Plant ecologist Mark A. Davis will not participate in this year’s “Buckthorn Roundups” around his St. Paul, Minn., neighborhood. ­Davis will not tag along as these intrepid crusaders set out to eradicate the common and glossy buckthorn, two ornamental shrubs imported in the 19th century from Europe. The nonnatives have now taken over some Midwestern forests, prairies and wetlands. That is why eco-minded volunteers eagerly wrench young weeds from the soil, hack away at thick stems and douse remaining stumps with herbicides. Their hope: a return of Minnesota to its primeval state.

At one time, Davis, too, could see the logic in eradicating these “invaders.” He even advocated planting only Minnesota native plants on the Macalester College campus where he teaches. That changed in 1994, when he read an essay by journalist Michael Pollan in the New York Times Magazine that made his blood boil. He bristled at Pollan’s statement that turning the “ecological clock to 1492 is a fool’s errand, futile and pointless to boot.”

After Davis cooled down, he started to think carefully about the problem. Gradually, he reconsidered his assumptions and developed a more nuanced position on the threat from nonnative species. In line with his new view, he gave benign nonnatives the name “LTLs”—as in something we should “Learn to Live” with­—which vexed some colleagues. He argues here, as he does in his book Invasion Biology (Oxford University Press, 2009), that the field needs less emotion and more science. The transcript has been edited for clarity.

Scientific American: You’ve tried to establish yourself as a skeptic in the debate over the impacts of introduced species. What would you say makes particular species a problem?
Davis: A species is a problem when humans define it as a problem. Organisms are just organisms. They’re not moral or ethical; they are just living. Good or bad, that’s completely a human declaration. The problem I have is when species are not health threats, are not causing any significant economic cost, yet people claim they have undesirable ecological effects. And that’s where I think it’s very important that we challenge ourselves: “Wait a minute. Is this harm, or is this just change?” The fact that certain native species may become less abundant, is that really harm, or is that just change? It’s socially irresponsible to call those changes harm. Once we declare something as invasive or harmful, it makes society obliged to reduce or mitigate this harm, which draws on scarce resources. I don’t believe we can justify using social resources to support projects that are often little more than claims of personal preference.

Some critics of your argument would say that the introduction of the brown tree snake on the island of Guam, which has wiped out 10 bird species, is more than just change. That’s irreversible damage.
I absolutely agree. The one environment where introduced species can and absolutely have caused lots of extinctions are in these insular environments such as oceanic islands or freshwater lakes. The species that cause those extinctions are almost always either a predator or a pathogen, and in these sorts of small, insular environments, there aren’t any refuges for the prey or the host. So the introduced predator actually is able to wipe out prey. In those sorts of habitats, absolutely, introduced species are a major threat to species survival, and I certainly support dedicating resources to prevent that.

Are you suggesting that the impact of some nonnative species has been exaggerated?
Few nonnative species come close to causing the damage of the brown tree snake. We’ve been studying garlic mustard at the Macalester field station, and there’s quite a bit of it that has spread into the oak forest. This past summer we sampled lots of plots and looked at the number of species that were in plots that had garlic mustard and the number in plots that didn’t have garlic mustard. Now, what you generally hear about garlic mustard is that it’s the evil enemy, pushing out the native species. In fact, we found no relationship between the abundance of garlic mustard in a plot and the number of other plant species.

Science cannot be motivated by ideology. It has to be driven by good data. I’m not against values at all, but when scientists express values, we need to make it clear that these are values we’re expressing, as opposed to conclusions based on actual data. We’ve seen what’s happened with climate change. As a scientific community, the worst thing we can do is to provide science skeptics more fuel for their arguments that scientists can’t be trusted.

Aren’t those who want to avoid the spread of nonnative species just being prudent?
The precautionary principle is used all the time, and it basically means, “Well, because we don’t know, because of our ignorance as scientists, we have to act accordingly.” The fact is, the world is changing in many ways. With climate change, some species are going to move to new areas, and there are going to be new combinations of species together in different conditions, different meteorological regimes. We can’t predict the behavior of those species either. The future is unpredictable with native species and with nonnative species. Native species in the past that haven’t been problematic might very well become problematic in the future. So the concern for future harm applies for all species, not just nonnatives.

You mean that native species can also be harmful to the environment?
Yes, of course. The best current example in North America may be the mountain pine beetle, a native insect in Western coniferous forests. This is a species that, probably partly because of a warming climate, has exploded in numbers in recent years and has been responsible for killing half the timber trees in some areas of British Columbia. No doubt about it—this is major economic harm. 

But without natural enemies, a nonnative species has an advantage over natives. Haven’t scientists documented their populations exploding exponentially after landing on new territory?
Nineteenth-century American botanist Asa Gray said that under the right conditions, any species could become a weed. Whether or not a species is able to become very abundant is going to depend on a combination of the traits of that particular species and the nature of the environment that it is established in.

Now, there have been many dozens of papers trying to determine whether there really are some predictable traits that distinguish species that are invasive versus noninvasive. For the most part, there hasn’t been a lot of success.

One of the most important recent findings with respect to invasive plants is that the longer an invasive plant has been in an area, the greater the negative feedback becomes between itself and the soil environment. In other words, the number of pathogens in the soil that are able to infect it increases, resulting in a reduction in the abundance of the invasive plant. It’s important for people to remember that just because a species comes in and becomes abundant, that doesn’t mean it’s going to remain abundant. In fact, if you believe in evolution, this is exactly what you would expect. Now, if the species is causing great economic or health harm, we can’t wait around for the natural processes to take place and let the numbers begin to slide back down. We need to intervene.

The native prairie habitats where you work are also prime territory for biofuel production. In the hunt for the optimal biofuel crop, would you have a preference for a native over a nonnative?
In some respects, no, I don’t have a preference, because concerns have been raised over even the native species that have been looked at, like switchgrass. For example, to get enough energy out of biofuel, the amount of land area that’s going to have to be planted will need to be quite large and probably will require the planting of the grass outside of its current range. On the other hand, if we find a native species and there’s enough of it, well, sure, I’d go with that first. My focus, and what I’m trying to argue, is that there should simply be less preoccupation with where a species is from and more of a focus on whether or not it really is a problem. I believe, in time, historians of science will view this preoccupation with native versus nonnative as very much a 20th-century phenomenon.

Where did this fear of the nonnative originate? Is there a moment when humans first started realizing they were shaping the distribution of species?
We don’t have documentation of it, but I’m sure that when humans were spreading from island to island in the South Pacific, they were aware they were transporting plants and animals with them, some of which would become naturalized in their new environment. Botanists during the Greek times were aware that travelers would sometimes bring back plants from other areas and would plant them. In 1850 we have explorer Alexander von Humboldt, who pointed out that the American prickly pear cactus Opuntia had been spreading throughout Europe, the Middle East and North Africa.

So when was there a shift from documenting these movements to thinking about preventing species from spreading and attempting to eradicate them?
Here in the U.S., our attitude toward nonnative—not just species but non-American things in general—has varied over the past several hundred years. When the country was younger, there was an interest in showing the world that we weren’t just a frontier backwater. We could be cosmopolitan, too. We could be worldly. There was actually considerable interest in bringing in stuff, whether it was music or opera or art, as well as plants and animals, from other parts of the world. Some of those species that were brought in, for example, pest insects and weeds, turned out to cause problems, primarily by negatively affecting the country’s agriculture. These observations gradually led to the shift in perspective at the federal level and efforts to control what species were brought in and to control those that were already here. With the advent of supposedly miracle chemical pesticides after World War II, such as DDT, the focus shifted from trying to just manage species to actually trying to eradicate them with DDT. We know how that turned out.

Has the pendulum begun to swing back in the other direction?
I think so. On the invasive species Web page of the Minnesota Department of Natural Resources is a statement emphasizing that most nonnative species are not problems. So the perspective and message are gradually becoming more nuanced now.

These days, more than ever, we need to spend society’s fiscal resources wisely and strategically. The number of species that will be transported around the world is just going to increase. We need to focus our resources on those species that are truly causing serious harm. The others we need to learn to live with.