Bucking Convention to Track the Upside of Invasive Species

By Asher Elbein, a writer based in Austin, Texas. His work has appeared in The Oxford American, the Texas Observer, and The Bitter Southerner. Originally published at Undark.

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.

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, 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. While some biologists were interested by the findings, Gleditsch said, 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 told Undark in a recent interview. “And it showed how emotionally charged this topic can be.”

Over the last century, 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, like 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, said Daniel Simberloff, a prominent invasion biologist now at the University of Tennessee, and the results didn’t look good. Insects like the emerald ash borer and fungi like chestnut blight have devastated American forests. Mesquite took over South African rangeland. Rabbits and cats chewed up flora and fauna in the Australian bush, while feral hogs rooted through farmer’s 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 said, and global trade continually brings organisms into new landscapes. In 2014, 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, or 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, said Jens-Christian Svenning, a researcher with the Center for Biodiversity Dynamics in a Changing World at Sweden’s Aarhus University. A few critics of the field saw 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 said, noting potential biases in how scientists measured damage, and asked 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, said Giovanni Vimercati, an invasion biologist at the University of Fribourg, Switzerland and lead author on 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 over 100 alien organisms with reported positive impacts: giant tortoises that had helped native trees on the Mascarene islands, east of Madagascar, vegetation like exotic lovegrass and saltcedar that helped birds thrive in Arizona, and alien trees in Puerto Rico that helped repair soil structure and provide vital cover for native animals.

Vimercati and his colleagues scored the papers based the replicability of the data and the intensity of the aliens’ effects on native populations, through mechanisms such as providing food, shelter, or helping species disperse. “We’re measuring the impacts on native species,” Vimercati said, “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 — 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 ladybugs helped to devour pests afflicting native trees on the island of St. Helena.

One issue the study has run into, Vimercati acknowledged, 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 of the papers that do exist, some suggest benefits of invasive species without providing much supporting evidence.

But there’s also a bit of unconscious bias at play, Vimercati said. 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 added, “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, said Simberloff. To name one example: In 2010, the United States Department of Agriculture called for a halt on eradication efforts of the invasive saltcedar, a deciduous shrub found throughout the southwest. While the saltcedar 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 said. EICAT+ simply represents a formalization of that process.

“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 said. “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 who participated in shaping the EICAT. People are “less conservative,” said Ross Shackleton, a biologist with the Swiss Federal Institute for Forest, Snow, and Landscape Research. Previously species have been labeled as harmful simply because they weren’t native, he said, 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 that alien species should be regulated as innocent until proven guilty (76 percent agree) and that the field desperately needs more consistency and clarity on definitions for terms like “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 they 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 try to control them early. “There’s a lot that we don’t know,” Meyerson said. But we do know that some invasions have been incredibly harmful and expensive, and have caused many extinctions in the past, she said: 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 said, 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 said, 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 said, “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 said. “And that’s why we think we need this kind of scheme.”

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About Lambert Strether

Readers, I have had a correspondent characterize my views as realistic cynical. Let me briefly explain them. I believe in universal programs that provide concrete material benefits, especially to the working class. Medicare for All is the prime example, but tuition-free college and a Post Office Bank also fall under this heading. So do a Jobs Guarantee and a Debt Jubilee. Clearly, neither liberal Democrats nor conservative Republicans can deliver on such programs, because the two are different flavors of neoliberalism (“Because markets”). I don’t much care about the “ism” that delivers the benefits, although whichever one does have to put common humanity first, as opposed to markets. Could be a second FDR saving capitalism, democratic socialism leashing and collaring it, or communism razing it. I don’t much care, as long as the benefits are delivered. To me, the key issue — and this is why Medicare for All is always first with me — is the tens of thousands of excess “deaths from despair,” as described by the Case-Deaton study, and other recent studies. That enormous body count makes Medicare for All, at the very least, a moral and strategic imperative. And that level of suffering and organic damage makes the concerns of identity politics — even the worthy fight to help the refugees Bush, Obama, and Clinton’s wars created — bright shiny objects by comparison. Hence my frustration with the news flow — currently in my view the swirling intersection of two, separate Shock Doctrine campaigns, one by the Administration, and the other by out-of-power liberals and their allies in the State and in the press — a news flow that constantly forces me to focus on matters that I regard as of secondary importance to the excess deaths. What kind of political economy is it that halts or even reverses the increases in life expectancy that civilized societies have achieved? I am also very hopeful that the continuing destruction of both party establishments will open the space for voices supporting programs similar to those I have listed; let’s call such voices “the left.” Volatility creates opportunity, especially if the Democrat establishment, which puts markets first and opposes all such programs, isn’t allowed to get back into the saddle. Eyes on the prize! I love the tactical level, and secretly love even the horse race, since I’ve been blogging about it daily for fourteen years, but everything I write has this perspective at the back of it.


  1. Steve H.

    Value judgments are relative with regard to descent with modification. With regard to the nativism argument, it seems to me that the invasive species is initiating a relationship in which it is the subject and the locals are made objects. That’s a power differential.

    “People for whom people are things.”

    1. MT_Wild

      I’ve been working on invasive species issues for over 20 years now, and there is a lot of intellectual dishonestly and half-truths presented here. Would be happy to write a full on rebuttal for the benefit of all but don’t have time this morning.

      In Invasive species biology we make a clear distinction between species that are simply “non-native” vs. “Invasive” i.e., having been shown to cause economic or ecological harm. This is a Bio 100 level distinction, taught in intro classes, yet the authors use the words interchangeably as suits their purpose. As such in North America, feral pigs are considered invasive, but pheasant are non-native.

      Salt cedar and SW willow fly catcher (SWFL). In riparian areas of the SW where a 100 years of dams and channelization and salt cedar invasion have practically extirpiated native vegetation, SWFLs will nest in salt-cedar. There are no other trees. You could put out a fake Christmas tree and they would nest in that too. The whole issue came up with the release of the tamarisk beetle, and as it defoliated and killed salt cedar the SWFLs nested in the dead trees because there was no other vertical structure. Although they nest in salt cedar, they don’t feed in them. Other than the tamrisk beetle, not many if any insects feed on salt cedar. So there are no insects for the SWFL to feed on or feed to their chicks and reproductive success is very low and the salt cedar “habitat” is essentially a population sink. There are many publications out there on this because the issue became well known about 15 years ago. They also neglect to mention that the purpose of removing salt cedar was to reestablish native vegetation for SWFL.

      Hawaii. The state is a basket case in regards to invasive species, non-native species we’re not sure about, and just ecological collapse. And so to be fair it depends. If we are talking about Japanese red-billed leothrix and they are spreading the native plant seeds of kanawao or okala, then maybe it’s a benefit. But one that would be better served by moving a native thrush like the puahohi from Kaui to another island to do the same thing. If we’re talking about a non-native bird like the kalij pheasant and it’s moving the seeds of strawberry guava, then it’s clearly a negative effect.

      I could go on and on with the potential interactions with non-native birds and invasive grasses on the dry side of the islands but will end it here.

      Y’all are my go-to source for economic and political commentary, but would plead that you please dive in a little deeper into this topic.

      1. Eclair

        Thanks, MT_Wild!

        Invasive plant species in Chautauqua County, New York are a big problem.

        Rosa multiflora, Japanese honeysuckle, Japanese knotweed and wild mustard are the invasive species I deal with on our patch. The deer population, not kept in check by hunters and automobiles now that the wolves are gone, browse down the tasty tiny, native maple and oak seedlings that spring up each year under mature trees, but turn up their noses at roses, honeysuckle and knotweed. So, things get totally out of balance. Our little forest is ‘unhealthy’ because all the trees are pretty much the same age, about 50 years old, and the deer keep the forest floor browsed clean; except for the invasives. Result: no forest regeneration. And a couple of multiflora rose, left undisturbed, will result in an impassable thorny thicket in a few years.

        A couple of articles I read compared the berries on multiflora rose and Japanese honeysuckle to junk-food for birds. Looks and tastes great, but has minimal nutritional value.

        I plant some non-natives around the house: lilacs and forsythia …. and the deer aren’t fond of them. And, of course, our veg garden is full of non-native species. But they don’t run wild and take over the landscape.

      2. Carolinian

        Thanks for your comment. Here in the Southeast we have a plant friendly climate and tons of introduced Asian species (most famously Kudzu) some of which would be considered “invasive” per your definition. That said, I’m not sure how successfully such a thing can be fought and to many the Endangered Species Act has been seen as a tool to save wilderness in general with the excuse of preserving this or that species. I’ve been reading a book about extinctions and how species have always come and gone depending on the ever changing environment (remains to be seen whether we can control that change either). But the situation should be described as it actually is.

        1. Harold

          The Kudzu invasion didn’t happen naturally. It was intentionally planted in the thousands during the Great Depression in a misguided attempt to help the economy (it contains an edible and quite valuable starch/thickener. Same with many others, such as multiflora roses — food for wildlife, supposedly. Highway, agriculture departments and others spread such plants.

  2. rick shapiro

    This is a nothingburger. It is trivially obvious that in a closed ecology, more is better. That is, the local ecology becomes more resilient if all the world’s species are allowed to compete.
    It should also be obvious, however, that species diversity in the world is reduced, as species that can exist only in the absence of more effective competitors are eliminated.

    1. The Rev Kev

      Yeah, nah! Sometimes the costs can be just too high-


      Ask me about the time some English guy released two dozen rabbits on his property for a bit of jolly good shooting in the 19th century in Oz and how well that worked out. Your way actually does not lead to species diversity but species monocultures. And should I even mention the present health of American forests as a counter to your argument?

  3. Mangelwurtzel

    Anecdata: We have been “managing” (lol) a 40 acre farm surrounded by forests and lake for the last 20 years. Formerly an abandoned orchard, it had become nearly overrun by “invasive natives” Sumac, Virginia Creeper and Poison Ivy. After much effort land clearing, bare following, replanting of pasture/hayfield, and new orchards, the land is now also sprouting quite a few additional “invasives” like Autumn Olive, Bittersweet, and Amur Corktree. What’s the common thread here? These are all bird-vectored species, every single one. Who am I to say that the birds don’t know what they are doing? Maybe they are the real managers. When it comes to rapid climate warming, the birds will be leading the way, planting their preferred foods further and further north.

    1. Steve H.

      And selecting for a vine phenotype. That’ll rip through the conifers in the taiga.

      In the evolutionary equation, this is advancing by altering the covariants of selection. And bird-time is moving faster than terrasaur-time.

    2. MT_Wild

      Like economics, in ecology we need to examine first, second, and third-order effects.

      There’s a pretty well accepted understanding that altering food resources in any given environment will restructure the community. If you are a snorkelers or diver, You may have visited a reef site in the past where they feed the fish. If you’ve been to that same site more recently you may have noticed that they’ve discontinued the practice and even put up signs asking you to not feed the fish That’s because it’s been shown that feeding the fish restructures the reef fish community in the local area to the benefit of a few large aggressive species and the detriment of others.


      The same thing happens with birds and bird feeders. Generally in a suburban urban area we don’t really care that the over abundance of cardinals and blue jays displaces a lot of small brown birds because those environments are already artificial. But if you went to a more natural area and set up a whole bunch of bird feeders as an experiment as many grad students have done, then what you would see is those same cardinals and blue jays that increase in abundance over a few years drive out a lot a lot a smaller more secretive birds.


      So when we’re talking about oriental bittersweet, or autumn olive, Russian Olive etc., we don’t know to what extent we’re creating the same effect. A couple of those plants in your yard will probably only temporarily drive out some bird species when the flock of robin’s, waxwings, or starlings come in to feed. But if those same non-native or invasive plant species become a more abundant or predominant food source on the landscape, will those temporary and localized changes to the bird community become permanent?

      We don’t know. Generally speaking if we don’t know, we try to act out of an abundance of caution. But that general principle gets lost in translation.

      1. anon in so cal

        n=23 for the cited study on bird feeders?

        “We found evidence of the feeding regime negatively affecting native biodiversity, with native species richness remaining lower at feeding properties during the feeding regime compared with before, whereas an increase was observed at nonfeeding properties. It is arguable, though, whether the effect detected is biologically significant, given that the difference in species richness was less than 0.5 species. [italics added] “

        1. MT_Wild

          It’s a wildlife study not a pharmaceutical one. That was the 1st one I found when i looked and was easy to link to, There’s a lot more or and with a little searching you can see that it’s a pretty widely regarded phenomenon.

          And as far as it goes Proceeding of the national Academy in Science is a pretty big deal for publishing along the lines of Science or Nature as far as journals go.

      2. Chet G

        Blue jays at feeders: I live next to a PA game land, so it is a theoretically diverse area although this particular game land is also used for GM crops. For my first 17 years here, blue jays were morning feeders; this present year they’re here off and on throughout the day (I suspect due to the lack of insects and other food sources). Blue jays do public bird service by being the first to spot and call out predators (although the occasional blue jay cheats by calling an alarm simply to scatter everyone else; however, blue jays are also altruistic, insmuch as least twice I’ve seen a single blue jay chase away a sharp-shinned hawk that was hunting another bird).
        For me, a large part of the problem is the green desert, whether lawns or GM fields. Looking at a field in the spring where GM has grown the previous year is to see dead land.
        As to whether something should be considered invasive or non-native, the dividing line for me is whether bees and butterflies will dine on it.

      3. juno mas

        There’s a pretty well accepted understanding that altering food resources in any given environment will restructure the community.

        For urban dwellers this is quite obvious. More discarded fastfood leads to an expanding crow population. (And their even caw-caw-cawing in the evening trees—so much for the song birds.)

        Most folks have no idea how much the North American landscape has changed since 1492.

  4. Wukchumni

    We used to have native California Quail by the thousands here with their favorite trick being that they’d run up as close as possible to your wheel wells when doing 30 mph and somehow survive the would-be suicide attempt. I never hit one.

    A common sight was a conga line of 17 of them following mom at a rapid pace, but that was then and this is now and maybe i’ve seen 100 this year.

    Invasive wild turkeys were here by the dozens when we moved to tiny town nearly 20 years ago, and now they are in the thousands…

    Quail & turkeys can fly but they’d rather walk, and quail are unusual in that their nests are laid on the ground and you’ll see around 20 eggs in them, easy pickings for gobbles & poultry in motion in the bird reich.

  5. SittingStill

    I’ve overseen many invasive species management efforts over the decades, most of which have either failed or been of limited value. While not irrelevant, this debate sidesteps a massive (IMO) overarching issue – the extremely limited agency we have in successfully “managing” invasive species problems (at least those where the impacts are severe enough to warrant investment in managing). We simply don’t have the ability in most cases (but there are a limited number of successes) to mitigate damages to the natural world on a scale that is meaningful. The ecological changes that are manifesting over time are massive and unimaginable to most non-ecologists (google “crazy jumping worms” – just one of many little-known slow-moving catastrophes) – it is easy to come to this conclusion after paying attention and observing ecosystems for a number of decades. The rate of the movement and establishment of exotic species is simply too large to be kept up with, barring the unlikely upscaling of financial resources allocated conservation to military-like levels.

    Like it or not, natural world is going to be dramatically altered/damaged by this issue regardless of the outcome of this debate, which is mostly focused on defining the most useful framework for making decisions on when and where intervention is warranted.

    1. MT_Wild

      As I’m sure you are aware, the other part of this is managing for what we want, and not necessarily what we had. I’m currently knee deep in sagebrush conservation issues in the West where invasive annual grasses like cheatgrass, Medusa head, etcetra are fostering rapid ecological change.

      We’re not going to be able to bring back the sagebrush community in areas of the West where it has been completely lost and replaced with invasive annual grasses. But that’s not to say we have to leave it as a cheatgrass monoculture. There might be other “tame” non-natives that will provide ecological or economic benefit without the fire risk. We’re going to have to do this with other biomes too. And thinking about the potential impacts of climate change these decisions are going to be ones we will have to face all too soon.

  6. anon in so cal

    Bookmarked for later, thanks!

    Just skimming (and not yet reading), my birding friends and I have a standing joke
    that birds prefer non-native plants and trees. Just kidding!

    Of course, Live Oaks and other native trees, plants, shrubs are crucially important for birds, insects, wildlife. Besides the wildlife benefit, nothing compares to the beauty of native Western Sycamore, Live Oak, Laurel Sumac, Goldenrod (in bloom right now), ETC.. Nothing is worse than a “landscaped” look, (ficus, queen palms, pygmy date palms, etc).

    Having said that, here in southern California, we notice that Toyon berries are typically passed over. Black-headed Grosbeaks love non-native Silk Oak blooms. Aleppo Pines are very popular with migrating Hermit and Townsend Warblers.

    For the most part, though, birds prefer natives. It’s heartbreaking to see the destruction of coastal sage (home to Wren Tits, Thrashers, California Gnatcatchers (critically endangered), and the ongoing extirpation of fields and hillsides replete with many other crucial natives.

    Links to another issue: deliberate razing (destruction) of California chapparal by CAL Fire and the endless construction of housing subdivisions.


  7. Harold

    I personally haven’t seen a lot of Japanese honeysuckle in recent years. When I was a child it was ubiquitous and practically would come in the doors and and windows of houses. Same with Ailanthus — the tree of heaven. Eventually native bugs and other wildlife (and diseases) learn to prey on them, I guess. (I wish this would happen to English sparrows, cute and cheeky as they are, they have displaced the native purple finch, which were even cuter, if less aggressive. (I do notice native sparrows in among the English ones, at times.)

    1. Aaron

      Thanks for this reply. I find the whole invasive debate short sighted and limited in scope. In Michigan the autumn olive is much hated for being highly “invasive”. But, I typically only see it growing in human disturbed areas, roadsides, abandoned fields, etc. If you cut open a trunk, you realize the wood is useless for firewood; autumn olive is a quick growing pioneer shrub. It has three main positive points to me, edible berries which birds love and are highly nutritious for humans, nitrogen fixing roots which help repair the soil for the next generation of plants, and a short life, a decade or two until they are overtaken by other growth. I cannot fathom the hate for it, besides xenophobia. I love them.

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