In the past several months, however, that idea has come under blistering attack. In a polemical essay that appeared in the leading science journal Nature in June, a biologist from Macalester College in Minnesota named Mark Davis led 18 other academics in charging that the movement to protect ecosystems from non-native species stems from a “biological bias” against arbitrarily defined outsiders that ultimately does more harm than good. According to Davis and his co-authors, the fight against invaders amounts to an impossible quest to restore the world to some imaginary, pristine state. The world changes, they argue, and in some cases, the arrival of a new plant or animal can actually help, rather than hurt, an ecosystem. The whole idea of dividing the world into native and non-native species is flawed, the article says, because what seems non-native to one generation might be thought of as a local treasure by the next. Instead we should embrace “novel ecosystems” as they form, and assess species based on what they do rather than where they’re from.

The reasons to fight invasive species may be economic, or conservationist, or just practical, but underneath all these efforts is a potent and galvanizing idea: that if we work hard enough to keep foreign species from infiltrating habitats where they might do harm, we can help nature heal from the damage we humans have done to it as a civilization.

The European water chestnut is considered an invasive species, one of the 1,500 or so plants and animals across the United States that have ended up settling in places where they don’t belong because of human activity. It’s a dubious distinction - one that most of us associate with evil carp overpowering local fish populations in the Mississippi River Basin, stubborn zebra mussels clogging pipes and killing birds in the Great Lakes, and the Asian longhorned beetle wiping out trees here in Massachusetts. Controlling the spread of such creatures has been a priority among ecologists and conservationists since roughly the 1980s. In that time, projects like the one on the Charles have proliferated around the world, forming a movement to patrol the natural environment and protect its fragile native ecosystems from intruders.

EARLIER THIS MONTH, a troop of volunteers in Newton piled into canoes and went to war in the name of the Charles River. They wore gloves to protect themselves from their enemy: a thorny aquatic plant called the European water chestnut, believed to have invaded the Charles a century ago after escaping from the Harvard botanical garden. The plant spread swiftly, growing so thick in some areas that it overwhelmed the waterway entirely. For the past four years, the Charles River Watershed Association has led the effort to get rid of the pest, recruiting concerned citizens to pull the unwanted plants out by their roots and collect them in plastic laundry baskets.

It wasn’t until the rise of environmentalism in the late 20th century that the American public caught onto to the idea that our natural ecosystems were being overrun by species that were never meant to be there. That was when people in America started hearing about things like snakehead fish, killer algae, and zebra mussels. And the problem was getting worse: as humans moved around more, so did plants and animals, by getting rides out of China to New York in a wooden crate, or in ships’ ballast tanks, or even on the bottom of someone’s shoe. “You can overwhelm a system by having so many new arrivals,” said UMass-Amherst entomologist Roy Van Driesche.

THOUGH BOTANISTS first started talking about the idea of nativeness back in the 1830s, for most of history people didn’t worry much about the risks of species moving from one place to another. The 1870s even saw the formation of the American Acclimatization Society, a group of wealthy hobbyists and animal-lovers who wanted to populate North America with species of European animals and plants they thought “useful or interesting.” The chairman of the AAS, Eugene Schieffelin, hatched a scheme to bring every species of bird ever mentioned in a Shakespeare play into America.

This flare-up has reawakened a debate over non-native species that goes back more than a decade. And while it would appear that the two sides are badly mismatched - those who oppose the targeting of non-native species are still very much a minority - their disagreement highlights questions about mankind’s relationship to nature that are far from settled. If we’re going to help restore a more natural environment, how do we decide what in the world is “natural” and what is the result of artificial forces? Why do some species get to stay, while others get pulled out by the roots? Their clash points up the fact that as humans take upon themselves the job of managing a changing natural world, there’s no obvious way to know which version of nature we should be aiming for.

The article in Nature joined similar arguments that had recently appeared in the journal Science as well as the op-ed page of The New York Times, where an anthropologist who had recently become a naturalized US citizen likened the control of invasive species to the anti-immigration movement. These critiques of so-called “ecological nativism” inspired equally spirited responses by scientists, including a letter in Nature signed by 141 scientists arguing that Davis and his cohort had downplayed the dangers of non-native species while distorting the work of ecologists and conservationists.

“Newcomers are viewed as a threat because the world that you remember is being displaced by this new world,” Davis said recently. “I think that’s a perfectly normal and understandable human reaction, but as scientists we need to be careful that those ideas don’t shape and frame our scientific research.”

While Pollan said in an interview that he now regrets resorting to the Hitler button to make his point, he maintains that there is something worrying about the zeal with which some environmentalists seek to keep foreigners out of places where they think they don’t belong.

One of the first people to publicly make this “anti-nativist” argument was, somewhat surprisingly, the journalist Michael Pollan, author of “The Omnivore’s Dilemma” and hero to locavores everywhere. He wrote an essay about it in the New York Times Magazine in 1994, focusing on the native gardening movement that was sweeping the United States at the time. Proponents of natural gardening had been calling on their fellow green thumbs to stop planting exotic species in their backyards; Pollan did not mince words in communicating his distaste for the practice, suggesting it came out of an impulse that was “antihumanist” and “xenophobic,” and even tracing its history back to a “mania for natural gardening” in Nazi-era Germany.

“We need to learn to accommodate change, and change our attitude rather than try to garden nature and keep things the way they are,” Davis said recently. Species migrate, he said, and some end up thriving while others go extinct. This would happen whether people were involved or not, and Davis emphasizes there’s no reason to believe that the best version of an environment - whether that’s defined as the most diverse, or the most useful for humans - is the one that happened to exist just before we meddled with it. Lots of flowers that are now considered as local as can be, for instance - including the state flower of New Hampshire, purple lilac, and the red clover of Vermont - originated in Europe.

That simplicity is also where Mark Davis and supporters come in and say, “not so fast.” As a biologist, Davis studies competition between plants, focusing on what makes some ecosystems more vulnerable than others to invasion, and how certain species of trees and grass interact. The author of the 2009 Oxford University Press book “Invasion Biology,’’ Davis has been a leader in the small but vocal group of thinkers who argue that nativeness is simply the wrong lens to use when we think about the environment.

For environmentalists and anyone worried about a local lake or forest, trying to keep the potential carnage at bay seems like a no-brainer: if non-native species might destroy an ecosystem we cherish, then of course we should do what we can to suppress them. The simplicity of that idea is a big part of why projects like Operation: No More Water Chestnuts can attract 70 volunteers to the banks of the Charles on a Saturday morning.

“We should always be alert that even those of us who think they’re practicing pure science or pure environmental policy are sometimes influenced by other ideas, other feelings,” Pollan said. “And we should interrogate ourselves to see if that’s what’s going on.”

This point was echoed this past spring by Hugh Raffles, an anthropologist at the New School who wrote the essay comparing invasive species to immigrants. “We choose to designate some plants and animals as native because they fit with the way that we want the landscape to look,” said Raffles in an interview. If you call something native, he added, “you should realize you’re just making certain claims about what you want to see and what you think is important to preserve.”

THE SCIENTISTS WHO study non-native species and try to control them are called invasion ecologists, and they’re used to feeling embattled. But their opponents usually come from the political right, and can be counted on to dismiss most any effort at conservation as an expensive nuisance or an impediment to industry. This other contingent, though - the one that includes Davis, Pollan, and Raffles - comes from a less obvious place. Suddenly, these environmentalists who have always identified with progressive ideals are themselves being accused of being conservative, backwards - even intolerant.

Their reply is that, as scientists, their job is to save plants and animals from extinction, protect their habitats, and make sure that subsequent generations get to enjoy as much of the earth as possible. To suggest that the work has xenophobic connotations, they say, amounts to little more than academic noodling - a philosophical stance at best, and a harmful distraction at worst.

“They’re throwing up a straw man,” said conservation biologist Daniel Simberloff, from the University of Tennessee Knoxville. He added, “[They’re saying] there’s a huge amount of resources being wasted in trying to deal with introduced species that aren’t really having any impact, so we’re wasting our efforts. It’s not true.”

As the editor of the research journal Biological Invasions, Simberloff is a leader in the field, and over the years he has stepped up repeatedly to defend himself and his colleagues from what he considers the slander of ideologically driven contrarians. His most recent contribution was writing the Nature letter and collecting 140 signatures. He says that if he wanted to, he could have gotten 1,000 - that that’s how much of a non-debate this is within the scientific community.

The reason he bothers to respond at all, Simberloff said, is that he doesn’t want to give politicians who are inclined to oppose funding for conservation projects a real excuse to do so. “I felt that there had to be some response or else someone in a high policy-making position would be completely justified in saying, ‘Well, this is a different view, and we can stop supporting this kind of activity because the other guys aren’t even responding,’” he said.

When it comes to what we should actually do for the environment, the two sides of this debate might not be quite as far apart as their denunciations of one another might indicate. Just as most ecologists accept that only a fraction of non-native species are harmful, the anti-nativists, when pressed, will admit that unequivocally destructive species like the Asian longhorned beetle should be reined in. Their disagreement lies more in how we should talk about the issue, how we justify our interventions and how we label the species we want to eradicate.

Is the debate simply over rhetoric, then? If it is, its fierceness has highlighted just how important rhetoric is to the environmentalist movement, and how valuable the distinction between native and non-native is in terms of rallying people to the cause of conservation. Psychologically, it’s not hard to see why the anti-nativist position holds an appeal, and why it would worry environmentalists.

There is something undeniably comforting, even self-forgiving, about abandoning the idea that human beings are separate from nature - accepting that we are part of an ecosystem, too, and that we belong. If you went with the mainstream ecologists, you’d have no choice but to believe that human beings are the worst invasive species of all. Stand with Davis, Pollan, and the rest of the anti-nativists, on the other hand, and suddenly it’s not a given that we’ve even done anything wrong at all.

Leon Neyfakh is the staff writer for Ideas. E-mail lneyfakh@globe.com.

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