A lack of universally accepted terminology leads to much confusion, lack of consensus, and disagreement regarding invasive species on the part of ecologists, organizations, governments, and the general public. Terms used in this context should be explicit and specific because the use of overly simplistic terms to communicate complex ecological concepts can undermine management efforts, and lead to skepticism and debate (Colautti & MacIsaac 2004). This problem is particularly evident in studies of invasive species as they have also been called non-indigenous, non-native, exotic, introduced, and naturalized (Colautti & MacIsaac 2004). The terms alien, exotic, foreign, introduced, invasive, naturalized, non-native, non-indigenous, domesticated, weed, and pest can all be found in various scientific literature. These terms are often used interchangeably yet sometimes have different meanings, typically referring to non-native species that may or may not be invasive (Courchamp et al. 2017; Poland et al. 2021). Terms such as pests and weeds are still sometimes used and can apply to both native and non-native organisms (Poland et al. 2021; Richardson et al. 2000). There is a need for a universally agreed-upon definition of invasive species, yet redefining commonly used terminology has proved difficult because organizations, governments, and even individual authors are partial to the particular definitions with which they are already familiar (Colautti & MacIsaac 2004; Poland et al. 2021).
This lack of universal invasion terminology created issues such that the Federal Government of the United States now utilizes a set of terminology established by Executive Order (“Executive Order 13112 - Invasive Species” n.d.). This Executive Order established a universal set of key terms in hopes that public and private organizations would adopt the definitions as well (Poland et al. 2021). However, many sectors continue to use other definitions, and debates regarding terminology persist in scientific literature (Poland et al. 2021).
Executive Order 13112 defines invasive species as "an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health" and specifies that this includes only species for which the introduction and movement is “a result of human activity.” (Poland et al. 2021; “Executive Order 13112 - Invasive Species” n.d.). This definition intentionally included anthropogenic causes and excluded the background rates of introductions of species that occur naturally by non-human means such as animal movement, range shifting, and natural disasters (Poland et al. 2021). The specificity of this definition was met with both support and opposition. Many argued against an unconditional definition as it could overlap with processes that are not driven by human activities, such as the movement of species onto new islands, volcanic areas, fire-ravaged areas, and natural succession (Poland et al. 2021).
Definitions of invasive species that lack an explicit statement about their impact have been met with disagreement as well. Many scientists argue that the impact of invasive species must be considered in defining them. Richardson et al. (2000) state that the term ‘invasive’ must only be used for species that cause or have the potential to cause obvious damage to ecological and/or economic systems. Russell and Blackburn (2017) argue that invasive species must be defined by the negative impact they create because there is an overwhelming amount of evidence that invasive species are one of the major global challenges to the conservation of biodiversity in our time. The IUCN defines invasive species as those species introduced by humans outside of their natural range, that then become established and spread, causing a negative impact (Russell & Blackburn 2017). However, determining the negative impact of invasive species is dependent upon a combination of objective scientific research along with definitions of impact which can often be subjective and can vary greatly depending upon a variety of factors including the species in question, the habitat, proximity to humans, level of disturbance in the ecosystem, and more (Russell & Blackburn 2017). The inherent subjectivity of defining impacts can lead to disagreement over these impacts and therefore the classification of species as invasive, potentially delaying or limiting the availability of programs and funding to prevent the spread and mitigate the impact of such species (Russell & Blackburn 2017).
Many important terms used in invasion biology vary greatly and have traditionally been quite subjective. The terms ‘weed’, ‘pest’, and ‘invasive’ have been used to mean a variety of things from displeasing to nuisance to a vector of disease (Colautti & MacIsaac 2004). Sometimes species are labeled as ‘weed’ simply because they are not wanted in a particular area despite being important to an ecosystem or as a ‘nuisance’ or ‘pest’ because they were labeled as such elsewhere (Colautti & MacIsaac 2004). In these cases, the terms are not only extremely subjective but are tied to human emotions and perceptions about the species rather than scientific evidence or ecological impacts. Because of this ambiguity in terminology, ‘invasive’ has been used as a taxonomic description rather than as a term that describes the ecological phenomenon of invasion (Colautti & MacIsaac 2004). Subjective terms can lead to misconceptions and can even complicate investigations into invasions and management of invasive species (Colautti & MacIsaac 2004).
Further, the term ‘invasive’ is often dynamic (Courchamp et al. 2017). A species can have both beneficial and harmful effects. An example of this is the introduction of the mosquitofish Gambusia affinis in many parts of the world because of its appetite for mosquito larvae. While the fish controls the mosquito population, depending on the ecosystem, it can also have a negative impact on native fish, amphibians, and other insects (Colautti & MacIsaac 2004). Because species can become invasive in one habitat and not another, analyses done at the species level can help identify potential invaders. However, the proclivity toward invasion depends on a variety of factors and not just the traits of the species, as invasions occur at the population level, not the species level (Colautti & MacIsaac 2004; Mack et al. 2000; Lockwood et al. 2013). By focusing on invasion as a series of stages, we can view invasions as biogeographical rather than taxonomic phenomena (Colautti & MacIsaac 2004). That being said, problems have arisen in deciding whether a species should be labeled as ‘naturalized’ or ‘invasive’ as the stages of invasion are not discrete (Richardson et al. 2000). A species may move from stage to stage over time and space and as ecosystems change. A species might move from ‘naturalized’ to ‘invasive’ if limiting factors and opportunities in an ecosystem change in their favor (Richardson et al. 2000). This fluidity between the stages of the invasion process further demonstrates the need for early recognition and monitoring efforts so that species can be properly managed or eradicated early while still possible.
For all the previously mentioned reasons, the scientific messaging surrounding invasions can be complex, unclear, and even contradictory (Courchamp et al. 2017). A set of clear, standard definitions would make the understanding of invasive species more efficient and effective (Courchamp et al. 2017). Terminology stemming from the invasion process and its impacts, rather than the origin of the species or specific genetic traits could lead to a universal understanding of invasions and reduce confusion related to invasion terms and concepts (Colautti & MacIsaac 2004). A consensus on terminology is necessary to dispel misconceptions about invasive species and bring clarity to both the field of invasion ecology and the general public. Lack of strong consensus and debate among scientists leaves space for denialism to take root at a time when strong action against harmful invasions is necessary to protect biodiversity, ecosystem services, economic interests, and human health.