Q&A: Regulating invasive terminology 

Non-native species occur nearly everywhere in the world. They travel in ship cargo, escape from aquariums into nearby waterways and occasionally even float to new islands on top of hurricane debris.  

Many of these species don’t last long in their new home—the conditions usually aren’t right for them. Others thrive, filling ecological niches that native species don’t occupy, or outcompeting them in others. A subset of the latter species can cause serious issues to native species, human health and infrastructure, and native ecosystems as a whole.  

But how do scientists define these terms? When does a non-native species become invasive? And what constitutes an alien species?  

To delve more into these questions, The Wildlife Society connected with Ismael Soto, a PhD student at the University of South Bohemia in České Budějovice, who recently published a paper on the subject in Biological Reviews titled “Taming the terminological tempest in invasion science.” The Wildlife Society caught up Budějovice to find out how to tackle these questions.  

Why is it important to agree on terms “alien species,” “invasive species” or “non-native species?” 

In science—and everywhere—it is crucial to define terms clearly and understand their meanings. The use of polysemous—words with multiple meanings—and/or redundant terms in our field can hinder progress. I will start with “invasive.” This term is by far the most problematic, with an increasing number of definitions emerging each year. We have reached a point where we don’t truly understand what “invasive” means. For example, at a conference, if someone mentions they are working with the invasive species “X,” it is unclear whether they are referring to the species spreading, having a significant impact, both or neither, thus creating confusion. This ambiguity dilutes the useful potential of the word, which was initially intended to describe a subset of non-native species of particular concern. Furthermore, it appears that the terms “invasive” and “non-native” are now used synonymously, which they should not be. Probably the root of the problem lies in the three main ways of defining these terms—impact, spread, and impact and spread—each having its deficiencies.  

Regarding “alien/non-native,” the issue is simpler and of less concern. These terms are interchangeable as they share the same meaning. The problem with “alien” is that for some audiences, it might convey images of extraterrestrial beings like the classic green silhouette or subjects from the movie Alien vs. Predator. The main reason I avoid using “alien” is due to its strong negative connotations with things like immigration policies. Indeed, the term has been used for a long time, but as language evolves, we have to adapt to new contexts. “Non-native” is currently my preferred term, as it is not politically charged. 

Although I have my definitions and reasons for considering them the best, I do not claim they are the absolute truth. However, I believe in the necessity of clearly defining and limiting these terms. While a global consensus may seem naïve or impossible, there have been successful cases in related fields such as the Venice System, which is used to classify different types of water according to their salinity.  

When does a non-native species become an invasive species?  

The answer depends on who you ask, and this is because there are over 25 different definitions of “invasive.” While not all of these definitions are currently in use, they certainly contribute to confusion in the field. In my personal opinion, although it’s far from perfect, the term “invasive species” should be linked to the concept of “spread.” Therefore, I believe that an invasive species is a non-native species that spreads beyond its point of introduction and establishes new populations. However, I also acknowledge that some people have chosen to stop using the term due to the vagueness of the definitions we currently have. 

When does a non-native species become a native species? 

By definition, this should never happen. Even in cases of old invasions—whether dating back to colonialism or even ancient Australasian migrations—the effects of these invasions are still visible today. However, it is true that some non-native species have become part of the culture in certain region, such as with crayfish festivals, and as a result, little or no management is applied to them. Some people also refer to certain species as “naturalized,” meaning that the species can survive and reproduce. However, this is another controversial term with multiple definitions. 

Are there regulatory repercussions for not having a standardized set of definitions for these terms? 

Definitively, how can we manage these populations if we are unclear about the terms we are using? The concurrent definitions of several terms complicate our understanding and actions. Moreover, how should we prioritize these populations—based on impact, spread or both? A recent example from a colleague highlighted that in Australia, there is no unified list of non-native or invasive species, and the definitions—and therefore the terms—vary by state, leading to a lack of communication in the same country. Imagine this at a global scale. Another consideration, just as food for thought, is that our classifications of native, non-native or invasive species are often based on political borders, which are meaningless to the species themselves. This discrepancy can lead to curious anomalies. 

The IUCN has defined terms that many other countries also use—categories like “critically endangered” or “vulnerable.” Should the IUCN or a similar international regulatory body set guidelines for the meaning of invasive species terminology? 

All efforts to harmonize terminology are greatly welcomed, as I consider inconsistent definitions a critical issue. However, such efforts must be implemented on a global scale and take into account diverse perspectives and points of view, from early-career researchers to professors. Without this inclusive approach, the effort may prove fruitless or, even worse, lead to greater confusion. Or it may become just another failure to harmonize the terminology. 

What can be gained by improving the agreement on the definition of these terms? 

As we say in Spanish, “everything to win, nothing to lose.” A field that progresses in consensus will undoubtedly accelerate the rate of innovation, not merely by focusing on new ideas but by fostering a shared understanding. While these definitions should remain open to debate and not become dogmatic, we need solid arguments to support or challenge these discussions. Biological invasion is a global issue with complex solutions, if any. Thus, any effort we make to mitigate impacts and prevent future expansions is already an advancement.