We recently republished an article from the Garden Rant blog about the overuse and misuse of the word “invasive” to describe plants. That article objected to the exclusive use of that pejorative word to describe non-native plants, when native plants often behave in exactly the same way. The article also pointed out that the behavior of plants varies depending upon local conditions such that labeling any plant “invasive” beyond a specific locality is bound to be inaccurate.
Today we will tell our readers about a new study which explains why the word “invasive” is causing confusion.(1) Four scientists in Zurich, Switzerland conducted structured interviews with 26 academic invasion biologists and landscape professionals and found no consensus about the definition of “invasive.” Nor is there any agreement about the definition of “native,” which is surely contributing to the confusion about the appropriate use of the word “invasive.” Finally, the authors of the study compared this lack of consensus with the scientific literature of invasion biology. They found a conspicuous discrepancy between the uncertainty expressed in the interviews with experts and the conservation policies that are theoretically based on the scientific literature.
What is the difference between native and non-native species?
Although there was some agreement amongst invasion biologists that non-native species arrived with the help of humans, there was less agreement about the timing and location of arrival. Some said non-natives arrived after the last glacial period (about 10,000 years ago) and others said after 1500 A.D. (post-Columbian exchange). Landscape professionals were more likely to say that species are native which arrived prior to the life-span of humans, a significantly shorter period of time, obviously. This is consistent with the tendency for landscape professionals to consider human perception the source of such categorization.
There was little agreement about a spatial definition of native species. Some invasion biologists define native species within the context of political units (such as countries) while others use biogeographic definitions such as continents or on either side of a continental divide.
There is also disagreement about the means of movement used to define a species as non-native. Some invasion biologists consider species non-native if their range has changed as a result of climate change because of the anthropogenic origin of contemporary climate change. Such movement is now rapidly occurring so this particular definition is likely to result in the changed status of many species presently considered native, a change that is likely to make them targets for eradication.
When is a non-native species “invasive?”
There was little agreement about the criteria for calling a non-native species “invasive.” Some invasion biologists did not think that an “invasive” species need behave any differently than a native species to be categorized as “invasive.” Others believed it is appropriate to call a non-native species “invasive” if it spreads, even if that species has no negative impact on the ecosystem. When asked to evaluate the impact of non-native species, more invasion biologists and landscape professionals considered the impact “neutral” (56%) than those who considered the impact “negative” (32%). Invasion biologists tended to assess the affect of non-native species more negatively than landscape professionals.
The interviewees were then asked on what they based their judgment of the impact of non-native species. This is perhaps the most telling question of all. Both groups of experts lamented the absence of empirical evidence of the impact of non-native species: “In almost a third (32%) of all assessments, experts could not recall any effects of non-native invasive species on ecosystem services.” Most admitted that their judgment was based on “intuition” informed by their “general knowledge” or “extrapolating” from related knowledge: “most experts were prepared to assume that non-native invasive species have a generally negative effect upon native biodiversity.” (emphasis added)
Are non-native “invasive” species a serious problem?
This is the question for which the answers of invasion biologists and landscape professionals were most divergent. Invasion biologists consider non-native “invasive” species a serious problem which is underestimated by the public and politicians. In contrast, landscape professionals said the problem is overestimated: “…particularly due to anxiety and xenophobic feelings among the public.”
The consequences of this lack of clarity
The authors of this study then examined the publications of invasion biologists to see if this lack of consensus is apparent in the scientific literature of invasion biology. They observed that invasion biologists start their publications with a definition of non-native and invasive species, but “in the rest of the text this definition was rarely strictly applied.” They tend to use the terms “non-native” and “invasive” interchangeably. They compare the spreading of native species to non-native species without indicating that the native species is also invasive.
The authors conclude that both the categorization of species as native or non-native and their designation as “invasive” are largely value judgments that reflect cultural values, not scientific judgments. They suggest that invasion biologists “acknowledge the uncertainties and engage transparently with stakeholders and the public in deliberations about conflicting opinions. Here invasion biologists should take the role of ‘honest brokers of policy alternatives,’ taking into account different prevalent values and policy preferences rather than adopting the role of ‘issue advocates.’”
In other words, the conservation policies which are theoretically based on the scientific literature of invasion biology should acknowledge the uncertainty that pervades the discipline. Given that uncertainty, the value judgments of the public should be on an equal footing with the value judgments of those who have a vested interest in the projects that are destroying existing landscapes. There are alternatives to those destructive projects and the public’s opinion must be taken into account in considering those alternatives.
Of course, we agree. As we have said many times on Million Trees, invasion biology is not a scientific discipline. Rather it is a set of value judgments based on a belief that native species (whatever they are) are superior to non-native species. This is not just a question of semantics. Many species are being killed because of what someone chooses to call them and irreparable damage is being done to our environment in the process of killing them.
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Conservation Sense and Nonsense 
It will always be a debate. There is a definition to follow, but it gets muddled often.
It seems “invasive plant” is usually a worldwide sinister gimmick since several scientists have shown that “invasive plants” mainly grow on soil that humans disturb–agricultural, landfills, deforested areas, and near highways and other infrastructure. The “invasive plants” are also enjoyed/beneficial to many wildlife. Also, many of the “invasive plants” are ornamentals from Asia, as most ornamentals (80% of ornamentals such as fruiting/flowering plants in the USA are originally from Asia). The term “invasive” reads militant? The whole “invasive plant” scene appears very racist also, not to mention they spray toxins like pesticides to supposedly get rid of “invasive plants” in some areas of the world? In the USA, the government does not propagate endangered plants and put them on the market for people to buy, and this is quite suspicious also. More racial tolerance and civil rights attorneys are needed worldwide? There may be some truly invasive plants, but usually they are not as invasive as they say, and the lists would probably be reduced to 90% of what they are now, if there was honesty and ethics in that part of the field?