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