Alien Invasion? An Ecologist Doubts the Impact of Exotic Species

Many conservationists have dedicated their lives to eradicating invasive plant and animal species, but Mark Davis wants them to reassess their missions


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Last November, Mark Davis spoke at a special meeting in South Africa to honor the 50th anniversary of Charles Elton's seminal book, The Ecology of Invasions by Animals and Plants. From Asian kudzu conquering the U.S. South to brown tree snakes wiping out birds on Guam, the ecological havoc wrought by exotic plants and animals has become—along with habitat destruction and climate change—one of the most talked about problems in species conservation.

But as Davis took the podium, he let his sport coat swing open to reveal his flip-flopped necktie, the skinny end stretching to his belt buckle—a not-so-subtle jab at the late Charles Elton, a notoriously inept dresser.

Though he may have earned a few chuckles from the in-joke, Davis's shenanigans did little to loosen up a crowd of scientists who viewed him as an exotic species, an alien and, perhaps, an invader. Davis is a plant ecologist at Macalester College in St. Paul who has spent the past 15 years challenging the most cherished principle of restoration ecology: that nonnative species should be eradicated from the landscape.

[Slideshow: Invasive Species Don't Get No Respect]

Earlier this spring, he published a bombshell of a book with Oxford University Press called Invasion Biology. Davis claims that alien species have been demonized and resources wasted on purported "invasives" could be better spent protecting habitat. More than that, he disputes the maxim that invasive species are the second-leading cause of species endangerment after habitat destruction, impacting some 42 percent of threatened and endangered species. Such concerns are particularly timely as ecologists debate the risks of relocating species to save them from climate change. In June, one reviewer wrote that Davis "dares to touch the third rail of invasion biology," slaughtering some of its "sacred cows."

A contrarian is born

Mark Davis was not always a contrarian. In the early 1990s he was the chair of the environmental studies department and spoke with the head of facilities at Macalester to suggest that the campus plant only native Minnesotan species. But as he began to think more carefully, the distinction between native and nonnative species no longer made biological sense. Instead, he realized it is more reasonable to talk about "undesirable and harmful" species, particularly when one considers that half of all agricultural pests are homegrown.

In his book Davis picks apart the claim that invasive species are the second-leading cause of extinctions. He traces that meme back to a 1998 paper by Princeton ecologist David Wilcove and colleagues in the journal Bioscience, which he derides for being based on the "opinions" of field researchers. Moreover, most species said to be imperiled by invaders were located in Hawaii and on other islands, not the mainland U.S., where he is skeptical that alien species can gain a foothold. "There have been thousands of nonnative species introduced in the United States," he says, "and they have not caused one native species to go extinct."

Davis is not alone in his call for reason: In South Africa he shared the stage with another invasion biology gadfly named Matthew Chew, a researcher at the Center for Biology and Society at Arizona State University in Tempe. Chew has made it his duty to restore dignity to the much-maligned tamarisk tree, aka salt cedar. Initially introduced in the U.S. Southwest for erosion control, the salt cedar rapidly established itself along desert waterways and was soon vilified for displacing native vegetation, sucking up scarce water resources, and releasing salt into the soil.

As Chew and his co-authors point out in the March issue of Restoration Ecology, salt cedar was just a scapegoat in the water wars that have gripped the Southwest. Today, many early claims have been refuted and the exotic plant is considered a critical habitat for endangered bird species, such as the southwestern willow flycatcher that nests in its branches.
Even the cane toad—that monster of invasion ecology depicted in the madcap 1988 documentary, Cane Toads: An Unnatural History—may not be as bad for Australia as some have claimed. Imported from South America in the 1930s, Bufo marinus soon spread from its gateway in Queensland, and researchers have since documented a much-ballyhooed trail of devastation in its wake. When molested, the amphibian secretes a milky blend of neurotoxins from glands on the back of their heads powerful enough to kill any crocodile or quoll (a catlike marsupial) that tries to eat it. Volunteers now conduct military-style operations to hinder the toad's progress, and government scientists have spent millions of dollars to build a virus to exterminate it.

But the most respected voice in Australian herpetology, Richard Shine of the University of Sydney, says cane toad hysteria is overblown. Not a single organism has gone extinct due to the cane toad, and many have adapted to its presence. Shine has found that some snakes, such as the red-bellied black snake, are evolving to have smaller heads, which forces them to prey on smaller, less toxic cane toads. Some species of birds and rodents have also learned how to eat the toads safely by flipping them over on their bellies to avoid the toxins before devouring their organs. Life, in other words, goes on.

Balancing act

Off the top of his head, Dan Simberloff can run through a list of devastating invasive species from Brazilian pepper in Florida to gray squirrels in the U.K. to zebra mussels clogging water pipes in the Great Lakes. Simberloff is a well-known ecologist, the founder of the Institute for Biological Invasions at the University of Tennessee Knoxville, and has been the editor of Biological Invasions for the last decade. Few things puzzle the eminent ecologist more than Mark Davis. "He's like those global warming skeptics or people who say cigarettes don't cause cancer," Simberloff says. "He holds a radical view that very few people favor."

He says that Davis's argument that not all exotics are invasive is impractical. "A number of introduced species have been innocuous for decades and they [can] suddenly explode and become problematic," he says. He points to the example of ornamental figs in Florida, which arrived in the early 1900s. Confined to backyards until their pollinating wasp showed up 25 years ago, figs have now invaded Everglades National Park.

Other ecologists have taken a more nuanced view of the invasive question. "The extent of their harm may have been overstated," says Princeton's David Wilcove, who first tallied up the threat of invasive species and still stands by it. "Maybe the issue is: Are we being strategic in the way we combat invasive species? And I think that's a fair question to raise."

That's one point that Davis might just agree with. "Given that we have scarce conservation resources," he says, "we need to be sure we are targeting species truly causing harm."

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