Can Invasive Species Ever Be Good?

Some scientists say fear of invasive species is nativist; others call that criticism “unconvincing if not tortuous.”

Flowers blooming in black and white
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This article was originally published in Undark Magazine.

A little over a decade ago, Jason Gleditsch was removing Asian honeysuckle when he noticed the birds. Robins and gray catbirds flocked around the thickets in autumn, attracted by the fat, ripe fruits. Originally introduced as ornamental plants in the early 20th century, Asian honeysuckle rapidly spread across the Eastern Seaboard of the United States, growing in dense stands that can shade out native plants. Conservationists often destroy these clumps. But Gleditsch, then an undergrad at Pennsylvania State University, noted something interesting: Native bird species seemed more drawn to the honeysuckle patches than to other plants.

Intrigued, Gleditsch conducted a set of experiments—removing honeysuckle in some places, placing potted native plants in others, and counting birds. Not only did many native bird species adore honeysuckle, he wrote in a 2010 paper, but they also consumed the fruits of native plants placed near the thickets at a noticeably higher rate, potentially helping increase the spread of their seeds.

The response was immediate. Although some biologists were interested by the findings, Gleditsch says, others dismissed them as unimportant. And angry emails and handwritten letters arrived calling him a hack. “It was kind of difficult, to be honest,” Gleditsch says. “And it showed how emotionally charged this topic can be.”

For decades, biologists have warned of the dire consequences of introducing alien organisms into new ecosystems; a whole field of study, called invasion science, has extensively cataloged the resulting damage. When some researchers, such as Gleditsch, have suggested that certain alien species may help ecosystems, they’ve been met with fierce debate from invasion scientists who worry that such studies are a distraction from real damage. Researchers on the other side of the debate, however, argue that the relentless focus on the damage risks missing equally important data, and could lead to management decisions that do more harm than good.

In August, an international group of experts in invasion biology weighed in, suggesting in the journal PLOS Biology a new framework for classifying the positive impacts of alien species. It’s an attempt some researchers feel marks a shift in the field toward a more holistic approach—and one that many agree could have serious implications for how invasive species are studied and managed.

The field of invasion science arose in the late 1950s, a time of growing awareness that ecosystems across the world were under serious threat. Introduced species were a kind of natural experiment, says Daniel Simberloff, a prominent invasion biologist at the University of Tennessee, and the results didn’t look good. Insects such as the emerald ash borer and fungi such as chestnut blight have devastated American forests. Mesquite took over East African rangeland. Rabbits and cats chewed up flora and fauna in the Australian bush, and feral hogs rooted through farmers’ fields throughout the American South.

Conservationists have tried to tamp down these flare-ups with targeted campaigns of suppression and extermination. But these strategies can be expensive, Simberloff says, and global trade continually brings organisms into new landscapes. In the 2010s, the need for a simple, usable framework to measure harm caused by non-native organisms—and thus help with their control—led to the Environmental Impact Classification for Alien Taxa (EICAT), a standardized global tool for assessing the species’ negative effects.

But while the damage done by some invasives was clear, the notion that they were always an inherent threat to native ecosystems troubled some researchers, says Jens-Christian Svenning, a researcher with the Center for Biodiversity Dynamics in a Changing World at Denmark’s Aarhus University. A few critics of the field saw what they believed was a heavy, unthinking bias toward nativism and, perhaps, xenophobia. Others pointed out that ecosystems might be more receptive to some alien species than the field assumed, Svenning says, noting potential biases in how scientists measured damage and asking whether the benefits alien species might offer were going unstudied.

Some of these criticisms provoked furious responses. The nativism question in particular rankled: In a fiery 2003 paper, Simberloff accused those raising such criticism of ignoring damage done by invasives and declared nativism allegations “unconvincing if not tortuous.” A 2016 paper raised hackles by declaring much of the criticism a form of science denial.

Still, the possibility that positive impacts were being overlooked wasn’t so easily ignored, and a growing movement of researchers began asking for a framework that could help measure them, says Giovanni Vimercati, an invasion biologist at the University of Fribourg, in Switzerland, and the lead author of the recent PLOS Biology paper. Vimercati worked with a team of prominent invasion biologists—including several who’d helped write the EICAT—to create the EICAT+ in order to collate these sorts of findings. The team gathered papers on more than 100 alien organisms with reported positive impacts: giant tortoises that had helped native trees on an island east of Madagascar; vegetation such as exotic lovegrass that helped birds thrive in Arizona; and alien trees in Puerto Rico that helped repair soil structure and provide vital cover for native species.

Vimercati and his colleagues scored the papers based on the replicability of the data and the intensity of the aliens’ effects on native populations, through mechanisms such as providing food or shelter, or helping species disperse. “We’re measuring the impacts on native species,” Vimercati says, “which is allowing us to compare species that aren’t closely related to each other.”

Minor changes to a new ecosystem—an alien plant attracting pollinators to nearby native plants, for example (alien plants can also outcompete native ones for pollinator attention)—might help individuals from a native species without boosting their population numbers. A moderate change, such as Pennsylvanian birds thriving on Asian honeysuckle fruit, helps grow a native-species population. The much rarer major and massive changes occurred when aliens led to native species expanding their territory or being saved from extinction—for example, when introduced lady beetles helped devour pests afflicting native trees on the island of St. Helena.

One issue the study has run into, Vimercati acknowledges, is a severe lack of data. In general, most introduced species haven’t been subjected to close scrutiny. (There are only so many grad students willing to devote a decade to studying an animal that may or may not be a problem, and small things—from insects to microbes—often get short shrift.) And some of the papers that do exist suggest benefits of invasive species without providing much supporting evidence.

But there’s also a bit of unconscious bias at play, Vimercati says. For decades, invasion biologists have focused their attention on negative impacts and, as a result, accumulated a lot of data pointing in that direction. Now, he says, “we’re reaching a moment where we’re considering positive impacts that in the past were simply overlooked.”

For some researchers, the idea that invasion science is shifting at all is overblown. The field has long acknowledged the potential for positive impacts, Simberloff says. To name one example: In 2010, the United States Department of Agriculture called for a halt on eradication efforts of the invasive salt cedar, a deciduous shrub found throughout the Southwest. Although the salt cedar may reshape desert streams, it also serves as habitat for the endangered willow flycatcher. “Usually, we’re trying to deal with the negative impact, but I can’t think of a case where we didn’t bear in mind that there may be other factors, especially for species that are long established,” Simberloff says. EICAT+ simply represents a formalization of that process.

Indeed, thinking about efforts such as the EICAT+ as part of a major shift could be misleading, says Laura Meyerson, an ecologist at the University of Rhode Island and a co–editor in chief of the journal Biological Invasions.

“It suggests that ‘Oh, we’ve been doing it all wrong; we’ve been thinking about it all wrong all this time. You see, invasive species are really good,’” she says. “There’s a camp that’s been beating that drum for quite a long time, and there are many of us in the field that find that problematic. I think that in terms of language, we have to be very careful about how we describe EICAT+.”

Other researchers, however, pointed out a noticeable change over the past few years in the tenor of both arguments and research questions. Several noted that the 26 authors of the EICAT+ are prominent, well-respected invasion scientists, many of whom participated in shaping the EICAT. People are “less conservative,” says Ross Shackleton, a biologist with the Swiss Federal Institute for Forest, Snow, and Landscape Research. Previously, species have been labeled harmful simply because they weren’t native, he says, whereas today there’s increasing acceptance that some species are worse than others.

In May 2022, a survey of 698 invasion scientists and practitioners published by Shackleton and colleagues—including Vimercati—suggested that the field is still split by active and contentious debate. But notably, the survey found broad agreement (76 percent) that regulating alien species as innocent until proven guilty could be appropriate (although 64 percent agreed that a guilty-until-proven-innocent approach was also valid), and that the field desperately needs more consistency and clarity on definitions for terms such as invasive (81 percent agree). And more than half of respondents supported the idea that invasiveness should be defined by impact, not just by spread, and that such species may have biological benefits.

Despite these findings, the field will still explore the negative effects of invasive species. Several researchers pointed out that it’s often better to hedge when it comes to newly emerging non-native species and to try to control them early. “There’s a lot that we don’t know,” Meyerson says. But we do know that some invasions have been incredibly harmful and expensive, and have caused many extinctions in the past, she says: Her concern is “that we don’t disregard that—we work carefully, rationally, and reasonably, and deal with what the data’s telling us.”

But just as recognizing potential positive impacts doesn’t cancel out negative ones, Vimercati says, the reverse is also true: Alien organisms’ interactions with native ecosystems can be extremely complex, and taking a more holistic view is vital for making careful management decisions. What if, for example, researchers seeking to remove alien species accidentally get rid of an organism that’s helping declining natives? In the Ogasawara islands of Japan, a 2010 study found that native land snails were hiding from invasive rats in groves of invasive trees. Where the trees were cut down, the snail populations declined.

Or consider the disturbed rainforests of Hawaii, Gleditsch says, where most native birds have long since been wiped out, and alien birds now act as the primary seed dispersers. “So if you remove the non-native birds,” he says, “there’s not going to be anything to disperse seeds, which is critical for forest stability and functioning.”

“This is where everything becomes complicated,” Vimercati says. “And that’s why we think we need this kind of scheme.”