Invasion biology is the scientific discipline that spawned the native plant movement. Charles Elton published a book in 1958 that is considered the origin of the modern version of invasion biology, although there are precursors centuries earlier. These are the basic tenets of modern invasion biology:
- Plants and animals that are “native” to a specific location are considered members of an ideal ecosystem that have co-evolved over thousands of years so that members of the community are dependent upon one another.
- Plants and animals introduced to an ecosystem by humans are assumed to disrupt the equilibrium balance of the community and threaten its existence because introduced plants and animals do not have predators that would control their spread. All introduced plants and animals are therefore considered potentially invasive.
- Animals are believed to be dependent upon the plants with which they evolved—and only these plants–and these mutually exclusive relationships are disturbed by the introduction of new plants and animals.
- Adaptation and evolution of introduced plants and animals is believed to be too slow for introduced plants and animals to successfully enter the food web.
- Native members of the ecosystem are presumed to be inherently superior to introduced plants and animals. Invasion biology does not acknowledge that introduced plants and animals are often functional members of the ecological community.
- Native ecosystems are said to be in “balance” and introduced species are presumed to cause “imbalance.” Introduced species must be eradicated to restore balance to the ecosystem, presumed to be the ideal for a particular location.
Hundreds of empirical studies have been conducted since the 1960s to test these assumptions. Little scientific evidence has been found to support them. Current knowledge of ecology explains why the assumptions of invasion biology are mistaken.
What is native?
The native plant movement defines native as the plant species that lived in a specific location prior to the arrival of Europeans. In the San Francisco Bay Area, “native” is defined by native plant advocates as the plants and animals that lived here prior to 1769 when Europeans first laid eyes on San Francisco Bay. When Europeans arrived, the San Francisco Bay Area was already occupied by indigenous people who had arrived approximately 10,000 years earlier.
The arbitrary selection of the pre-European settlement period to define the ideal landscape was based on the mistaken assumption that the indigenous human population had not radically altered the land. Anthropological and paleontological research informs us that the landscape was essentially gardened by the indigenous population to provide food and cultural implements.
The landscape found by Europeans at the end of the 18th century was not “natural.” It was altered by humans to serve humans who lived as hunters and gatherers. Since modern society no longer hunts and gathers for its food and shelter, the landscape that served that lifestyle cannot be maintained without mimicking the land management practices of native people such as frequent burning of the landscape and grazing by animals. Indigenous people in California did not have domesticated animals (except dogs), but the grassland was grazed by wild deer, elk, and antelope.
Plants and animals have migrated around the world without the assistance of humans since life began. The seeds of plants are carried in the stomachs of migrating birds and on the winds of storms. Animals, including humans, move to wherever they can find what they need to survive. Migration is natural and often necessary for survival. Making a distinction between species moved by humans and those moved by natural forces is pointless and usually impossible to distinguish.
Climate change renders the concept of “native plants” meaningless because when the climate changes, the vegetation changes. The plants that live in tropical climates will not survive in arctic cold and vice versa. Introduced plants are often better adapted to current climate conditions than their native predecessors because the climate has changed and it will continue to change.
Mistaken assumptions about evolution
Animals rarely depend upon a single plant species for survival. Such mutually exclusive relationships rarely exist in nature because they are evolutionary dead-ends. Animals can, and often do, adapt quickly to changes in the environment. Transitions from native to introduced plants are routinely made by animals, including humans. Indigenous hunter/gatherers quickly incorporated plants introduced by European settlers into their diets. Plants in the same family and genus are often chemically similar, making the transition more likely.
Native plant advocates assume that evolution only occurs slowly, over thousands of years, but evolution can be faster than they assume. Rapid environmental change accelerates the speed of evolution because extreme weather events caused by climate change increase the speed of natural selection, the primary tool of evolution. When cataclysmic events such as hurricanes, droughts, floods, extreme temperatures kill many members of a species population, these are selection events in which the fittest members survive to breed and the next generation inherits the genetic traits that helped their parents survive. The classic example of this principle is the finches in the Galapagos Islands who died if they didn’t have big enough beaks to eat the seeds of the only plant that survived extreme drought. The next generation of finches had bigger beaks.
Evolution occurs when genetic changes enable future generations to inherit the genetic change. Adaptation occurs when animals respond to environmental challenges by changing behaviors that aren’t necessarily inherited by the next generation. Adaptation to changed environmental conditions is even more rapid than evolution and equally effective to ensure survival. Genetic changes are not required for an insect to make the transition from a native host plant to a chemically similar introduced plant. Extreme temperatures require that plants and animals move to more temperate climates. “Native” ranges must change to survive changes in the environment. A plant or animal that cannot survive extreme heat will migrate (if it can) into regions where temperatures are not as warm. They should not be prevented from doing so.
Plant and animal species with large populations and short lives, such as insects, evolve more quickly. This more rapid pace of evolution enables a more rapid transition from native host plants to closely related introduced plants.
Nativism and the native plant movement
The native plant movement is based on the belief that native plants are superior to introduced plants, that native plants are somehow “better” than immigrant plants. That assumption of superiority is the definition of nativism. It is as specious an assumption in the natural world as it is in human society and it is equally dangerous.
There are pros and cons to everything living in the natural world and there is no right answer to the question of which species is “best.” When evaluating introduced plants, nativists consider only the negative aspects. They refuse to acknowledge that there are also advantages and a death verdict should take both into consideration. For example, native plant advocates want all eucalyptus trees in California cut down because they were planted here after European settlement. This negative judgment of eucalyptus does not take into consideration that 75% of monarch butterflies who spend the winter in California use eucalyptus trees for their safe haven. Also, eucalyptus blooms in California from November to May, providing nectar to butterflies, hummingbirds, and bees at a time of year when native plants are not blooming. Eucalyptus trees are also nesting homes of owls and other raptors. Cutting down eucalyptus trees simply because they are not native in California ignores the many benefits they provide to wildlife.
Confusing cause and effect
The native plant movement mistakenly assumes that the mere existence of introduced plants threatens the existence of native plants. They believe that native plants will magically emerge if introduced plants are eradicated. They have spent 25 years eradicating non-native plants and do not seem to have noticed that native plants have not returned. They make this mistake because they do not acknowledge the changes in the environment that make non-native species better adapted to current environmental conditions.
Many of the changes in the environment that are inhospitable to native species are caused by structural changes made to accommodate human activities, not by introduced species. For example, all the major rivers in California have been dammed to prevent floods and store water for use during the dry season. These dams have fundamentally altered the ecology of our rivers. There are no longer cleansing spring floods that clear rivers of accumulated mud and vegetation. Channeled rivers are deeper and warmer. Salmon can no longer get to their spawning grounds past the dams. The altered structural conditions are more hospitable to bass than to trout. Aquatic plants from tropical regions become invasive in warmer water. None of these conditions are reversed by spraying aquatic plants with herbicide or killing introduced bass.
Wherever “invasions” are observed, no thought is given to why. Instead, a convenient plant or animal scapegoat is found and poisoned. That death sentence doesn’t reverse the underlying reason for the invasion. Therefore, the invasion persists. Society is unwilling to make the sacrifices, even inconveniences, needed to address the underlying cause of the “invasion.” We have done little to address the causes of climate change. We are unwilling to destroy the dams and the system of supplying water to serve agriculture needs. Invasions are the symptom, not the cause of the changes in nature.
Conservation Sense and Nonsense 
Thank you very much for this important post. I’d like to add to your comments about indigenous people’s practices. Indigenous people also did not assault nature with an environment of concrete, petrochemicals, nuclear radiation, and dense populations with millions of structures filled with poisonous and flammable chemicals, etc. I don’t think plant nativists are suggesting removing all of that non-native infrastructure and presence. I’m not suggesting doing that either—except for the toxic chemicals, nuclear power, etc. But, apart from all the excellent reasons you give, it especially makes no sense to attack non-native plants while leaving this gargantuan destructive non-native reality in place.
It certainly would make more sense for nativists to put energy into stopping the use of pesticides, for instance, instead of promoting their use against non-native plants. Best of all, how about organizing non-native people to respect and protect the rights of indigenous people, to improve their chances for survival and health here in their own land.
Webmaster: Thank you for this opportunity to engage directly with a proponent of invasion biology. I begin by suggesting that you read this recently published article about the unfortunate state of the on-going debate about invasion biology, which concludes, “The field [of invasion biology] is at a stage where no firm pronouncements should be made without clear and compelling evidence to support them. And, as we have seen, such evidence remains in short supply. In its place, we have found a number of rhetorical and logical moves that distort the tone of the debates involved and stand to impede the development of constructive dialogue in the field. It has been the modest goal of this paper to draw attention to this state of affairs in a way that will hopefully encourage balanced and respectful discourse as we move forward.” (R.C. Guiasu, C.W. Tindale, “Logical fallacies and invasion biology,” Biology & Philosophy, 2018, 33(5).
This article makes a lot of assumptions and at best they’re half truths. I’ll try to go by paragraph:
Yes, the pre-European model has been used for a long time to denote what is natural or not. No, it is not rigidly adhered to as it once was. Many ecologists consider how the natives changed their environment to be just as unnatural and potentially damaging as European practices. It’s called shifting baselines.
Webmaster: I have observed that shift in emphasis, but do not consider it a step forward. “Natural” is still defined by such projects as some specific point in time that projects attempt to replicate. Here in California, for example, more native coyote brush is being destroyed than non-native shrubs that encroach on grassland in the absence of fire or grazing. Likewise, native Douglas fir is being destroyed when it encroaches on grassland. Why? Because grassland is the preferred ecosystem. Why? Because grassland existed prior to the arrival of Europeans. There are similar examples elsewhere, such as the destruction of native juniper trees in Oregon because they weren’t there in the 18th century. Whether the destroyed plants and trees are native or non-native, it amounts to the same thing, i.e., the pointless destruction of existing habitats.
There is a difference between plants moved by humans vs natural forces. Humans move around a lot more frequently, and in ways that don’t put other organisms (whether intentionally brought or not) under the same kinds of stressors they would experience without humans. It also is possible to distinguish between plants moves by humans vs not. Since every plant species has a limited number of ways to disperse its seeds, some travel distances or dispersal past certain barriers can span the breadth of likelihood from “unlikely” to “impossible”. Genetic analysis also allows us to determine the genetic origin of a particular plant or population of plants, thus giving insight to its most likely geographic origin.
Webmaster: Even the best educated biologist cannot distinguish between “native” and “nonnative” plants by observing them in the wild. They are just plants living in their environment. To know that a plant is “non-native” you must know its history. Or, to paraphrase a knowledgeable ecologist: “To call a plant ‘native’ is only to say you don’t know when it arrived.” Theoretically, molecular analysis could make such a determination, but the fact is, it’s not done by “restoration” projects. Eradication projects are not based on molecular analysis and mistakes are frequently made. For example, bush lupine was planted by dune “restorations” in Northern California over 25 years ago by mistake. It became “invasive” before someone determined that it is native to Southern California, not Northern California.
Climate change does not render the concept of “native” meaningless. If there are appropriate corridors of habitat and intact dispersal syndromes then the distributions of native plants will shift accordingly.
Webmaster: Yes, plants and animals will move (if possible) to find the environment needed to survive. Unfortunately, practicing invasion biologists try to prevent such movements. For example, Monterey pine has a very narrow native range and its survival in the native range is threatened by a pathogen that is killing it. Unfortunately, Monterey pine is destroyed by native plant “restorations” outside of its native range. It is not being “allowed” to move as needed to survive.
There are often animal and plant relationships that depend exclusively on each other. The author is displaying a common observational bias due to the fact that most of the species humans commonly see are generalist species which use a variety of habitats and resources. Most species are in fact specialists and may depend exclusively on a particular plant for part or all of is life cycle. Thus degree of specialization is particularly prevalent among insects.
Webmaster: Specialization by insects is greatly exaggerated by invasion biologists. For example, Doug Tallamy claims that “More than 90 percent of all insects sampled associate with just one or two plant families.” (Karin Burghardt, Doug Tallamy, et. al., “Non-native plants reduce abundance, richness, and host specialization in lepidopteran communities,” Ecosphere,November 2010) There are over 600 plant families and thousands of plant species within those families. Most plant families include both native and non-native plant species. An insect that uses one or two plant families, is therefore capable of using both native and non-native plant species. For example, there are 20,000 plant members of the Asteraceae family, including native sagebrush (Artemisia) and non-native African daisy. In other words, an insect that confines its diet to one family of plants is not very specialized.
Rapid environmental change does not accelerate the speed of evolution. The speed of evolution for each species is determined by its generation time (how long it takes the organism from “birth” to be able to reproduce). Shorter generation times mean faster evolution.
Webmaster: You are mistaken. Although larger populations with shorter lifespans evolve more rapidly than species such as humans (as the article on which you are commenting says), rapid changes in the environment accelerate the rate of adaptation and evolution. The more challenges to survival, the greater the pressure on species to adapt. Here’s a recently published study that provides the urban environment as an example: “Recent research has demonstrated that the species inhabiting cities are capable of rapidly changing in response to anthropogenic environments…The realization that species are capable of quickly evolving in response to urban contexts has driven a flurry of academic and popular interest in urban evolution in recent years.” (M.R. Lambert, C.M. Donihue, “Urban biodiversity management using evolutionary tools,” Nature Ecology & Evolution, May 11, 2020)
Native plants are better for the environment since their presence allows the continuation of evolutionary relationships that have been developing for millions of years. Trying to imply favoring native plants is akin to racism might be the most unscientific piece of garbage I’ve ever heard.
Webmaster: As you may know, Stephen Jay Gould is one of the most important evolutionary biologists of the 20th century. This is his scientific opinion of the concept of “native plants: “a notion [that] encompasses a remarkable mixture of sound biology, invalid ideas, false extensions, ethical implications and political usages both intended and unanticipated.” (“An Evolutionary Perspective on Strengths, Fallacies, and Confusions in the Concept of Native Plants,” Arnoldia, journal of Harvard University’s arboretum). Gould does not come to this conclusion solely from his knowledge of the principles of evolution and his desire for the public to correctly understand its mechanisms. He is also concerned about the “slippery slope” of nativist ideology from application to plants to application to humans. This is not a theoretical anxiety on his part. It is based on historical precedents. In Nazi Germany and in the United States around the same time, horticultural theories abounded about the superiority of native landscapes and those theories were inextricably linked to the belief that non-native humans were also inferior.
There is a somewhat subjective answer as to which plants are “best” in an environment, and that is whichever species combination supports the most biodiversity. Due to a long past of evolutionary relationships, this is mostly or entirely native community. Cutting down all of the eucalyptus in California without replacing them with native trees would be stupid. There are native trees that owls can nest in, and certainly native trees that flower during the winter.
Webmaster: Eucalyptus was planted in California because much of California was virtually treeless prior to settlement. Native trees will not survive in most places where eucalyptus trees are being destroyed. In fact, that isn’t the goal of the projects that destroy eucalyptus. Grassland is the goal because that’s the habitat that existed before non-native trees were planted. (What trees native to California flower in the winter?)
Native plants unfortunately will not magically reappear when non natives are removed. The native species have to at least be in the area for a chance to disperse to the site in question, and due to habitat destruction, many natives simply aren’t there and have to be reintroduced.
Webmaster: Agreed. Native plants do not magically reappear and they are rarely planted after non-natives are destroyed. That’s why these “restoration” projects are usually failures. In the few cases where natives are planted, they must be irrigated to survive. The naturalized landscapes they replace did not require irrigation. After 8 years of extreme drought these projects are an irresponsible use of a scarce resource. My opinion of these projects is based on actual experience with the projects over 25 years. Your theoretical ideas about how the projects should be done are not consistent with actual experience.
Often times invasives gain a foothold because of anthropogenic disturbance. That’s because most invasive species are disturbance specialists. But every environment will have native disturbance specialists. The issue is that the natives were removed during or prior to the disturbance in question, while non natives were planted in the vicinity. Most invasive plants originate as garden plants meant to look pretty, or attract certain wildlife species. The non natives then have the means to disperse to the newly disturbed habitat, whereas the natives are no longer nearby. The underlying disturbance event is unfortunately ignored by the public sometimes, but I can all but guarantee it weighs heavy on the kinds of actual ecologists.
This author conveniently forgets to mention that only around 4% of introduced plant or animal species become invasive where they are introduced. Those 4% however can cause major problems for the environment. The author also conveniently forgets that the main criteria for being considered invasive is that a species reduces overall biodiversity where it is introduced.
Webmaster: The California Invasive Plant Council has designated nearly 300 species of plants as “invasive.” They are all targets for eradication. I fail to see the logic in the claim that eradicating 300 species of plants will increase biodiversity.
Excellent, clear discussion of the topic.