives on the basis of how they interact with surrounding species. Ragtag assemblages of exotics and natives quickly adapt and perform about as well.

Except in insular environments, such as islands and lakes, exotic species rarely drive natives to extinction. Throughout the United States, local ecosystems have perhaps 20 percent more plant species than they once did because of the addition of foreign species. “How many species of plants in the U.S. have gone extinct because of the thousands of nonnative plants that have been introduced?” asks Davis. “Zero!”

The existence of nonnative organisms is not a sign of a degraded or unhealthy ecosystem. “There isn’t such a thing as a healthy ecosystem or a sick ecosystem,” Davis says. It would be difficult to know how to define such a thing. “When someone is referring to a healthy ecosystem, what they are referring to is an ecosystem the way they want it to be. It’s really kind of a way to manipulate the audience, because who can be opposed to ecosystem health?”

The world will increasingly be made up of “novel ecosystems,” a stew of old and new. Davis wouldn’t invest much effort or money in fighting species such as buckthorn or purple loosestrife that are already established and are not doing much damage. He suggests instead employing triage: vigorously trying to stop threats to human health (such as avian flu) and the economy (gypsy moths and emerald ash borers) and not worrying about the rest. “For certain targeted species we can make a difference and it definitely is worth the effort. You have to focus.”

Invasion biology has cut itself off from other ecological sciences by investing in battling exotic species rather than examining the function of nonnatives in ecosystems. Even language among scientists, such as the term invasion itself, reveals biases that make for poor science. “All I’ve been arguing for is a more nuanced characterization of what’s been happening,” says Davis. And he’s finding traction for his ideas: “People are thinking more carefully about the words they are using, the assumptions they might be bringing in.”

Not everyone. Dan Simberloff—professor of ecology at the University of Tennessee, director of the Institute for Biological Invasions, and one of the most prominent voices in the field—counters that invasions of exotic species do threaten natives. Chestnut blight, caused by an introduced fungus, swept across the eastern United States a century ago, almost exterminating the native chestnut. Moreover, Simberloff says, “We know it caused global extinction of at least seven species of moths that were host-specific only on American chestnuts.” Nonetheless, he calls Invasion Biology “a really good book.” He adds, however, “It has a number of peculiar aspects to it. They almost all revolve around Davis’s odd views that invasions aren’t really so problematic and there’s something xenophobic about people who worry about them.”

But Davis isn’t alone in his beliefs. One of the more famous champions of this view was the late evolutionary biologist Stephen Jay Gould, who argued that the movement of species around the globe should be viewed as part of the evolutionary process, not necessarily a destructive force. The discussion about native plants, Gould wrote, “encompasses a remarkable mixture of sound biology, invalid ideas, false extensions, ethical implications, and political usages.”

Davis’s unorthodox take on biology may follow from his unorthodox path to the discipline. “What I find so interesting,” he says, “is that so many of these issues are seen immediately by people outside the field.” People like Davis himself, who earned a bachelor’s degree at Harvard, writing a thesis on Spanish baroque medicine. “I have a checkered background,” he notes. Literally: he drove a Checker cab with dreams of writing the great American novel. He even flew hot-air balloons for a while.

After returning to Harvard for a master’s in education, Davis worked part time in the university’s primate labs, rekindling an interest in wildlife that may have begun on family bird-watching outings. “I really wanted to study animals in their natural habitat,” Davis recalls. So the summer after receiving his master’s, he signed up for a tern research and management program on Cape Cod. He couldn’t believe that this—a summer on the beach with birds—was really a job.

Back in Boston driving cab, Davis waited for a sign about what to do with his life. “I was driving into the Callahan Tunnel on the way to the airport. I went in not knowing what I was going to do and came out having decided to apply to graduate school to work with birds.”

Established researchers treated Davis as though he were an exotic species himself. “They were trying not only to keep new species out. They were trying to keep new ideas out,” says Davis.

Applying to Dartmouth despite his lack of an extensive biology background, he sold himself as a renaissance man. It worked. He graduated with his PhD, having studied first shorebirds, then insects, and later plants.

When he came to Macalester as an assistant professor of biology in 1981, Davis began studying plant ecology, often at Cedar Creek, the University of Minnesota field station. He churned out papers on the ecology of prairie, woodlands, and savannas, on subjects ranging from wildfire to gophers. Over time, he wondered why old fields were so resistant to colonization by native oaks. What are the characteristics that make an area either vulnerable or resistant to invasions?

“This has been a Macalester story!” Davis exclaims, by which he means it began with the questions of an inquisitive seminar student. She had written a report on a study that noted that the places with the most native species are also richest in exotic species—a fact at odds with the prevailing orthodoxy that a tight-knit ecosystem of natives would hold nonnatives at bay. The student said the paradox reminded her of Davis’s work at Cedar Creek.

“It was that student who actually made the connection,” says Davis. He found himself wrestling the problem as he returned from a conference. “Between the time we started the descent and we touched down—10 minutes—I had written out the fluctuating resource hypothesis for invasiveness.” That hypothesis states that if the availability of resources such as nutrients or sunlight fluctuates, an area will be colonized by new species—both native and nonnative.

“I thought it was a good idea. But I was left with this challenge. I wasn’t really very well known at that point.” He found two enthusiastic co-authors at England’s University of Sheffield, senior researcher Ken Thompson and professor emeritus Phil Grime. Grime in particular was well known in the world of plant ecology. Both added important points to bolster Davis’s arguments. The paper was published in the Journal of Ecology in 2000. “It’s so commonsense it really couldn’t be wrong,” Davis says. Even though it challenges the idea that a diverse assemblage of native species is the best defense against invasion, researchers have been quick to cite the paper in their own work because it explains in direct mechanistic fashion how invasions occur. “There’s been enormous support for it.”

With that paper, Davis had leaped into the cage match of invasion biology. He challenged the definition of invasion itself (“a very unfortunate choice of words”) and received spirited rebuttals that, he felt, veered toward ad hominem attacks on his inexperience in the field. Established researchers treated Davis as though he were an exotic species himself. “They were trying not only to keep new species out. They were trying to keep new ideas out.”

Rather than back down, Davis wrote several rebuttals, including another paper with Thompson and Grime that warned that so-called invasion biologists were limiting their own scientific development and isolating themselves from the rest of ecology by making artificial, unsubstantiated, and value-laden distinctions between native and nonnative species. Another response in the Bulletin of the Ecological Society of America seemed to mock his adversaries with the title “Newcomers Invade the Field of Invasion Ecology: Question the Field’s Future.”

“They just picked the wrong guy. I’ve been called a contrarian. Which is always hard to respond to. Because if you say, No, I’m not!” Davis pauses and smiles. “Then I guess it’s true.”

Many of Davis’s outside interests revolve around the natural world. He continues his childhood pastime of bird-watching. “In fact, that is how my wife and I met. I actually proposed on Hawk Ridge in Duluth!” He often goes camping and canoeing.

He sees his broad background, with a healthy dose of liberal arts, as a “perfect match for Mac, where the students in my biology classes have broad liberal arts interests and enjoy the multidisciplinary approach I bring to classes. If I had to pick one thing that has brought the most enjoyment in my teaching over the years, it would be turning on students to birds and plants. I often begin my field botany and animal behavior and ecology classes with the warning: Be careful about taking this class, because it will change your life!”

“He really knows what he’s talking about,” says Courtney Jones ’10, a biology major and teaching assistant for an evolution class. “Years of experience have given him a wealth of knowledge that I can only hope to reach someday. He is also extremely driven to learn and teach. He seems to pick up on the talents of students and hold them to very high standards,” she says.

With the publication of Invasion Biology, Davis plans to keep on studying nonnative species. He will continue to work on the prairies and woodlands of Cedar Creek, investigating why some plant communities seem more subject to invasion than others, how resources affect plant competition, and how environmental factors such as rainfall and carbon dioxide affect forest and grassland succession.

He also would like to reinvigorate a Macalester field site, the Katharine Ordway Natural History Study Area, along the Mississippi in Inver Grove Heights, and use it with students to continue similar research into species invasions and competition.

The exotic species flap has also stirred his interest in broader issues of ideological beliefs within the scientific community. He argues that values and age-old religious attitudes toward nature frame scientific study and debates more than most scientists would acknowledge. “There’s clearly an emotional side, but most scientists don’t want to admit it,” says Davis. Ecologists, for example, particularly value “native” ecosystems not only for the long-term interspecies relationships that exist, but also out of a sense of original order, that “species have their place,” a notion that begins to sound downright religious, as if we were trying to preserve an Eden-like world.

“That’s a lot of what I’ve been challenging,” says Davis. “People can get addicted to paradigms. Then paradigms become an ideology. Belief and conviction are very difficult adversaries since they are little affected by data and evidence.”

Greg Breining writes about travel, science, and nature for Audubon, Natural Geographic Traveler, and other publications. 

July 30 2010

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