BAD REPUTATION: Kimberley Toadbusters catch and kill invasive cane toads in Western Australia, but some scientists think fighting exotic species can be a waste of time and resources
BRENDAN BORRELL

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.