When biologist Mark A. Davis talks about exotic species, he eventually comes to LTL, his shorthand for Learn to Love them. Flying in the face of the conventional wisdom among biologists that exotic species are harmful to native ecosystems, Davis and a small cohort of biologists espouse a heretical viewpoint: Exotic species are here to stay, so get used to them, and forget about ripping out the fast-spreading shrub, buckthorn, on a large scale or throwing Asian carp on the bank to die.
If the newcomers are only changing the ecosystem but “not causing significant harm,” then “altering one’s perspective is certainly much less costly than any other sort of management program,” Davis writes in his recently published book, Invasion Biology.
“It’s amazing how extensive the indoctrination has been: ‘Non-native species are bad — we’ve got to get rid of them,’” says Davis, chairman of the biology department at Macalester College in St. Paul, Minnesota. “Boy, if you want nature to stop, you’re going to be miserable.”
Today, Americans continue to wage war on alien species that have taken root by accident or design.
Davis and his like-minded colleagues contend that the rigid attitudes, and militaristic metaphors, that characterize the debate about exotic species make for poor science and policy-making. Assuming that exotic species areinherently bad, that ecosystem “integrity” can be measured by the number of alien species, or even that newly arrived species are functionally different from longtime residents, simply isn’t supported by science, says Davis. Many introduced species, he notes, have proven valuable and useful, including crops (from apples to wheat), horticultural plants (hostas and Norway maples), and game species (ring-necked pheasants and brown trout).
He holds firm to that position, even while conceding that some newcomers, such as kudzu (“the vine that ate the South”) can be terribly destructive.
“I’m very careful to say that lots of invasive species are causing great problems,” says Davis, who says he supports control programs where damage is great and controls stand a chance of success. One highly destructive exotic species causing great economic harm, and for which Davis supports eradication programs, is the emerald ash borer, a beetle — recently introduced into the U.S.’s upper Midwest from Asia — that has already killed 2 million ash trees in Michigan and nearby states.
“All I’ve been arguing for is a more nuanced characterization of what’s been happening,” says Davis. And he claims that 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.”
Davis’ work is in the tradition of some scientists — including the late evolutionary biologist Stephen Jay Gould — who have argued that the movement of species around the globe should be viewed as part of the tumultuous evolutionary process, and therefore not necessarily a destructive force. Gould wrote in 1998 that the discussion about native plants “encompasses a remarkable mixture of sound biology, invalid ideas, false extensions, ethical implications, and political usages.”
“But we will not achieve clarity on this issue,” Gould continued in the journal Arnoldia, “if we advocate a knee-jerk equation of ‘native’ with morally best, and fail to recognize the ethical power of a contrary view, supporting a sensitive cultivation of all plants, whatever their geographic origin, that can enhance nature and bring both delight and utility to humans.”
Today, Americans continue to wage a war on alien species that have taken root by accident or design. Zebra mussels, Dutch elm disease, cheat grass, purple loosestrife, Eurasian water milfoil, spiny water fleas, gypsy moths — the United States is beset by exotics invading native forests, spreading across prairies, and clogging streams and lakes.
But Dov F. Sax, assistant professor of ecology and evolutionary biology at Brown University, says a growing number of scientists are listening to iconoclasts like Davis. James Brown, professor of biology at the University of New Mexico, says he generally agrees with Davis’s view of alien species but acknowledges that “almost certainly most ecologists and conservation biologists would not agree with either one of us.”
One of those scientists, Dan Simberloff — a professor of ecology at the University of Tennessee, the director of the Institute for Biological Invasions, and one of the most prominent voices in the field — called Invasion Biology “a really good book.” He added, however, “I’m going to say 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.”
Among the widely accepted precepts that Davis challenges are the following:
Because native species evolved in a specific ecosystem and exotics didn’t, natives are better suited to their niche. In fact, says Davis, a scientist who didn’t know the history of individual organisms would have difficulty distinguishing natives from non-natives. The Galapagos Islands — a world heritage site of biodiversity and the scene of Darwin’s important discoveries leading to the theory of evolution — provided just such an example. For years conservationists had fretted over invasive weeds overrunning the islands. Because the weeds were out of control, conservationists assumed they were non-natives, recently introduced by human traffic. Research showed, however, that the plants were native. They belonged. In fact, they had been present since prehistoric times.
Assemblages of exotics and natives quickly adapt and perform about as well.
Diverse communities of native species resist invasions by alien species. While carefully controlled experiments on small plots have confirmed this age-old belief, studies in the tumult of natural forests and grasslands show quite the opposite, says Davis. Natural disturbances, such as disease or fire, and fluctuations in resources in diverse communities offer plenty of footholds for invaders.
Pristine ecosystems are highly evolved and well-ordered. Again, ecologists have little evidence for this, Davis says. Ragtag assemblages of exotics and natives quickly adapt and perform about as well. “If you view [nature] as a continually changing tumult, the introduction of new species isn’t necessarily looked at as a huge threat.” He cites the example of regeneration of forests in Puerto Rico, where non-native species are facilitating the re-establishment of native species.
Which brings up the dire warning of orthodox conservation biologists: The spread of exotic species threatens to drive natives extinct. Rarely happens, Davis says, except on islands, in lakes, or in other insular environments. Usually it’s the opposite — the appearance of exotics increases species richness, he maintains. Throughout the United States, local ecosystems have perhaps 20 percent more plant species than they once did because of the addition for 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!”
Finally, exotics signify a “degraded” ecosystem. “There isn’t such a thing as a healthy ecosystem or a sick ecosystem,” Davis says. “Ecosystems are just out there. There’s no particular goal or purpose. They’re just the species and the physical and chemical processes taking place.”
Other ecologists beg to differ. Dan Simberloff counters that invasions of exotic species do threaten native species with extinction. The chestnut blight, caused by an introduced fungus, swept across the eastern U.S. a century ago, virtually exterminating the native chestnut tree. In addition, Simberloff says, “We know it caused total global extinction of at least seven species of moths that were host-specific only on American chestnuts.”
Even if exotics don’t drive native species to extinction, they can completely transform ecosystems, many biologists argue.
Land managers can’t begin to control the thousands of exotics that reach our ecosystems.
“There are some biotic communities that have entirely disappeared from the U.S. because of invasive species,” says Simberloff. “There are many others that have drastically changed over large areas.” For example, he notes that Eurasian cheat grass now dominates millions of acres of Western range, displacing native bunch grasses and reducing the value of the range for livestock. Asks Simberloff, “If thousands of people work to deal with the impact of these, even aside from any moral or aesthetic issue, is Mark [Davis] saying they’re all nuts?”
James Carlton, professor of Marine Sciences at Williams College and a Pew Fellow in Marine Conservation, attacks Davis’s assertion that biologists can’t distinguish natives from exotics. “We can easily distinguish natives from many (not all!) nonnatives many times in many ways, based very much on how they function in an ecosystem. Native species are often finely tuned physiologically to the environment, whereas nonnative species may possess a repertoire of adaptations that clearly do not match the environment.”
Still, a sizable minority of biologists are more sanguine about the impact of exotic species. Dov Sax says he began to question exotic species orthodoxy as an undergraduate at the University of California, Berkeley. A professor leading a field trip described the Bay Area’s abandoned plantations of Australian eucalyptus trees as a “biological desert.” Says Sax, “There was all kinds of stuff growing in there. I found there were really a similar number of species in both [native oak and eucalyptus] woodland types. Exotics weren’t always doing the awful things people seemed to think they were doing.”
Sax says that land managers can’t begin to control the thousands of exotics that reach our ecosystems through globalized trade and travel. “A lot of conservation biology in the past has been built around the idea of preventing change,” says Sax. “That old mantra is going to get thrown out because it’s going to be impossible to prevent change.”
The future landscape will be home to “novel ecosystems,” never-before-seen agglomerations of species, of which exotics will be a key — and often valued — component, Sax and others say. “If we lump them into this category of all being evil or awful in some way,” he says, “we may blind ourselves to those situations where they’re actually providing a benefit either for humans or for biological conservation.” For example, a recent study of two nonnative wetland plants, phragmites and hydrilla, suggests they provide waterfowl habitat, biomass production, and nitrogen retention that equals that of native species.
Overselling the threat exotic species pose is bound to lose credibility as exotics make up ever more of the biota around us, says Davis. And it will lead to misguided spending on projects as fruitless as ripping out buckthorn from thousands of acres of parks, when we should focus instead on disease organisms, agricultural pests, and other more pressing threats, Davis believes.
“It’s very important,” he says, “to distinguish harm from change.”