Invasive species are bad news, or so goes the conventional wisdom, encouraged by persistent warnings from biologists about the dangers of foreign animals and plants moving into new territories.
Conservation organizations bill alien species as the foremost threat to native wildlife. Cities rip out exotic trees and shrubs in favor of indigenous varieties. And governments spend millions on efforts to head off or eradicate biological invaders.
“I think the dominant paradigm in the field is still a ‘when in doubt, kill them’ sort of attitude,” said Dov Sax, an associate professor of ecology and evolutionary biology at Brown University.
But a growing number of scientists are challenging this view, arguing that not all invasive species are destructive; some, they contend, are even beneficial. The assumption that what hails from elsewhere is inherently bad, these researchers say, rests more on xenophobia than on science.
“It’s almost a religious kind of belief, that things were put where they are by God and that that’s where they damn well ought to stay,” said Ken Thompson, an ecologist and retired senior lecturer at the University of Sheffield in England, who wrote the 2014 book “Where Do Camels Belong: Why Invasive Species Aren’t All Bad.”
“We’re actually moving plants and animals around the world all the time,” he said. “We have been for centuries.”
Dr. Thompson and other scientists have called for a more nuanced approach to evaluating whether the presence of a species is harmful or beneficial. Eradicating most invasive species is virtually impossible in an era of globalization, they note. And as climate change pushes more species out of their home ranges and into new areas, the number of so-called invaders is likely to multiply exponentially.
Yet the notion that a species should not be judged on its origins is highly controversial, as Mark Davis, a biology professor at Macalester College in Minnesota, discovered when he and 18 other researchers submitted an article in 2011 saying just that in the journal Nature.
The response was immediate — and signed by 141 scientists, many of them specialists in the field known as invasion biology. Their approach, they said, was already sufficiently “nuanced,” thank you very much.
“Most conservation biologists and ecologists do not oppose nonnative species per se,” wrote Daniel Simberloff, a professor of environmental science at the University of Tennessee, who led the group that wrote the rebuttal. He added that Dr. Davis and his colleagues had vastly played down the severe harm that alien species caused.
But in the five years since that contentious exchange, the idea that invasive species should be assumed guilty until proven innocent has begun to wane, the shift prompted in part, Dr. Davis speculated, by concerns over the use of chemical pesticides and the disruption of landscapes caused by many eradication efforts.
Some alien species are undeniably harmful, a fact that neither Dr. Davis nor others who share his view dispute. The fungus that causes chestnut blight, for example, decimated thousands of trees and changed the American landscape in the early 1900s. The Zika virus is invading new regions, carried by infected mosquitoes that some say are being driven northward by warmer temperatures. The vampirelike lamprey, sneaking into the Great Lakes in the 19th century, gradually champed its way through the fish population.
Islands and mountaintops are especially vulnerable to damage from invaders because their native species often evolved in isolation and lack natural defenses against predators or immunity to exotic diseases. The brown tree snake, accidentally transported to Guam, has virtually eliminated the bird population there.
But, Dr. Davis noted, “all species have negative impacts on something,” and the danger, he said, is often exaggerated.
A study published Feb. 17 in the journal Biology Letters, for example, concluded that alien species “are the second most common threat associated with species that have gone completely extinct” since 1500 A.D.
But the study, Dr. Davis and other experts said, relies on subjective judgments about extinction and does not distinguish between island species — which are far more vulnerable — and land or ocean species.
In some instances, nonnatives offer clear benefits. In California, for example, monarch butterflies prefer to spend their winters in the branches of the eucalyptus, an exotic tree transplanted to the state more than 150 years ago and viewed by some as an invasive fire hazard. In Spain, non-native crayfish serve as prey for migratory wetland birds, including some endangered species.
And some notorious invaders can have positive effects. Western states have spent a fortune trying to eradicate the tamarisk tree, which many experts believe hogs more than its share of water and damages the habitat of native species.
But Julian D. Olden, an associate professor in the School of Aquatic and Fishery Sciences at the University of Washington, said tamarisks had been found to provide shelter for birds like the southwestern willow flycatcher. Some studies have also concluded that the tree’s water use is not significantly different from that of other tree species.
The antipathy to foreign plants and wildlife is relatively recent. While the distinction between native and non-native species dates to the 18th century, the term “invasion” was first used in a 1958 book — “The Ecology of Invasions by Animals and Plants,” by Charles Elton — that drew on the militaristic vocabulary of the post-World War II era.
But the moniker did not achieve its full derogatory weight until the 1990s and early 2000s, when academic interest in the subject peaked and the number of papers on the subject generated by invasion biologists grew proportionately.
To biologists like Dr. Simberloff, taking action to head off alien species early on makes sense, allowing governments to address threats before invaders take firm hold. Non-native species are far more likely to do harm than native plants and animals, he argued, adding that the debate was “a phony controversy.”
Whether a species is viewed as native, however, often depends on when you arrived on the scene. Much of what Americans eat was originally imported: The horse, an icon of the American West, for example, was reintroduced by the Spanish thousands of years after the original North American horse became extinct. Several states list the honeybee as their state insect. But like many other state fish, insects and flowers, the bees are in fact immigrants.
In at least one case, a species that was long extinct in its native range was treated as an interloper when it finally returned home.
Beavers were common in Britain until they were hunted to extinction centuries ago. But when a group of the toothy dam builders took up residence along the River Tay in western Scotland several years ago, local farmers and fishermen greeted the animals with hostility, saying they posed a threat to farmland and salmon runs and were potential carriers of disease.
Scottish Land and Estates, an organization representing landowners, insisted that the beavers’ centuries of absence from Britain nullified their resident status, the Independent reported in 2010.
“It’s just silly,” Dr. Thompson said, of the reaction to the Tay beavers. “I don’t think we would have ended up in this ridiculous situation if we hadn’t been so bombarded by propaganda about invasive species.”
Often, he and others say, “invasion” is just another word for “change.” And the only thing that is certain is that more change is to come. Already, the flora and fauna of countries around the world are more homogeneous than they once were, as globalization has, accidentally or intentionally, moved exotic species from one place to another.
“From birds to plants to fish to mammals, there’s strong evidence that things are becoming more similar,” Dr. Olden said, likening the phenomenon to “the popping up of big-box retailers and the loss of mom-and-pop shops.”
As more species migrate, new quandaries are likely to arise. And as the human population increases, driving more animals and plants toward extinction, a species’ second home may be the only one it has.
In a paper published last month in the journal Conservation Biology, two scientists in California, Michael P. Marchetti and Tag Engstrom, describe the “paradox” of species that are under threat in their native range but are viewed as invasive in other places they have settled.
They include the Monterey pine, endangered in California and Mexico but treated as a pest in Australia and New Zealand, and the Barbary sheep, endangered in Morocco and other countries but running rampant in the Canary Islands and elsewhere.
“This is a challenge,” Dr. Olden said. “If we identify a plant or animal that might not be able to respond to climate change, do we roll the dice and intentionally move that species northward, or up in elevation?”
“We’re playing a little bit of ecological roulette here,” he added.