The elusive meaning of the term “invasive species"
It's not as simple as it sounds
The following is a draft chapter from the book that Nikki Hill & I are writing, tentatively entitled, “Don’t Blame the Messengers: A critique of the ‘invasive plant’ narrative.” So far, I’ve only been posting these for paid subscribers, but this one’s for everyone, first because the concepts are important, and second because it makes a good teaser as it refers to so many other chapters in the book. Paid subscribers will be getting more chapters, plus a PDF of the completed draft. (At which point we’ll be seeking a publisher.)
Alien species seem practically designed to excite public concern. Almost by definition they are most abundant, and most visible, in the most highly human-modified habitats, such as towns and cities. Personal encounters with aliens are routine, so everyone has an opinion, and it’s often ‘obvious’ that aliens are actively supplanting natives, even if that isn’t what’s happening at all. It’s equally ‘obvious’ that something must be done, even if it’s not clear what that should be, and even if ill-judged intervention might only make things worse.i ~Ken Thompson
What is an “invasive plant?” Colloquially, some gardeners call any plant that thrives and spreads with little or no care “invasive.” They will cast the term on both “weeds” (which are often native plants) and ornamental nursery plants that spread outside their apportioned area. Such gardeners are seeking to uphold a particular aesthetic and their impulse to direct and control yields results ranging anywhere from elegant to cloying. If this were the only way that people used the word, “invasive,” it would be harmless, and there’d be no need for this book.
Going to the dictionary for the meaning of “invade,” Merriam-Webster gives us: “to enter for conquest or plunder, to encroach upon, infringe,” and for “invasive”: “tending to spread especially in a quick or aggressive manner” and “relating to, or characterized by military aggression.” The Cambridge Dictionary defines “invade” as: “to enter a country by force with large numbers of soldiers in order to take possession of it; to enter a place in large numbers, usually when unwanted and in order to take possession or do damage; to enter an area of activity in a forceful and noticeable way." For “invasive” it says: "moving into all areas of something and difficult to stop."
That’s clear enough. But what is the definition of an an “invasive species,” whether plant or other?
The short answer is that there is none.
The longer answer is that there are many definitions crafted by various policy-makers, non-governmental organizations and advocacy groups. Here are a few:
US federal government: “an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health.”ii
The United Nations Environmental Program: “introduced species that become established in a new environment, then proliferate and spread in ways that are destructive to human interests and natural systems.”iii
The International Union for Conservation of Nature (IUCN): “animals, plants or other organisms that are introduced by humans, either intentionally or accidentally, into places outside of their natural range, negatively impacting native biodiversity, ecosystem services or human economy and well-being.”iv
Convention on Biological Diversity: “species whose introduction and/or spread outside their natural past or present distribution threatens biological diversity.”v
These definitions might seems straightforward, but if we unpack them, a lot of ambiguity emerges.
The key words—“harm,” “destructive,” “negatively impacting,” “threatens”—beg the question of what constitutes “harm.” As we shall explore in this book, that’s not at all simple to answer in terms of ecological interactions. It’s even more difficult to identify what is “likely to cause” harm, as the feds put it. As invasion biologists have discovered, predicting the results of species introductions based on the attributes of the species doesn’t have a successful track record. Also, if “harm” is being alleged then the possibility of benefit must also be admitted, a point completely ignored not just in these definitions but in most discussions or media coverage about “invasives.”
Economic concerns or “human interests” are named first by the US and UN. which might surprise people who think of the issue of “invasive” species as solely an environmental one. But commercial interests have had a strong hand in drafting the official policies that guide the “management” of “invasive” species. As we discuss in a later chapter, Monsanto and other chemical companies played a part in the shaping US federal policy around “invasive species” because they profit from the sale of toxic pesticides used in their control. As for measuring the bottom line, one might assume that economic harm is readily quantifiable, but it’s also tricky as we explain in the chapter, “Lies, Damned Lies, and Statistics.”
“Alien” and “introduced” are synonyms for “non-native” (as are “exotic” and “non-indigenous”). The very concept of a “native plant” is a recent invention, though, and is another subject of deliberation. We delve into the origins of the concept in the chapter, “A Brief History of Invasion Biology.”
“Introduced” is shorthand for “introduced by humans either intentionally or accidentally.” It doesn’t include the roles of animals, weather, or other non-human agents in transporting species. It also doesn’t usually include indigenous humans, not because their own actions in moving around species haven’t been significant or are considered qualitatively different than those of settler-colonialists, but simply because they are generally not considered at all. This narrative omission is significant because it narrows the realm of possibilities for both human activity and human nature. This is another point we will be returning to in depth.
A cursory sweep of these definitions demonstrates that we only need to scratch the surface to reveal a lot of grey area in what is presented as a black and white issue.
In some cases, the non-native requirement has been dropped entirely. The State of New York includes the native Cup Plant (Silphium perfoliatum), on their “Prohibited and Regulated Invasive Plants” list because they characterize its growth as “aggressive.”vi
Similar accusations of “encroachment” by native flora are currently playing out with horrific results in the western US, where Pinyon-Juniper forests are being destroyed to expand rangeland for cattle.vii Juniper is now being called a “native invasive” by some.viii See our chapter, “’Native invasives’: The Pinyon-Juniper tragedy.”
Further muddying the picture, the term “noxious weeds” is often used interchangeably with “invasive species” by organizations and individuals alike. The USDA has a list of “Introduced, Invasive, and Noxious Plants.”ix Lists of noxious weeds are typically drawn up by local government agencies in support of conventional farming and ranching, so a particular plant—native or introduced—will be added because of its purportedly negative effect on domesticated crops or animals. Thus, St. Johnswort (Hypericum perforatum) is bad because it can cause phototoxicity in sheep; Chervil (Anthriscus sylvestris) because it might carry a rust fungus that can affect cultivated carrot crops; and Pigweed (Amaranthus palmeri) because it grows in cornfields. Note that in all three cases the plant or animal species supposedly threatened are themselves not native, and are products of a system—industrial agriculture—that is severely detrimental to the environment wherever it is practiced. All three of these “weeds” also happen to be edible or medicinal for humans, so it’s a matter of priorities not usefulness. The primary motivation of noxious weed management is economic, not ecological—despite recent glosses of “sustainability” applied by some of its adherents—and more often than not involves pesticides.
What about science? How does invasion biology itself define “invasive?”
Scientists are well-known for disagreeing about definitions, and “invasive” is no exception. One biologist summed up the discussion by stating that there is “no indication that the field [will] be able to achieve uniformity in language in the near future.”x
But these are some of the concepts that have been used or proposed:
a species that successfully establishes itself in a new place
such a species that establishes and has impact
such a species whose impact is negative
a species that, regardless of impact, establishes and spreads rapidly
a species that does any of the above which is non-native and introduced by humans
a species that does any of the above which is non-native, regardless of how it was introduced
a species that does any of the above which is native
These can’t all be true at once, obviously. There are too many contradictions.
Whether or not to use the word, “invasive” at all is an ongoing debate in invasion biology. Historically, scientists have used the word “invade” and “colonize” as a simple descriptor that merely described the movement of a species into a new area, or just the establishment of a species in a recently opened area, like after a fire, flood or landslide. But many scientists now recognize that the word has undeniably negative connotations that can’t be avoided, which has implications not just within the field, but for social discourse. Writes biologist Mark Davis, in his book, Invasion Biology:
I have never liked the term ‘invasion’ and think the field would have been much better off had it never been adopted, along with its accompanying military metaphors. Although the usage of military language may help to attract a group of highly motivated supporters, this same language may help foment a strongly confrontational approach, making it much more difficult to negotiate and resolve conflicts.xi
This “confrontational approach” has certainly been a feature of too many discussions about introduced plant species. We have both been saddened to watch as online native plant forums, once a place to goodnaturedly share photos and get IDs, have become toxic places, where the word is wielded like a club. Such hateful rhetoric is the hallmark of dogma, not reason or thoughtfulness. We wonder how different the attitudes would be if “invasive” had never been used and we’d only ever had something neutral like “introduced.”
What’s not defined as “invasive” is at least as important as what is.
Excluded from virtually everyone’s definition are the hundreds of non-native domesticated plants important to agriculture. This is highly significant given that over one fifth of the land in the lower 48 states of the US is cropland. That's nearly 400 million acres of what was originally habitat for many, many native plant species. The excuse of “We need to eat!” doesn't fly here; only 20% of that cropland is devoted to growing food directly for people; the remainder is for things like ethanol production, livestock feed, and export industries.xii
These stats should put the “invasive” dialogue into perspective. Picture the Midwest’s horizon-to-horizon corn and soy crops, California’s vast rice fields and almond orchards; or Oregon’s expanses of grass seed. These monocrops of non-native plants displaced prairies, forests and wetlands brimming with native flora and fauna. They are maintained with toxic pesticides and fertilizers, unsustainable water use, heavy machinery, fossil fuels, and oppressive labor practices. They dominate more than just their local geographical footprint, sucking in resources from distant places. According to the “invasive plant” narrative, the only “invasives” in these abused landscapes are the weeds coming up in the ditches or between the rows, a perspective we find perverse. If the concern is the health of native plants, what is the real problem here? The Pigweed between the rows of corn? Or the corn? The Bindweed coming up in the orchard? Or the orchard? The Blackberry in the grass seed field? Or the grass seed field? The Pigweed, the Bindweed and the Blackberry wouldn’t even be there if not for the disturbances made by these agricultural projects.
Other exemptions apply to the “invasive” label. As the US Department of the Interior's Invasive Species Advisory Committee points out: “Kentucky bluegrass would be considered an invasive species in Rocky Mountain National Park in Colorado, but considered non-invasive a mere 60 miles away at a golf course in Denver.”xiii If anything is “invasive,” how is it not the golf course? In the Coachella Valley of southern California, over 140 irrigated golf courses represent the complete destruction of the desert habitats that they replaced, and are a significant draw on precious water.
The topic of golf courses leads us to another big omission: lawns, which in their emerald-green ideal are monocultures of non-native grass. If lawn grass were categorized as an irrigated crop in the US, it would be ranked #1 in land area and water use.xiv Yet it is the Dandelion that sprouts up in the front yard that’s “invasive.” But the “invasive plant” narrative is not ultimately about logic or facts. It’s about cultural beliefs and prejudices, which we will explore in several chapters.
In sum, the term “invasive plant” is thrown around so carelessly these days that it doesn’t really mean anything beyond some vague notion of “bad plant.”
But there are no “bad plants.” There are just particular plants that particular people in particular places at particular times have considered undesirable for particular reasons. That’s a lot of particulars. Too many to brand a whole species of plant with a pejorative label as if that’s its entire, intrinsic nature. Does it make sense to weed a garden? Of course. But that’s no reason to elevate our subjective calls—useful as they might be in a particular place and particular time—to the level of a universal constant, and manufacture an army of villains in doing so. That just encourages our own misguided tendencies and puts off the day when we resume healthy connection and relating with life on this planet.
A very real invasion commenced in 1492, and with it came what some Anishnaabe tradition-bearers call “invasive land-ethics.”xv These ethics drove the cutting of forests, draining of wetlands, mining of mountains, plowing of prairies, damming of rivers, and slaughter to near extinction of Buffalo, Prairie Dogs, Beavers, Bears, Cougars and Wolves. These misbegotten ethics are the true threat to the planet’s environment, and without addressing them, the destruction will only worsen.
Great stuff, keep it coming. The idiotic "war on invasive species" is so out of control and based on non-science, it's depressing. Even many well-meaning, self-described "environmentalists" have gulped down the Invasives Kool-Aid. As one fellow activist pal put it so succinctly, "We just want them [the "nativists"] to stop killing things." Indeed. In our war on nature, we will inevitably find ourselves on the losing side. Hence my creating more resources on this critical topic: https://treespiritproject.com/invasionbiology/
This was a wonderful piece, thanks. I really enjoyed the way that you brought up our raping of the land for monocultures as an example of ‘invasive species’. Don’t get me started on golf courses - I hadn’t thought about them as being a giant example of ‘invasive species’. I think back to my master gardener courses and the feeble attempts to define plants as invasive while most of these also have medicinal value, etc.
I look forward to release of your book. You have a confirmed sale here ☺️