We have often wondered why so many plants and animals introduced to North America become invasive, compared to species introduced to Europe. In California, there are about 200 plants on the inventory of “invasive” plants. In Britain, there are only about a dozen plants considered “invasive.” In past articles, we have speculated that Americans are using different standards to determine invasiveness and that may be a factor. But now scientists, Jason Fridley and Dov Sax have recently reported the empirical evidence that suggests some regions are more vulnerable to invasion than others because of competitive advantages of species from regions with longer evolutionary histories. In fact, Charles Darwin is the original author of this theory:
“Darwin (1859) observed that because ‘natural selection acts by competition, it adapts the inhabitants of each country only in relation to the degree of perfection of their associates, such that, we need feel no surprise at the inhabitants of any one country…being beaten and supplanted by naturalized productions from another land.’ Darwin’s view, one of the earliest on biological invasions, presents invasion as an expectation of natural selection – a view largely absent from modern invasion biology. Darwin further suggested that species from larger regions, represented by more individuals, has ‘consequently been advanced through natural selection and competition to a higher stage of perfection of dominating power’ and therefore be expected to beat ‘less powerful’ forms found in other regions.” (1)
Based on Darwin’s speculation, Fridley and Sax formulated the evolutionary imbalance hypothesis, based on three postulates:
- Evolution is essentially an infinite series of experiments as each generation is tested by the conditions they encounter. The more tests the species passes by surviving and reproducing, the more fit the species is to face the next test.
- The number of such experiments vary by region that differ in size and biotic history, which influences the intensity of competition each species encounters.
- “Similar sets of ecological conditions exist around the world” thereby facilitating the movement of species from their native ranges to new ranges.
It follows from these postulates that when species from previously isolated habitats are mixed, some species will be more fit than others for any given set of conditions. In other words, they have an evolutionary advantage by virtue of having faced more competition for a longer period of time. These are the environmental conditions that are likely to confer such an evolutionary advantage:
- Larger regions with large expanses of habitat usually have larger populations of species. Larger populations have more genetic variation, which provides more opportunities for natural selection to choose a “winning” genetic combination.
- Also, more stable environments enable lineages to survive for longer periods of time. The longer the opportunity for natural selection to operate, the more fit the surviving lineage.
- The greater the competition each species experiences, the more fit the surviving species is likely to be. Therefore, species occupying diverse habitats are likely to be more fit than species in less diverse habitats.
The authors of this new study tested these hypotheses in three geographic areas that have well-documented non-native floras, including Eastern North American, the Czech Republic, and New Zealand. For example, the climate of the Northeast of America is similar to East Asia. Some of the most destructive invasive species in the Northeast are from East Asia, such as the emerald ash borer. Yet species from North America do not become invasive when introduced to East Asia. Species from East Asia have a much longer evolutionary history than species native to the Northeast because much of the United States was buried in glaciers during the Ice Ages, while East Asia was not. (2) The longer evolutionary history of East Asia makes East Asian species “fitter” and more likely to be successful in North America, while North American species are less successful in East Asia.
Failure of the competing theory
Invasion biology is the competing theory of why introduced species become invasive when introduced outside their native ranges. It is a theory that turns its back on evolutionary theory by assuming that plants and animals are incapable of adapting to changed conditions. Invasion biology assumes that introduced plants become invasive because they leave their predators behind. This is the predator release theory which also implies that introduced plants are not useful to native animals.
The problem with the predator release theory is that there is no empirical evidence that supports it. For example, equal numbers of insects are consistently found in native and non-native habitats. And when empirical studies claim to have found evidence of predator release, sampling errors have discredited those studies:
“For example, one study found fewer parasitic worms in introduced starlings in North America than in the entire native range of Europe and Asia. But once allowance was made for the actual local source of the starlings, the difference disappears: various evidence suggests starlings arrived in North America via Liverpool, and American starlings have most of the parasites of Liverpool starlings, plus quite a few others, either American natives or European parasites introduced with other birds. In fact, American starlings have more parasites than are found in the likely source population.” (3)
“Resistance is futile”
And so we add the evolutionary imbalance hypothesis to the long list of reasons why we are opposed to fruitless attempts to eradicate well established non-native species of plants and animals:
- The methods used to eradicate non-native species are harmful to the environment: pesticides, prescribed burns, destruction of species that are performing valuable ecological functions such as healthy trees.
- Attempting to eradicate non-native species rarely addresses the underlying cause of their existence. Therefore, natives rarely return when non-natives are eradicated because they are no longer adapted to the changed environmental conditions. For example, tamarisk dominates in the southwest because there is no longer enough water available to support the willows that were native to riparian corridors.
- The climate has changed and will continue to change. Therefore, native plants and animals are no longer adapted in much of their historical ranges.
And now we know that many invasive species have evolutionary advantages over the native species they have displaced: “The evolutionary imbalance hypothesis…could have a grim implication for conservation biologists trying to preserve native species: They may be fighting millions of years of evolution. If that’s true, the phrase ‘Resistance is futile’ comes to mind.” (2)
- Jason Fridley and Dov Sax, “The imbalance of nature: revisiting a Darwinian framework for invasion biology,” Global Ecology and Biogeography, 23, 1157-1166, 2014
- Carl Zimmer, “Turning to Darwin to Solve the Mystery of Invasive Species,” New York Times, October 9, 2014
- Ken Thompson, Where do camels belong?, Greystone Books, 2014
Conservation Sense and Nonsense 
Many of the plants considered “invasive” are horticultural introductions, tweaked for hardiness and resilience to disease, which gives them an advantage, not only within the new ecosystems into which they’ve escaped, but also more than they would have had originally. Humans have created, in a sense, super-weeds. You would apparently flood the world with them. Bizarre.
Webmaster: Million Trees encourages everyone to plant whatever they wish. We do not begrudge anyone their horticultural preferences. We only object to the pointless destruction of existing landscapes because of the harmful methods they use.
I also have to wonder about your environmental ethics, philosophy and aesthetics, if you would sacrifice the world’s biodiversity for the same plants and animals everywhere. Would you also prefer to have cats and dogs rather than lions and elephants in Africa?
Webmaster: As we have said to our readers many times, there is no evidence that plant introductions have caused decreases in biodiversity anywhere in the world. Unfortunately, gw continues to believe something for which there is no empirical evidence.
How does the indiscriminate use of pesticides in the environment put gw and her allies on a higher moral plane? We don’t consider indiscriminate pesticide use ethically, philosophically, or aesthetically superior to our objections to nativist ideology.
GW’s accusation that we might prefer cats and dogs to lions and elephants in Africa is a classic reductio ad absurdum argument which is both ridiculous and illogical.
The beauty of evolution is the biodiversity that gives us a fabulous array of life forms around the world. You argue for great losses and extinctions, not due to the natural process of evolution, but human carelessness, overpopulation, invasiveness and exploitation. Darwin would roll in his grave.
Webmaster: Darwin might be unhappy about the speed with which anthropogenic changes in the environment have accelerated evolution, but he would understand that evolution moves inexorably forward and cannot be stopped by human ideology. Unfortunately, native plant advocates have a fantasy that evolution is a historical process that can be stopped by the interference of humans. They are mistaken.
Milliontrees/webmaster says: “As we have said to our readers many times, there is no evidence that plant introductions have caused decreases in biodiversity anywhere in the world.”
I’d encourage your readers to do their own research, as this is well-documented and occurs all over the world. I’ve offered comments with links to specific research, which Milliontrees has refused to include in the comment section.
Webmaster: We encourage our readers to do their own research as well. GW provided a link to an article about Doug Tallamy which we did not publish because we don’t consider him a source of accurate information. Here are two critiques of Tallamy’s work published by Million Trees in the past:
https://milliontrees.me/2012/08/14/doug-tallamy-refutes-his-own-theory-without-changing-his-ideology/
https://milliontrees.me/2014/03/18/are-non-native-plants-ecological-traps-for-birds/
Milliontrees/webmaster says: “How does the indiscriminate use of pesticides in the environment put gw and her allies on a higher moral plane?”
No restoration work I know of employs “indiscriminate use of pesticides.” Every biologist, ecologist and naturalist I know of is extremely cautious with use of herbicides, targeting only the invasive plant. It’s not only insulting, it’s counter-intuitive to say that those who care most about nature would be reckless and indiscriminate.
Webmaster: Here is the report of a survey of land managers about the methods they use to eradicate plants they consider “invasive” which was conducted by the California Invasive Plant Council:
https://milliontrees.me/2014/02/09/methods-used-by-land-managers-to-control-invasive-plants/
Please note that 94% of land managers report using herbicides. Ten percent use them “always” and 62% use them “frequently.” We are not exaggerating the use of herbicides in the San Francisco Bay Area. Perhaps herbicides are used less frequently where GW lives in Missouri. More likely, GW is unaware of how frequently herbicides are used because local and state laws do not require the posting of herbicide applications and the reporting of herbicide use as laws in California do.
Milliontrees/webmaster says: “GW’s accusation that we might prefer cats and dogs to lions and elephants in Africa is a classic reductio ad absurdum argument which is both ridiculous and illogical.”
Actually, it quite reflects your argument, using the example of domesticized, non-native animals instead of plants. If the example strikes you as absurd, you may want to consider the absurdity of your own position.
Milliontrees/webmaster says: “Unfortunately, native plant advocates have a fantasy that evolution is a historical process that can be stopped by the interference of humans. They are mistaken.”
Evolution is not the issue. The issue is the introduction of non-native plants, which through horticultural tweaking for hardiness and a lack of natural enemies, escape cultivation and become invasive in natural landscapes, interfering with the local ecology and evolutionary processes of native species.
“The two great destroyers of biodiversity are, first habitat destruction and, second, invasion by exotic species.” –E.O. Wilson.
But then, I guess Milliontrees is more of an expert than E.O.Wilson?
Webmaster: Actually, evolution IS the issue. The plants and animals that live in our environment are best adapted to current conditions, such as climate, air quality, soil conditions, etc. Unless those underlying conditions are altered, those plants and animals will be most competitive.
E.O. Wilson is a member of the “old guard” of invasion biologists. They are rapidly being discredited by the next generation of biologists and ecologists who are using empirical methods to test the hypotheses of invasion biology. The hypotheses of invasion biology are being overturned by these empirical studies:
https://milliontrees.me/2012/09/18/support-for-hypotheses-in-invasion-biology-is-uneven-and-declining/
Million Trees does not know more than E.O. Wilson. The scientists who are testing his assumptions and finding them unfounded do. Here is a critique of the bogus claim that invasive species are the “second greatest threat to biodiversity” by Professor Mark Davis:
https://milliontrees.me/2011/05/22/non-native-species-are-not-the-second-greatest-threat-to-species-in-peril/
The Million Trees article, and the Fridley/Sax paper it is based on, are dealing with one narrow question in “invasion biology.” Asymmetry of invasion success. Why are European plants so successful in North America while North American plants are relatively less successful in Europe? Gw’s “tweaked for hardiness” theory doesn’t explain this asymmetry, and has no on-the-ground evidence behind it. Fridley/Sax’s “different evolutionary histories” theory explains it, and is based on good science.
Eighteenth century Britain enthusiastically imported exotic plants from around the world, especially from North America. In contrast, America’s founding fathers (e.g. Washington and Jefferson) were also enthusiastic gardeners, but specified that they would use American species, not imports. A “horticultural source” theory would imply that this asymmetric importation exchange should lead to more alien plants in England and fewer in North America—which is the opposite of what actually happened.
The greater success of European plants in America was noticed early. Charles Darwin (English) and Asa Gray (American) discussed it, in a friendly rivalry sort of way. Darwin: “Does it not hurt your Yankee pride. . .that we thrash you so confoundedly?” Gray: “…our weeds give up to yours…[they are] modest retiring things, and no match for the intrusive, pretentious, self-asserting foreigners.”
So, it has been long known that European plants are more successful in America than American plants are in Europe. “Tweaked horticultural plants” doesn’t explain this asymmetry; Fridley and Sax offer good scientific evidence that the answer lies in evolution. Yes, evolution, even though gw thinks “evolution is not the issue.” Different evolutionary history explains why the European plants are more “intrusive” and “self-asserting.” I prefer the scientific evidence over gw’s absolutely no evidence.
And, as I think about the non-native plants that so annoy native plant fanatics in the Bay Area (eucalyptus, broom, yellow star thistle, Mediterranean annual grasses, …) they are not tweaked horticultural plants. Just exotics that have been brought in, both intentionally and unintentionally, and thrived because their evolutionary history gave them adaptive advantages.
Below from Carl Zimmer’s article.
David Tilman, an ecologist the University of Minnesota, called the study “a wonderful extension of Darwin’s hypothesis.” But he cautioned that the work raises a paradox. While predators and pathogens can wipe out native species, it’s rare for an invasive competitor to do so.
“The most common outcome is coexistence,” said Dr. Tilman. The new hypothesis doesn’t explain why. We may never know why.”
Coexistence, thats an important concept, and one that the human species really needs to learn to apply to its own diversity.
But I would like to point out that American agricultural plants, eg.potatoes have survived and flourished in Europe and Russia, and perhaps many other parts of the world. And as I understand it most potatoes came from Peru. It has always seemed to me that the nativists have left out issues relating to agricultural plants and animals , because they realize that it would force more intellectual honesty on this entire issue.
But I have another concern. And this is re Kudzu. There was a farmer in Tennesse I believe who started a company that converted Kudzu into oil that could be used to supply a farms energy needs. He had a web site that only existed briefly. I wish someone would look into this to see why a promising use of a plant that many hate– that they say grows a mile a minute–seemed to suddenly disappear.
I hope I have not added more confusion to this discussion. I share the concerns of Million Trees about the abundant use of pesticides that are unleashed as a result of the dominance of the nativist philosophy , chemicals that end up in our rivers and streams and ocean.
You might find this interesting:
“Whether you like them or not, weeds are here and they’re going to stay here. You can try to get rid of them, but they keep coming back,” said Carlson, a doctor and ethnobotanist at UC Berkeley. “We might as well learn more about them.” http://www.sfgate.com/bayarea/article/Cal-professors-on-the-hunt-for-edible-nutritious-5846111.php