I have to respectfully disagree with this much of this article. However you distinguish plant and animal species that arrive suddenly, most often by human action, in areas distant from their place of origin, they often have a devastating effect on the existing local ecosystem into which they are released. Ecosystems develop and change over time, with new species arriving or evolving, but the pace of this transformation is typically slow in Nature, so that species have a chance to co-evolve. In this process, they have an opportunity to genetically adjust, both to compete with each other or/and collaborate.
If, however, you take an Asian species of chestnut, load it into a ship and transport it, complete with its fungal hitchhiker to a zoo in New York City, you take a chance of releasing a pathogen that is completely novel to the North American chestnut tree and in a few decades extirpate a tree that is foundational to the local ecosystem, killing not only billions of American chestnut trees (formerly a fouyndational species of the local woodlands) but also depriving local wildlife of the nuts on which much of it depended seasonally for food, and causing the disappearance or even extinction of many species of insects that fed on American chestnut foliage. This has huge impacts up the food chain and destabilizes the whole ecosystem.
Non-indigenous plant species we casually release into our gardens and the surrounding landscapes typically provide much less in support to the local flora and fauna. This is precisely why so many of the new arrivals spread explosively. The indigenous insects commonly cannot feed on them, and the indigenous pathogens are not adapted to preying on them. The new arrivals are also commonly less vulnerable to many of these plants have allelopathic qualities that essentially poison indigenous indigenously evolved flora, further degrading thee local ecosystem.
Yes, there is considerable debate among ecologists about the impact of releasing non-native species of plants and animals into local ecosystems. That is the nature of science - argument and debate is how it refines its understanding of the data it collects. However, the overwhelming consensus among ecologists is that the casual introduction of non-indigenous species is a very hazardous practice.
With. regard to Dr. Shapiro's findings about the adaptation of Californian insects to introduced plants, I would like to point out that this is a process every ecologist to whom I have ever spoken has said would surely occur. Such adaptations are an intrinsic part of evolution. The pace of the introductions, however, is again an issue. Dr. Shapiro sent me a personal communication rebutting another ecologist who spoke on my radio program and pointed out that Asian "butterfly bush" (Buddleia davidii) while a useful nectar source for Californian lepidopterans did not in general provide a host for their caterpillars. Dr. Shapiro informed me that he had identified two subspecies of a California butterfly that did use Buddleia davidii as a larval host. Given that butterfly bush was released into North America over a century ago and the numbers of it that have spread through wildlands in California since then (crowding out native species that native insects had previously depended on), Dr. Shapiro's observation underlines how slow an ecosystem is to recover from such human interferences.
As we confront a mass extinction caused by our self-serving degradation of the global environment, I do not believe that we can afford to continue the sorts of radically destabilizing, ignorant behavior which has brought us to the brink of this disaster. We can argue about semantics and whether we are verbally discriminating against the species gardeners and others are casually transporting around the globe. But if you look at the data, the damage this has caused and still is causing is clear.
Thank you for respectfully disagreeing. Others have left out the respectful part, lol. I appreciate that so far Substack has for the most part been fostering that tone.
I started my deep dive into the topic of "invasive plants" in 2018 (alongside Nikki Hill, my co-author on a soon-to-be-finished book on the subject) because I wanted to know the science behind statements like "they often have a devastating impact." Given how common such statements are, we were surprised to find that the science is far more nuanced than that. It's an easy thing to say, but as it turns out, it's a trickier thing to prove. So that's complaint #1 about the popular rhetoric around "invasive plants" -- that its brashness is unscientific (and hence this post).
On the topic of evolution, I have been intrigued to learn that gradualism is being questioned. For example:
“Until a decade or so ago, evolutionary change was broadly assumed to happen on a vastly longer time scale than ecological change. As a corollary, our view on biodiversity and ecosystem functioning has often been static, trying to conserve biodiversity as it is, and preferably, as it once was. Just like our ecosystems, however, this paradigm is shifting. The closer we look at adaptive evolution, often with the aid of new biological insights and technological advances, the faster it seems to happen. Evolution and ecology are proving to be so heavily entwined that the distinction is becoming increasingly hard to make. This knowledge profoundly affects our thinking on how evolution affects patterns of biodiversity, especially in the face of global change. Adaptive responses to climate change, for example, have been shown to occur within a single generation. Contemporary evolution is probably more important than we assumed to date and is, therefore, likely to mediate the response of populations, species, communities and ecosystems to both gradual and sudden environmental change.”
[Mergeay, J. and Santamaria, L. (2012), Evolution and Biodiversity: the evolutionary basis of biodiversity and its potential for adaptation to global change. Evolutionary Applications, 5: 103-106.]
The case of chestnut blight is certainly tragic but of course the "invasive" species in that case is the blight, not Castanea crenata (the introduced Asian Chestnut). It's worth noting that the tree was imported for commercial purposes, so I point again at the ecological destruction from agriculture. As a side note, I've spent some time on a farm in Sonoma County where blight-free chestnuts still grow, as part of a long-term breeding project to produce blight-resistant trees.
The idea that introduced plant species thrive because they lack threats (pathogens, predators) in their new environment is the "Enemy Release Hypothesis." For a discussion of this hypothesis, I would refer you to Mark Davis' book, "Invasion Biology" (Oxford University Press, 2009), in which he attempts to summarize work in the field, including ideas that are contradictory. His section on ERH is too long to include here, but since you're already familiar with it, I'll post part of the section that questions it:
--begin excerpt--
It is interesting that the emphasis has mostly been on the absence of natural enemies. Just as likely, it would seem, would be the possibility that resident enemies in the new environment would attack the new colonizers, which, lacking in the appropriate defenses for these enemies, would fail in their colonization effort. Gilbert and Parker (2006) emphasized that native pathogens might still play an important role in the biotic resistance of an environment. Parker and Gilbert (2007) described several reasons why enemy release may not be an important factor in accounting for differences in invasibility:
(1) many enemies may have broad host ranges and thus may easily be able to accommodate new spe- cies in their ‘diet’, particularly if the new species have native relatives;
(2) many introduced species may be accompanied by their native enemies;
(3) the introduced species may encounter cosmo- politan enemies.
More and more instances of colonization and establishment being thwarted by resident ene- mies have been reported in recent years (Agrawal and Kotanen 2003, Colautti et al. 2004). In a meta-analysis of 63 manipulative field studies, Parker et al. (2006) found that native herbivores not only do not tend to avoid introduced plant species, they tend to suppress them. Native predators may also inhibit the establishment of non-native prey. In the coastal waters of New England, de Rivera et al. (2005) found that a native predatory crab, Callinectes sapidus, is inhibiting the spread of the European green crab, Carcinus maenas, thereby providing biotic resistance to the spread and establishment of the invasive C. maenas. In a meta-analysis review, Levine et al. (2004) found little evidence to support the contention that the success of non-native plant species in establishing in an environment was primarily due to the absence of enemies.
In the claim that "indigenous insects commonly cannot feed on them" I would call out "commonly" for further refinement. The idea that native insects might be dependent on native flora has intuitive appeal for cultural reasons, and has been popularized by Doug Tallamy, but again, is not so cut and dry. This discussion is hampered by unclear definitions of "generalist" and "specialist" for example. While there certainly are insects who focus narrowly, like the Yucca Moth (upon whom the Joshua Tree depends for pollination), others are quite able to take advantage of introduced species, like the Black Swallowtail butterfly and Cecropia moth caterpillars I raised in my backyard as a child. They fed on the garden's Dill and an ornamental Pussywillow, respectively. This was a suburb where much of the native flora had been wiped out by the farmland that the suburb replaced. Researching this recently, I found that the Cecropia moth probably fed on Black Willow, which would've been common along the area's creeks, which were narrowed, channelized and otherwise disrupted by both these forms of development. Again, the facts on the ground are less equivocal than the "invasive" rhetoric suggests.
I agree with your statement: "That is the nature of science - argument and debate is how it refines its understanding of the data it collects." This is my broader point. The argument and debate are not reflected by the simplistic term, "invasive." Should industrial humanity be less casual in our treatment of the living things of the planet? I absolutely agree again, and hence my critique of the "invasive" narrative, with which we are trading one kind of casual disregard for another. I presume we will just have to agree to disagree about that!
Regarding your claim that "But if you look at the data, the damage this has caused and still is causing is clear" I will say that I *have* been looking into the data, and the deeper I look, the less clear it becomes. A subject that's gotten almost no attention outside the field of invasion biology is "data bias." Within the field, though, it's definitely discussed, especially recently because innovations in computing technology have allowed for meta-analysis of many studies, something that is onerous to do by hand. Here is an excerpt from the book I'm working on with Nikki Hill:
--begin excerpt--
Not until 2015, over three decades after invasion biology’s emergence, was there a central depository to compile a global list of successfully established introduced plants. This electronic depository, the Global Naturalized Alien Flora database, currently lists just under 14,000 species and subspecies of plants, which represents about 4% of the planet’s vascular plants. GloNAF, as it’s called, was started because “knowledge of the global spread and distribution of naturalized species (that is, alien species that form self-sustaining populations in new regions is still very limited”[ii] and existing data was “jaggy and incomplete.”[iii] The database is constantly updated. A caveat worth mentioning is the issue we brought up in an earlier chapter that terms like “native,” “alien,” “invasive,” etc., lack standardized definitions, and that baseline dates differ regionally. That is, “the classification of species as invasive can vary and be independent of its impact.”[iv] Nonetheless, GloNAF provides resources not previously available, and has proven useful in assessing various hypotheses in the field of invasion biology.
But surprisingly few of those ~14,000 species have received such thorough appraisals. Hulme et al. report:
"[R]obust quantitative assessments of ecological impacts have been undertaken for fewer than 200 alien plants, highlighting a considerable knowledge deficit. Are these studies representative of alien plant impacts as a whole? It does not appear so, given that only nine species account for one-third of all quantitative assessments of ecological impacts."[v]
(One of those nine species is Purple Loosestrife, the subject of a chapter in this book.)
We were surprised learn that the pool of data is this small and shallow and we expect that many readers will be as well. The authors also pointed out that several “high-profile” plants are not included in this list of 200, including Miconia (Miconia calvescens) and the undeservedly notorious[vi] Kudzu (Pueraria montana) and that others have been the subject of only one study, like Strawberry Guava (Psidium cattleianum) and Brazilian Pepper (Schinus terebinthefolius). The authors warn that “results from single studies at single locations or years might not be widely generalizable.”
--end excerpt--
There is also the issue of failed invasions not getting attention in the literature. Another book excerpt:
--begin excerpt--
The title of a 2013 paper is telling all by itself: “The elephant in the room: the role of failed invasions in understanding invasion biology.”[vii] The authors write:
"Most species introductions are not expected to result in invasion, and species that are invasive in one area are frequently not invasive in others. However, cases of introduced organisms that failed to invade are reported in many instances as anecdotes or are simply ignored... Furthermore, we identified key research topics where ignoring failed invasions could produce misleading results."
On the same topic, Pyšek et al. remark: “Studying failure is just as important as studying success, and invasion ecology generally lacks good information on why some species fail.”[viii]
A 2017 paper of by Warren et al. entitled “A systematic review of context bias in invasion biology”[ix] refers to what they call “file drawer” bias:
"We did not account for file drawer bias (unpublished null or non-significant findings), which certainly would influence findings if there is a bias against invasive species (studies showing no invasive species impact may be less publishable or perceived to be less publishable)."
--end excerpt--
If there is an "overwhelming consensus among ecologists," then this data bias should give some of them pause. Nikki and I were certainly taken aback when I started running across all the papers covering this type of bias. We had both assumed the data was more robust than it actually is, and it seems we're not the only ones.
Citations from the data bias section:
[ii]Kleunen, Mark van, et al. “Global exchange and accumulation of non-native plants.” Nature 525 (2015): 100-103.
[iv]Lefebvre, Suzanne, Josiane Segar and Ingmar R. Staude. “Non‐natives are linked to higher plant diversity across spatial scales.” Journal of Biogeography (2024).
[v]Hulme, Philip E., Petr Pyšek, Vojtěch Jarošı́k, Jan Pergl, Urs Schaffner and Montserrat Vilà. “Bias and error in understanding plant invasion impacts.” Trends in ecology & evolution 28 4 (2013): 212-8 .
[vii]Zenni, Rafael Dudeque and Martin A. Nuñez. “The elephant in the room: the role of failed invasions in understanding invasion biology.” Oikos 122 (2013): 801-815.
[viii]Pyšek, Petr & Richardson, David & Pergl, Jan & Jarošik, V. & Sixtová, Zuzana & Weber, Ewald. (2008). Geographical and taxonomic biases in invasion ecology. Trends in ecology & evolution. 23. 237-44. 10.1016/j.tree.2008.02.002.
[ix]Warren RJ II, King JR, Tarsa C, Haas B, Henderson J (2017) A systematic review of context bias in invasion biology. PLoS ONE 12(8): e0182502.
I so enjoyed this thoughtful, well-researched dive into the word “invasive,” which strikes me as a fancy term for the equally misguided “weed.” Years ago, I read an essay by Michael Pollan, “Weeds Are Us,” that completely flipped how I think about all plants — and our relationship with them. When we step out of the center and see the bigger picture, all sorts of insights can come. Thanks for this.
You're very welcome, Julie! I totally agree it's really just a fancy term for "weed." (My co-author and I discuss that very topic in our upcoming book.) I definitely appreciate Michael Pollan, too. I like that he does so much research, then throws in his own personal experience and spin. He's a great communicator for sure.
Thank you so much for another wonderful and needed article. I am always trying to reach people who hate and kill plants, even while they plant certain popular (and often non-native) flowers in their yards. I so worry for the animals who need all the plants and will never forget the Black-throated Blue Warbler who flew thousands of miles to where he was happily eating non-native Fennel seeds while a group of us was frantically trying to get photos. Usually the Warblers I know are always moving, but he was just a couple of feet away, staying safely on the ground as we guarded him, while he (and other birds, including another rare one) were also eating the Fennel seeds.
Yes my concern is also people who hate and kill plants. Not just because it's bad for plants, but because it's bad for us as humans to behave that way.
So lovely to hear about the birds who love the Fennel seeds! Several species of Swallowtails who love everything in Fennel's family (Apiaceae, the Carrot Family) have benefited from Fennel, Dill and other plants in that family we introduced here. I learned this as a child, raising their caterpillars, and have since seen research that corroborates it.
I agree! It's much easier! I'm just a lifelong plant lover, fascinated by all of them and what they do.
It's been discouraging to see the nativist cult arise. It's really just another form of puritanism. I'm personally thrilled to visit a new place and meet the plants there, but now it's like there's some kind of litmus test.
It's worth mentioning that the culty-ness is not universal. It's most popular in the UK, the US, Australia & NZ, Japan, South Africa and Canada. Except for Japan, I am sure you can observe what those countries have in common, lol.
Yes, exactly! Very puritanical and xtian fanaticism. I so wish we could stop it. I fear the poison spraying is increasing. I joke I could be a millionaire if anyone would bet me ten thousand and I identify a sprayed area, along roads, parks, etc. It's so visible to me, but not others, and I haven't seen them spraying openly in years. But then this week I saw hired gardeners in uniforms spraying poison by the Alameda to SF Ferry parking lot. It's so dead-looking and manicured, and they don't even know how to care for the ornamentals (all non-native) because they shear/prune species that aren't to be cut at all. I think they banned most poisoning in Marin, and so wish they did everywhere. Sorry to go on!
I love all the little animals with the plants too. Beautiful spiders and insects, etc....
Thank you so much for pointing out the blindness of how humans approach environment as a whole. The invaders dictate what is invasive through a lens only they view. It’s really rich and continues to evoke a conquering mindset regardless of any underlying subtleties. I can’t stand the word myself as it is extremely narrow and provokes reactions that don’t serve us in a realistic sense. I’m also curious about the whole Darwin ‘survival of the fittest’ concept and the evolutionary ‘skill set’ required to regenerate in a place you may not have previously been. Isn’t what we see in Nature, what we say in Nature merely the playing out of an evolutionary process that serves the planet? As illustrated in the butterfly study you mention there is significance to life in all forms as it provides something for something else/ it may not be obvious to our mindset. Nature doesn’t serve humanity and our policies should not keep trying to put a lid on a box we opened long ago. It’s far less significant what we think really as at some point our lease will be up, but Nature through its many incarnations and the genius of itself, will continue on as if we never were here. Yes, invasive is a dangerous and deliberate term meant to eradicate, and even when used within scientific communities it comes with underlying biases based on funding, etc. ❤️
"provokes reactions that don’t serve us in a realistic sense" -- yes, exactly.
As for Darwin, he was brilliant, but also a product of his place and time, which was the rapidly industrializing England of the 19th Century, which was all about ruthless competition, and a ruling class wanting to justify their position. In a very real sense, what we call "social Darwinism" preceded actual Darwinism as a philosophy. We have a chapter in the book that dives into that subject. If you DM me your email address, I'll send it to you.
My wife and I rented a house with a garden for a while, and we decided to plant clover - instead of the usual grass lawn which all the other neighbours had, even though it's considered an invasive species.
Not only was it almost no maintenance, but it was actually an incredible help to the other plants we had there - as it's fixating nitrogen in the ground. Extra: it had flowers and the butterflies loved it, too!
Good for you! Clover is wonderful! Yes very popular with the insects, and improves the soil. Of course nobody every calls lawn grass "invasive" even though, if it were classified as an irrigated crop, would be the most irrigated crop in the US. Worrying about what's native or non-native in an urban area is kind of pointless to me. Any plant life helps make a city better!
Don't sweat it. It's so ubiquitous and, as I mentioned, jibes well with Western cultural ideas. I've read a bit about attitudes toward these species in Australia, and it seems like it's a hotbed for the topic (largely because of the Cane Toad?). You might appreciate this paper, which explores indigenous perspectives there:
Bach, Thomas Michael and Brendon M. H. Larson. “Speaking about Weeds: Indigenous Elders’ Metaphors for Invasive Species and Their Management.” Environmental Values 26 (2017): 561 - 581.
So well said! I do not use the term invasive and it drives me buts when people do. This is invasive and that is invasive and it's been here where I live longer than I have and I am 68 yrs old. Not every non-local plant or animal is actually invasive.
Thanks, Pamela! Sometimes people just use it in a colloquial sense. Like, "plant that thrives without care and spoils my landscape design or needs to be weeded out of the garden." Which, whatever. Unfortunately, as a policy word it's used to push eradication efforts, which more often than not use herbicides.
A lot of people don’t know that weeds are actually local wildflowers and plants. Thankfully in some areas policies like that are not as popular as they are in the past. Conservation is popular when I live.
Definitely true. People get the concepts of "weed" and "invasive" and "noxious" all mixed up, because they're all categories of "bad" plants. But yeah, as an experienced gardener and farmer, I've totally noticed that many weeds are local native species. Of course many are also medicinal or edible too. Honestly, I love lots of "weeds" and only remove them when necessary, not just on principle as is common.
Language matters, thank you for writing about how harmful it is for us to talk about plants this way. The way we talk about things determines how we see them. When we call something "invasive," that changes how we think about it.
This seems like an odd, pathologically altruistic, and generally backwards take typical of late-civilization liberalism.
Here’s the practical matter:
You could simply rename invasives “out-competitive, ugly, unbeneficial plants” - but that’s a lot of words for the same idea. Invasives gets it done.
You must not farm. Or must not farm much, or must not have ever had to recover land previously lost to invasive species. It must be luxurious!
In the eastern US, thousands of acres are lost to nearly ineradicable invasives every year. The problem is that the process of recovering this land is often impractical, too costly, or simply impossible. How to get the reeking, sneeze-inducing invasives out of the National Park with one’s favorite hiking trails? “Just enjoy the newly ugly and smelly trail! Mother Nature doesn’t make a mistake!” But mother Entropy is an entirely different animal.
So beautiful hiking trails get overgrown with plants that block all views and sunlight, deer ticks with lymes’ thrive, healthy trees fail to germinate much less ever grow to maturity—beauty leaves the world, never to return.
When we think about trees alone, and the benefits including carbon capture that they offer, invasives-advocates such as yourself often fail to consider it’s one or the other. The loss of so much of America’s tree canopy—-from Chestnut Blight, Dutch elm disease or the emerald ash borer - is due to non-plant invasive species from foreign lands with no competive factors or reproductive limits.
And let’s say you want to beautify an area — are you yourself going to remove the Japanese knotweed ? Do you find Japanese knotweed beautiful and prefer it to pine & hardwood forests that were once native to much of the American landscape?
I'm an experienced farmer, actually. Whether or not I personally find it to be a hassle to deal with certain weeds is a completely different subject than whether certain species are ecologically harmful.
My love for natural ecosystems is at the heart of my concern about this issue. The number one threat to native ecosystems and native species is habitat destruction, with agriculture being the main culprit. So much of the degradation of native ecosystems that is attributed to "invasive" plants is actually the product of a history of habitat destruction, be it deforestation, resource extraction, or agriculture. The introduced plants that thrive in these disturbed environments are, by and large, symptoms of the disturbances. "Passengers not drivers" as it was put in one well-known invasion biology paper.
More on this in an upcoming essay, "You don't hate 'invasive' plants--you hate human disturbance."
Thanks for sharing. There are parts of Africa where certain plant species (for example Lantana sp) definitely are invasive, and are rapidly destroying the natural habitat.
Excellent post my friend, I am going to be discussing "plant blindness" in a presentation I am giving at the R-Future conference on designing Biocultural Refugium and I will be sharing screenshots from this post in it.
Biologist here. Stupid, eyeball-roll-inducing premise and post.
For starters, if you're focusing on a scientific word usage rather than the situation it's describing, you're basically engaged in an unscientific intellectual jack-off.
Next, the butterflies in your example aren't considered invasive, they aren't harming any native butterflies, or any other butterflies for that matter, the way, for example, non-native starlings are harming native bluebirds in the US.
You abandon fundamental ecological principles for the sake of your clunky, inappropriate woke analogy to illegal immigrants which you don't even have the courage to type out. So, so cringe.
Please confine the woke bullshit to the Humanities. It's harmful to objectivity which is essential for good Science.
I have to respectfully disagree with this much of this article. However you distinguish plant and animal species that arrive suddenly, most often by human action, in areas distant from their place of origin, they often have a devastating effect on the existing local ecosystem into which they are released. Ecosystems develop and change over time, with new species arriving or evolving, but the pace of this transformation is typically slow in Nature, so that species have a chance to co-evolve. In this process, they have an opportunity to genetically adjust, both to compete with each other or/and collaborate.
If, however, you take an Asian species of chestnut, load it into a ship and transport it, complete with its fungal hitchhiker to a zoo in New York City, you take a chance of releasing a pathogen that is completely novel to the North American chestnut tree and in a few decades extirpate a tree that is foundational to the local ecosystem, killing not only billions of American chestnut trees (formerly a fouyndational species of the local woodlands) but also depriving local wildlife of the nuts on which much of it depended seasonally for food, and causing the disappearance or even extinction of many species of insects that fed on American chestnut foliage. This has huge impacts up the food chain and destabilizes the whole ecosystem.
Non-indigenous plant species we casually release into our gardens and the surrounding landscapes typically provide much less in support to the local flora and fauna. This is precisely why so many of the new arrivals spread explosively. The indigenous insects commonly cannot feed on them, and the indigenous pathogens are not adapted to preying on them. The new arrivals are also commonly less vulnerable to many of these plants have allelopathic qualities that essentially poison indigenous indigenously evolved flora, further degrading thee local ecosystem.
Yes, there is considerable debate among ecologists about the impact of releasing non-native species of plants and animals into local ecosystems. That is the nature of science - argument and debate is how it refines its understanding of the data it collects. However, the overwhelming consensus among ecologists is that the casual introduction of non-indigenous species is a very hazardous practice.
With. regard to Dr. Shapiro's findings about the adaptation of Californian insects to introduced plants, I would like to point out that this is a process every ecologist to whom I have ever spoken has said would surely occur. Such adaptations are an intrinsic part of evolution. The pace of the introductions, however, is again an issue. Dr. Shapiro sent me a personal communication rebutting another ecologist who spoke on my radio program and pointed out that Asian "butterfly bush" (Buddleia davidii) while a useful nectar source for Californian lepidopterans did not in general provide a host for their caterpillars. Dr. Shapiro informed me that he had identified two subspecies of a California butterfly that did use Buddleia davidii as a larval host. Given that butterfly bush was released into North America over a century ago and the numbers of it that have spread through wildlands in California since then (crowding out native species that native insects had previously depended on), Dr. Shapiro's observation underlines how slow an ecosystem is to recover from such human interferences.
As we confront a mass extinction caused by our self-serving degradation of the global environment, I do not believe that we can afford to continue the sorts of radically destabilizing, ignorant behavior which has brought us to the brink of this disaster. We can argue about semantics and whether we are verbally discriminating against the species gardeners and others are casually transporting around the globe. But if you look at the data, the damage this has caused and still is causing is clear.
[part 1 of 2]
Thank you for respectfully disagreeing. Others have left out the respectful part, lol. I appreciate that so far Substack has for the most part been fostering that tone.
I started my deep dive into the topic of "invasive plants" in 2018 (alongside Nikki Hill, my co-author on a soon-to-be-finished book on the subject) because I wanted to know the science behind statements like "they often have a devastating impact." Given how common such statements are, we were surprised to find that the science is far more nuanced than that. It's an easy thing to say, but as it turns out, it's a trickier thing to prove. So that's complaint #1 about the popular rhetoric around "invasive plants" -- that its brashness is unscientific (and hence this post).
On the topic of evolution, I have been intrigued to learn that gradualism is being questioned. For example:
“Until a decade or so ago, evolutionary change was broadly assumed to happen on a vastly longer time scale than ecological change. As a corollary, our view on biodiversity and ecosystem functioning has often been static, trying to conserve biodiversity as it is, and preferably, as it once was. Just like our ecosystems, however, this paradigm is shifting. The closer we look at adaptive evolution, often with the aid of new biological insights and technological advances, the faster it seems to happen. Evolution and ecology are proving to be so heavily entwined that the distinction is becoming increasingly hard to make. This knowledge profoundly affects our thinking on how evolution affects patterns of biodiversity, especially in the face of global change. Adaptive responses to climate change, for example, have been shown to occur within a single generation. Contemporary evolution is probably more important than we assumed to date and is, therefore, likely to mediate the response of populations, species, communities and ecosystems to both gradual and sudden environmental change.”
[Mergeay, J. and Santamaria, L. (2012), Evolution and Biodiversity: the evolutionary basis of biodiversity and its potential for adaptation to global change. Evolutionary Applications, 5: 103-106.]
The case of chestnut blight is certainly tragic but of course the "invasive" species in that case is the blight, not Castanea crenata (the introduced Asian Chestnut). It's worth noting that the tree was imported for commercial purposes, so I point again at the ecological destruction from agriculture. As a side note, I've spent some time on a farm in Sonoma County where blight-free chestnuts still grow, as part of a long-term breeding project to produce blight-resistant trees.
The idea that introduced plant species thrive because they lack threats (pathogens, predators) in their new environment is the "Enemy Release Hypothesis." For a discussion of this hypothesis, I would refer you to Mark Davis' book, "Invasion Biology" (Oxford University Press, 2009), in which he attempts to summarize work in the field, including ideas that are contradictory. His section on ERH is too long to include here, but since you're already familiar with it, I'll post part of the section that questions it:
--begin excerpt--
It is interesting that the emphasis has mostly been on the absence of natural enemies. Just as likely, it would seem, would be the possibility that resident enemies in the new environment would attack the new colonizers, which, lacking in the appropriate defenses for these enemies, would fail in their colonization effort. Gilbert and Parker (2006) emphasized that native pathogens might still play an important role in the biotic resistance of an environment. Parker and Gilbert (2007) described several reasons why enemy release may not be an important factor in accounting for differences in invasibility:
(1) many enemies may have broad host ranges and thus may easily be able to accommodate new spe- cies in their ‘diet’, particularly if the new species have native relatives;
(2) many introduced species may be accompanied by their native enemies;
(3) the introduced species may encounter cosmo- politan enemies.
More and more instances of colonization and establishment being thwarted by resident ene- mies have been reported in recent years (Agrawal and Kotanen 2003, Colautti et al. 2004). In a meta-analysis of 63 manipulative field studies, Parker et al. (2006) found that native herbivores not only do not tend to avoid introduced plant species, they tend to suppress them. Native predators may also inhibit the establishment of non-native prey. In the coastal waters of New England, de Rivera et al. (2005) found that a native predatory crab, Callinectes sapidus, is inhibiting the spread of the European green crab, Carcinus maenas, thereby providing biotic resistance to the spread and establishment of the invasive C. maenas. In a meta-analysis review, Levine et al. (2004) found little evidence to support the contention that the success of non-native plant species in establishing in an environment was primarily due to the absence of enemies.
--end excerpt--
[part 2 of 2]
In the claim that "indigenous insects commonly cannot feed on them" I would call out "commonly" for further refinement. The idea that native insects might be dependent on native flora has intuitive appeal for cultural reasons, and has been popularized by Doug Tallamy, but again, is not so cut and dry. This discussion is hampered by unclear definitions of "generalist" and "specialist" for example. While there certainly are insects who focus narrowly, like the Yucca Moth (upon whom the Joshua Tree depends for pollination), others are quite able to take advantage of introduced species, like the Black Swallowtail butterfly and Cecropia moth caterpillars I raised in my backyard as a child. They fed on the garden's Dill and an ornamental Pussywillow, respectively. This was a suburb where much of the native flora had been wiped out by the farmland that the suburb replaced. Researching this recently, I found that the Cecropia moth probably fed on Black Willow, which would've been common along the area's creeks, which were narrowed, channelized and otherwise disrupted by both these forms of development. Again, the facts on the ground are less equivocal than the "invasive" rhetoric suggests.
I agree with your statement: "That is the nature of science - argument and debate is how it refines its understanding of the data it collects." This is my broader point. The argument and debate are not reflected by the simplistic term, "invasive." Should industrial humanity be less casual in our treatment of the living things of the planet? I absolutely agree again, and hence my critique of the "invasive" narrative, with which we are trading one kind of casual disregard for another. I presume we will just have to agree to disagree about that!
Regarding your claim that "But if you look at the data, the damage this has caused and still is causing is clear" I will say that I *have* been looking into the data, and the deeper I look, the less clear it becomes. A subject that's gotten almost no attention outside the field of invasion biology is "data bias." Within the field, though, it's definitely discussed, especially recently because innovations in computing technology have allowed for meta-analysis of many studies, something that is onerous to do by hand. Here is an excerpt from the book I'm working on with Nikki Hill:
--begin excerpt--
Not until 2015, over three decades after invasion biology’s emergence, was there a central depository to compile a global list of successfully established introduced plants. This electronic depository, the Global Naturalized Alien Flora database, currently lists just under 14,000 species and subspecies of plants, which represents about 4% of the planet’s vascular plants. GloNAF, as it’s called, was started because “knowledge of the global spread and distribution of naturalized species (that is, alien species that form self-sustaining populations in new regions is still very limited”[ii] and existing data was “jaggy and incomplete.”[iii] The database is constantly updated. A caveat worth mentioning is the issue we brought up in an earlier chapter that terms like “native,” “alien,” “invasive,” etc., lack standardized definitions, and that baseline dates differ regionally. That is, “the classification of species as invasive can vary and be independent of its impact.”[iv] Nonetheless, GloNAF provides resources not previously available, and has proven useful in assessing various hypotheses in the field of invasion biology.
But surprisingly few of those ~14,000 species have received such thorough appraisals. Hulme et al. report:
"[R]obust quantitative assessments of ecological impacts have been undertaken for fewer than 200 alien plants, highlighting a considerable knowledge deficit. Are these studies representative of alien plant impacts as a whole? It does not appear so, given that only nine species account for one-third of all quantitative assessments of ecological impacts."[v]
(One of those nine species is Purple Loosestrife, the subject of a chapter in this book.)
We were surprised learn that the pool of data is this small and shallow and we expect that many readers will be as well. The authors also pointed out that several “high-profile” plants are not included in this list of 200, including Miconia (Miconia calvescens) and the undeservedly notorious[vi] Kudzu (Pueraria montana) and that others have been the subject of only one study, like Strawberry Guava (Psidium cattleianum) and Brazilian Pepper (Schinus terebinthefolius). The authors warn that “results from single studies at single locations or years might not be widely generalizable.”
--end excerpt--
There is also the issue of failed invasions not getting attention in the literature. Another book excerpt:
--begin excerpt--
The title of a 2013 paper is telling all by itself: “The elephant in the room: the role of failed invasions in understanding invasion biology.”[vii] The authors write:
"Most species introductions are not expected to result in invasion, and species that are invasive in one area are frequently not invasive in others. However, cases of introduced organisms that failed to invade are reported in many instances as anecdotes or are simply ignored... Furthermore, we identified key research topics where ignoring failed invasions could produce misleading results."
On the same topic, Pyšek et al. remark: “Studying failure is just as important as studying success, and invasion ecology generally lacks good information on why some species fail.”[viii]
A 2017 paper of by Warren et al. entitled “A systematic review of context bias in invasion biology”[ix] refers to what they call “file drawer” bias:
"We did not account for file drawer bias (unpublished null or non-significant findings), which certainly would influence findings if there is a bias against invasive species (studies showing no invasive species impact may be less publishable or perceived to be less publishable)."
--end excerpt--
If there is an "overwhelming consensus among ecologists," then this data bias should give some of them pause. Nikki and I were certainly taken aback when I started running across all the papers covering this type of bias. We had both assumed the data was more robust than it actually is, and it seems we're not the only ones.
Citations from the data bias section:
[ii]Kleunen, Mark van, et al. “Global exchange and accumulation of non-native plants.” Nature 525 (2015): 100-103.
[iii]“A brief history of GloNAF” https://glonaf.org/index.php/a-brief-history-of-glonaf/ [retrieved 27March 2024]
[iv]Lefebvre, Suzanne, Josiane Segar and Ingmar R. Staude. “Non‐natives are linked to higher plant diversity across spatial scales.” Journal of Biogeography (2024).
[v]Hulme, Philip E., Petr Pyšek, Vojtěch Jarošı́k, Jan Pergl, Urs Schaffner and Montserrat Vilà. “Bias and error in understanding plant invasion impacts.” Trends in ecology & evolution 28 4 (2013): 212-8 .
[vi]“The True Story of Kudzu, the Vine That Never Truly Ate the South” in Smithsonian, September 2015. https://www.smithsonianmag.com/science-nature/true-story-kudzu-vine-ate-south-180956325/
[vii]Zenni, Rafael Dudeque and Martin A. Nuñez. “The elephant in the room: the role of failed invasions in understanding invasion biology.” Oikos 122 (2013): 801-815.
[viii]Pyšek, Petr & Richardson, David & Pergl, Jan & Jarošik, V. & Sixtová, Zuzana & Weber, Ewald. (2008). Geographical and taxonomic biases in invasion ecology. Trends in ecology & evolution. 23. 237-44. 10.1016/j.tree.2008.02.002.
[ix]Warren RJ II, King JR, Tarsa C, Haas B, Henderson J (2017) A systematic review of context bias in invasion biology. PLoS ONE 12(8): e0182502.
I'll just add this because it's from something I happen to be reading today, by biologist Brendan Larson, of the University of Waterloo:
--
Invasion
biology relies upon a narrative of native versus non-native that is seldom
questioned by invasion biologists. In a recent critique of my research on
potential implications of a metaphorical war against IS, for example, a
well-known conservation biologist wrote: “The bottom line for me is that,
given the abundant, massive, and seemingly insurmountable global conser-
vation problems that we face, the semantics of dealing with invasive spe-
cies is a low priority.” This comment belies a scientistic view that over-
looks the extent to which this issue is inextricable from pre-existent
cultural lenses. These lenses force us to think primordially in terms of “us”
and “them”, which is reflected in the use of linguistic categories such as
“native” and “invasive”, respectively.
--
Available free here:
"Entangled biological, cultural, and linguistic origins of the war on invasive species"
https://www.academia.edu/4010885/Entangled_biological_cultural_and_linguistic_origins_of_the_war_on_invasive_species?email_work_card=title
I so enjoyed this thoughtful, well-researched dive into the word “invasive,” which strikes me as a fancy term for the equally misguided “weed.” Years ago, I read an essay by Michael Pollan, “Weeds Are Us,” that completely flipped how I think about all plants — and our relationship with them. When we step out of the center and see the bigger picture, all sorts of insights can come. Thanks for this.
You're very welcome, Julie! I totally agree it's really just a fancy term for "weed." (My co-author and I discuss that very topic in our upcoming book.) I definitely appreciate Michael Pollan, too. I like that he does so much research, then throws in his own personal experience and spin. He's a great communicator for sure.
Thank you so much for another wonderful and needed article. I am always trying to reach people who hate and kill plants, even while they plant certain popular (and often non-native) flowers in their yards. I so worry for the animals who need all the plants and will never forget the Black-throated Blue Warbler who flew thousands of miles to where he was happily eating non-native Fennel seeds while a group of us was frantically trying to get photos. Usually the Warblers I know are always moving, but he was just a couple of feet away, staying safely on the ground as we guarded him, while he (and other birds, including another rare one) were also eating the Fennel seeds.
Yes my concern is also people who hate and kill plants. Not just because it's bad for plants, but because it's bad for us as humans to behave that way.
So lovely to hear about the birds who love the Fennel seeds! Several species of Swallowtails who love everything in Fennel's family (Apiaceae, the Carrot Family) have benefited from Fennel, Dill and other plants in that family we introduced here. I learned this as a child, raising their caterpillars, and have since seen research that corroborates it.
It's so easy to love nature instead of hating it, and I keep trying to reach people, but nativism is another cult. Thank you again!
I agree! It's much easier! I'm just a lifelong plant lover, fascinated by all of them and what they do.
It's been discouraging to see the nativist cult arise. It's really just another form of puritanism. I'm personally thrilled to visit a new place and meet the plants there, but now it's like there's some kind of litmus test.
It's worth mentioning that the culty-ness is not universal. It's most popular in the UK, the US, Australia & NZ, Japan, South Africa and Canada. Except for Japan, I am sure you can observe what those countries have in common, lol.
Yes, exactly! Very puritanical and xtian fanaticism. I so wish we could stop it. I fear the poison spraying is increasing. I joke I could be a millionaire if anyone would bet me ten thousand and I identify a sprayed area, along roads, parks, etc. It's so visible to me, but not others, and I haven't seen them spraying openly in years. But then this week I saw hired gardeners in uniforms spraying poison by the Alameda to SF Ferry parking lot. It's so dead-looking and manicured, and they don't even know how to care for the ornamentals (all non-native) because they shear/prune species that aren't to be cut at all. I think they banned most poisoning in Marin, and so wish they did everywhere. Sorry to go on!
I love all the little animals with the plants too. Beautiful spiders and insects, etc....
Thank you so much for pointing out the blindness of how humans approach environment as a whole. The invaders dictate what is invasive through a lens only they view. It’s really rich and continues to evoke a conquering mindset regardless of any underlying subtleties. I can’t stand the word myself as it is extremely narrow and provokes reactions that don’t serve us in a realistic sense. I’m also curious about the whole Darwin ‘survival of the fittest’ concept and the evolutionary ‘skill set’ required to regenerate in a place you may not have previously been. Isn’t what we see in Nature, what we say in Nature merely the playing out of an evolutionary process that serves the planet? As illustrated in the butterfly study you mention there is significance to life in all forms as it provides something for something else/ it may not be obvious to our mindset. Nature doesn’t serve humanity and our policies should not keep trying to put a lid on a box we opened long ago. It’s far less significant what we think really as at some point our lease will be up, but Nature through its many incarnations and the genius of itself, will continue on as if we never were here. Yes, invasive is a dangerous and deliberate term meant to eradicate, and even when used within scientific communities it comes with underlying biases based on funding, etc. ❤️
"provokes reactions that don’t serve us in a realistic sense" -- yes, exactly.
As for Darwin, he was brilliant, but also a product of his place and time, which was the rapidly industrializing England of the 19th Century, which was all about ruthless competition, and a ruling class wanting to justify their position. In a very real sense, what we call "social Darwinism" preceded actual Darwinism as a philosophy. We have a chapter in the book that dives into that subject. If you DM me your email address, I'll send it to you.
I am guilty of using the word. I'm really glad you wrote this! Sandy
Thanks, Sandy! Of course it's easy to use the word because it is so common.
My wife and I rented a house with a garden for a while, and we decided to plant clover - instead of the usual grass lawn which all the other neighbours had, even though it's considered an invasive species.
Not only was it almost no maintenance, but it was actually an incredible help to the other plants we had there - as it's fixating nitrogen in the ground. Extra: it had flowers and the butterflies loved it, too!
Good for you! Clover is wonderful! Yes very popular with the insects, and improves the soil. Of course nobody every calls lawn grass "invasive" even though, if it were classified as an irrigated crop, would be the most irrigated crop in the US. Worrying about what's native or non-native in an urban area is kind of pointless to me. Any plant life helps make a city better!
I've used the word too, far to often, I am now ashamed to say.
Don't sweat it. It's so ubiquitous and, as I mentioned, jibes well with Western cultural ideas. I've read a bit about attitudes toward these species in Australia, and it seems like it's a hotbed for the topic (largely because of the Cane Toad?). You might appreciate this paper, which explores indigenous perspectives there:
Bach, Thomas Michael and Brendon M. H. Larson. “Speaking about Weeds: Indigenous Elders’ Metaphors for Invasive Species and Their Management.” Environmental Values 26 (2017): 561 - 581.
You can download the whole paper for free here:
https://www.academia.edu/28925123/Speaking_about_weeds_Indigenous_elders_metaphors_for_invasive_species_and_their_management
I am enlightened, as always. I bow to you for singing this tune. Keep singing!
Aw thanks, Rebecca! I will keep singing!
So well said! I do not use the term invasive and it drives me buts when people do. This is invasive and that is invasive and it's been here where I live longer than I have and I am 68 yrs old. Not every non-local plant or animal is actually invasive.
Thanks, Pamela! Sometimes people just use it in a colloquial sense. Like, "plant that thrives without care and spoils my landscape design or needs to be weeded out of the garden." Which, whatever. Unfortunately, as a policy word it's used to push eradication efforts, which more often than not use herbicides.
A lot of people don’t know that weeds are actually local wildflowers and plants. Thankfully in some areas policies like that are not as popular as they are in the past. Conservation is popular when I live.
Definitely true. People get the concepts of "weed" and "invasive" and "noxious" all mixed up, because they're all categories of "bad" plants. But yeah, as an experienced gardener and farmer, I've totally noticed that many weeds are local native species. Of course many are also medicinal or edible too. Honestly, I love lots of "weeds" and only remove them when necessary, not just on principle as is common.
I keep all native species in my flower gardens now. I have knowledge in medicinals and edibles.
Language matters, thank you for writing about how harmful it is for us to talk about plants this way. The way we talk about things determines how we see them. When we call something "invasive," that changes how we think about it.
This seems like an odd, pathologically altruistic, and generally backwards take typical of late-civilization liberalism.
Here’s the practical matter:
You could simply rename invasives “out-competitive, ugly, unbeneficial plants” - but that’s a lot of words for the same idea. Invasives gets it done.
You must not farm. Or must not farm much, or must not have ever had to recover land previously lost to invasive species. It must be luxurious!
In the eastern US, thousands of acres are lost to nearly ineradicable invasives every year. The problem is that the process of recovering this land is often impractical, too costly, or simply impossible. How to get the reeking, sneeze-inducing invasives out of the National Park with one’s favorite hiking trails? “Just enjoy the newly ugly and smelly trail! Mother Nature doesn’t make a mistake!” But mother Entropy is an entirely different animal.
So beautiful hiking trails get overgrown with plants that block all views and sunlight, deer ticks with lymes’ thrive, healthy trees fail to germinate much less ever grow to maturity—beauty leaves the world, never to return.
When we think about trees alone, and the benefits including carbon capture that they offer, invasives-advocates such as yourself often fail to consider it’s one or the other. The loss of so much of America’s tree canopy—-from Chestnut Blight, Dutch elm disease or the emerald ash borer - is due to non-plant invasive species from foreign lands with no competive factors or reproductive limits.
And let’s say you want to beautify an area — are you yourself going to remove the Japanese knotweed ? Do you find Japanese knotweed beautiful and prefer it to pine & hardwood forests that were once native to much of the American landscape?
I'm an experienced farmer, actually. Whether or not I personally find it to be a hassle to deal with certain weeds is a completely different subject than whether certain species are ecologically harmful.
My love for natural ecosystems is at the heart of my concern about this issue. The number one threat to native ecosystems and native species is habitat destruction, with agriculture being the main culprit. So much of the degradation of native ecosystems that is attributed to "invasive" plants is actually the product of a history of habitat destruction, be it deforestation, resource extraction, or agriculture. The introduced plants that thrive in these disturbed environments are, by and large, symptoms of the disturbances. "Passengers not drivers" as it was put in one well-known invasion biology paper.
More on this in an upcoming essay, "You don't hate 'invasive' plants--you hate human disturbance."
Thanks for sharing. There are parts of Africa where certain plant species (for example Lantana sp) definitely are invasive, and are rapidly destroying the natural habitat.
You're my Sunday reading today, and I cant describe how much I'm enjoying your work on "invasive".
Yes! Very good. I have followed Dawkins ever since college, and you can look at it as invasive or look at it as natural selection.
Excellent post my friend, I am going to be discussing "plant blindness" in a presentation I am giving at the R-Future conference on designing Biocultural Refugium and I will be sharing screenshots from this post in it.
Keep up the great work!
Biologist here. Stupid, eyeball-roll-inducing premise and post.
For starters, if you're focusing on a scientific word usage rather than the situation it's describing, you're basically engaged in an unscientific intellectual jack-off.
Next, the butterflies in your example aren't considered invasive, they aren't harming any native butterflies, or any other butterflies for that matter, the way, for example, non-native starlings are harming native bluebirds in the US.
You abandon fundamental ecological principles for the sake of your clunky, inappropriate woke analogy to illegal immigrants which you don't even have the courage to type out. So, so cringe.
Please confine the woke bullshit to the Humanities. It's harmful to objectivity which is essential for good Science.