Conservation scientists have traditionally viewed non-native species (NNS) as potential threats to native biodiversity. Here, we question whether alternative views of NNS exist in the scientific community that stand in contrast to the dominant narrative that emerges from the literature. We asked researchers from the biological, social, and environmental sciences to participate in an anonymous poll regarding the perceived values and threats of NNS. Some 314 individuals responded, approximately half of whom were biologists and half were social or environmental scientists. We grouped responses into three statistical clusters defined by shared responses. We then analyzed the correlation of responses to individual questions and membership of clusters with predictor variables age, gender, and field of work. Overall, a majority of respondents in our sample supported statements that the species-component of biodiversity should include all species (55%) or some types of non-native species (an additional 32%), which contrasts with the manner in which major biodiversity assessments and indicators are constructed. A majority of respondents in our sample (65%) also supported that measurement of the impact of invasive species should be based on the net biological, social, and economic effects, which also represents a marked departure from current methods that focus only on the adverse effects of a subset of NNS considered as invasive. Field of work and age were correlated with clusters and numerous individual responses. For example, biologists were three-times more likely than non-biologists to support a definition of species richness that included only native species. Two clusters (Cluster 1 and Cluster 3), mainly composed of non-biologists and biologists, respectively, differed in their support for statements that NNS would provide useful ecosystem services in the future (66% and 40%, respectively). Thus, a key result of this study is that a variety of normative stances regarding NNS is present within the scientific community. Current international indicators of progress (e.g., Aichi Targets) capture only a “nativist” set of values, which, if our sample is representative of the scientific community, appears to be a minority view. Therefore, we argue that indicators should be modified to integrate the diversity of views that exist within the scientific community.

conservation ethics, exotic species, invasive species, nativism, values

There is a consensus among scientists that humanity is experiencing a sixth mass extinction of species, which is resulting in an alarming decline of biodiversity (Barrett et al. 2018; Ceballos and Ehrlich 2018; Ceballos et al. 2015; Ceballos et al. 2017; Mayhew et al. 2007). Species considered as invasive are regarded to be one of the main direct drivers of biodiversity loss and recent extinctions (Bellard et al. 2016; Butchart et al. 2010; McGeoch et al. 2010). Although scientists largely agree on the biodiversity crisis and potential negative effects of invasive species, the debate is just emerging about the potential risks and contributions that should be attributed to the bulk of non-native species (NNS) that are not currently considered to be invasive.

Ambiguities in the definitions and understanding of terms such as “biodiversity” and “invasive species” have contributed to confusion among scientists, policy-makers, and the public. They may also contribute to opposing positions around NNS. Indeed, the term “biodiversity” can have both scientific and cultural meanings. Scientifically, biodiversity (biological diversity) refers to the variability among all living organisms and the ecological systems of which they are a part; this includes variation at the genetic, species, and ecosystem levels [Convention on Biological Diversity (CBD), Article 2]. Culturally, biodiversity refers to the dimensions of nature that we cherish and wish to conserve, wherein individual species have been categorized as more or less desirable in particular places (Fall 2014). For example, a large number of assessments and reports use the word “biodiversity” when in practice only the status, richness and abundance of native species are assessed (Schlaepfer 2018b). In such examples NNS are implicitly - and sometimes explicitly - given either no value or a negative value.

In a similar vein, the term “invasive species” is conceptualized differently across policy contexts. Primarily, species considered as invasive refer to NNS (also referred to as “alien species”) whose ancestors were introduced deliberately or unintentionally outside their natural habitats and which become established, proliferate, and spread in ways that cause damage to a range of interests, including biological diversity (Daisie-database; Shine 2007). The term “invasive species” is used in this sense in an international and political context (e.g. CBD, IUCN) and by government and intergovernmental organizations (e.g. the European Commission). A broader definition is used by some researchers and practitioners (landscape managers, botanists, conservationists, horticulturists, etc.) who refer to species “considered as invasive” as native species and NNS that are over abundant, even without any proven adverse effects on the environment (Colautti and MacIsaac 2004). Finally, Richardson et al. (2000) proposed that the term “invasive” should be used without any inference to environmental or economic impact, but rather on the capacity of species to produce reproductive offspring, often in very large numbers, at considerable distances from the parent and thus have the potential to spread over a considerable area.

The versatile meanings of “biodiversity” and “invasive species” highlight the ambiguous thinking on NNS within conservation scientists. This raises some fundamental questions: What do we wish to conserve? And for whom? (Mace 2014). Currently, NNS are not considered as (positive) contributions to biodiversity. For example, indices and metrics used to measure progress towards biodiversity conservation and sustainability goals focus solely on the “native” component of biodiversity (Schlaepfer 2018b). Thus, current indicators may capture only a limited range of values that exist in the conservation community and across society.

In practice, the justifications for conserving nature have changed over time, and the current conservation frame is a mix of different conservation models (Mace 2014; Sandbrook et al. 2019), with each model receiving attention from different groups of people depending on their personal values and expertise. These differences in knowledge and values may be contributing factors to the differing views between invasion biologists and the public (Genovesi 2008). Beyond the expertise and influence of the former group (invasion biologists), the importance of the opinion of the latter is increasingly recognized. Indeed, there is a call to include greater input from the public in an attempt to seek greater public support for the conservation goal and reduce the rate of biodiversity loss. In this context, an important aspect that could contribute to a constructive debate surrounding the importance of NNS in conservation is an awareness of the range of views regarding NNS within the conservation community and greater care to consider alternative views in conservation discourse and decision-making (Sandbrook et al. 2019).

Here, we enquired whether a variety of values exists within the scientific community regarding NNS. We also investigated different positions on the debate and to what extent they were associated with the demographic (age, gender) and professional (field of study) factors of participants. The existing perspectives on NNS among scientists (Guiașu 2016; Guiaşu and Tindale 2018; Schlaepfer 2018b; Simberloff 2003), including among invasion biologists (Humair et al. 2014; Young and Larson 2011), suggest the existence of factors (personal values, diverging conceptual understandings, knowledge gaps, etc.) likely to determine or influence attitudes towards NNS. Considering the evolution of conservation paradigms (Mace 2014), and knowing that individual attributes influence people’s perception of invasive alien species (Shackleton et al. 2019), we hypothesize that socio-demographic factors and field of work can potentially shape the opinions of scientists. Also, based on our personal experiences at workshops and conferences, we considered age and gender as potential explanatory factors.

This survey differs from previous opinion surveys, which focused mainly on invasive species issues (Andreu et al. 2009; Bardsley and Edwards-Jones 2007; Fischer et al. 2014; Guiaşu and Tindale 2018; Humair et al. 2014; Young and Larson 2011) and contributes to the broader question of how to value and manage NNS (including those considered as invasive). Bringing more diverse voices into our understanding of NNS and the debate on “conserving what and for whom” (Mace 2014) can help to inform policy and research directions in the current context of preparation for the post-2020 Strategic Plan for the Convention on Biological Diversity.

The primary aim of this study was to investigate whether a variety of perceptions and values regarding NNS exists amongst scientists. Further, we investigated whether a person’s opinions were correlated with their age, gender, or field of work (e.g. biologist or non-biologist). An important result of this survey is the demonstration that a great variety of normative positions exist beyond the nativist position, which currently dominates the scientific literature and the operationalization of biodiversity programs.

From the perspective of the scientific community, should NNS be considered as part of biodiversity along with native species? A key result from our study is that in our sample there was a poor consensus on which types of species belong to what is called “biodiversity” (Table 1, Statement 1). First, the fact that some respondents excluded NNS from biodiversity or conditioned its inclusion to some features stands in contradiction to the definition of biodiversity stated in Article 2 of the CBD agreement (CBD 1992). Second, this may indicate that some scientists perceive NNS species to have less value outside of their historic ranges. Yet, recent studies have documented that some NNS do have a positive contribution to biodiversity and human-well-being (Guiașu 2016; Schlaepfer 2018a; Schlaepfer 2018b; Schlaepfer et al. 2011). According to some respondents, the risk of accepting NNS as part of biodiversity will be that “Non-native species will be seen as normal. This is not the case.” (Respondent #45, Biologist, male, 45–54 years old) or “People turn their backs on native species if both native and non-native are seen as of equal value” (Respondent #26, Biologist, female, 35–44 years). Such attitudes of re-conceptualization and operationalization of the term “biodiversity” at the level of the scientific community may reflect a shift from a scientific to more vernacular or cultural meaning of the term biodiversity that increasingly takes into account the desirability of the public and the political vision. This attempt to re-conceptualize and operationalize the term biodiversity may also reflect the different aspects of biodiversity being emphasized (Russell and Kueffer 2019).

Nevertheless, we were surprised to note that responses in our sample tended to be at odds with the current practices of assessing progress in biodiversity conservation, which use metrics focused only on native species. For example, the vast majority of respondents did not support the statement that species richness is defined by only native species. Instead, they favor notions of species richness that also included some other NNS and – more surprisingly – a notion of species richness based on all species, including invasive species (Figure 1, Table 1). Scientists in our sample mostly find the distinction between NNS and native species as artificial and counterproductive. In practice, the distinction between which species are native to a region or not is a matter of uncertainty on the origin of the species and therefore impacts management options. Species whose geographic origins cannot be determined based on the available evidence are now referred to as “cryptogenic” species, a much less stigmatizing expression than “invasive” (Carlton 1996; Guiaşu and Tindale 2018).

Does the origin of a species really matter? In the field, both native species and NNS are identified as drivers of species’ extinction and, therefore, the theoretical or practical usefulness and importance of a distinction between native species and NNS to conservation may be questioned (Davis et al. 2011; Guiașu 2016; Venter et al. 2006). However, recent evidence indicated that the biogeographic origin of species is likely to worsen the ecological impact (Blackburn et al. 2019), suggesting that the distinction may be justified, especially in an island context.

Does the scientific community consider NNS as a threat to native species and human well-being? Respondents in our sample mostly believed that NNS represent a threat to the survival of native biodiversity and to human well-being and should be considered as potentially invasive, which is congruent with the position of many invasion biologists (Richardson and Ricciardi 2013; Simberloff 2003). This opinion indicates a leeriness towards NNS and may reflect the magnitude of an “a priori” negative association to NNS in the collective consciousness (Guiașu 2016), even though evidence shows that only a small percent of established NNS ever become problematic (IUCN 2019; Moore 2005; Primack 2012; Seebens et al. 2018; Vander Zanden 2005). Interestingly, the fact that some respondents thought that NNS do not, on average, represent a threat to native species reveals a shift in how NNS are viewed within the field of conservation.

The views of respondents in our sample on how to define an invasive species were also at odds with current practices. An introduced species for instance can be considered as invasive if “at least” one undesirable economic, biological, social, or human-health effect is reported (Daisie-database; Shine 2007). By contrast, Schlaepfer (2018a) defined a species as “invasive” if the negative effects exceed the positive effects. The finding that respondents mostly supported such an approach acknowledges a need to account for the net impact of NNS, including invasive ones (Young and Larson 2011) and to call for more realism and transparency in communication on NNS and invasive species to curb the social anxiety regarding NNS (Ernwein and Fall 2015).

How best to decide whether an NNS is desirable or not? Most of the respondents in our sample agreed on the importance of taking into account the effects of NNS on ecological functions, evaluating the net ecosystem services provided by NNS, and taking into account the effect of NNS on native species richness. This observation suggests that the scientists surveyed were therefore aware of the variety of positive and negative effects of NNS, similar to those of native species (Davis et al. 2011). However, they were frequently neutral on the opportunities to conserve NNS that may provide useful functions (“ecosystem services”) in the future. Such an attitude confirms the hesitancy about NNS but also suggests the versatility of this position, which may evolve with more information about the importance and contribution of NNS to biodiversity and sustainable development.

The scientific community is heterogeneous and our results indicate that the perceptions of NNS may be influenced by the demographic features and professional training of scientists, among others factors established to influence people’s perceptions of NNS, especially the invasive subset (Shackleton et al. 2019). More generally, the opinions on NNS were not significantly associated with gender. However, unlike non-biologists, biologists (mainly older ones) were more likely to distinguish NNS from native species and were more likely to exclude NNS from biodiversity. However, both groups (biologists and non-biologists) showed a great deal of heterogeneity, generating the lack of consensus on some aspects of NNS even among biologists (Young and Larson 2011). Curiously, non-biologists were more likely than biologists to associate NNS with potential threats to the survival of native biodiversity and to human well-being, and thus – by extension – to consider them as invasive. This trend in the opinion of non-biologists may have reflected a “fear of the unknown” maintained by communication around NNS to influence the opinions, perceptions, and actions of the public (McNeeley 2012; Larson 2008). Beyond demographic and professional factors, scientists’ perceptions of NNS may also be influenced by other individual factors (i.e., knowledge and value systems, relationship), the species itself, the effect of the species, the socio-cultural context, the landscape context, the institutional and policy context (Shackleton et al. 2019).

Overall, the mapping of opinions on NNS within our sample indicated three main groups of scientists in our sample corresponding to three leading opinions: two minority, extreme positions and one dominant, intermediate position. Scientists, practitioners, and policymakers within the “nativist” position are likely to focus excessively on the potential negative effects of NNS, declaring them as harmful or potentially harmful even in the absence of evidence (Ricciardi and Simberloff 2009). In opposition to this group, the group that we characterized as “liberal” was represented mainly by non-biologists, and its respondents were more open to the potential benefits of NNS. This position reflects the belief that NNS should be considered “innocent until proven guilty”. Between these two opposing opinions was the group characterized as “agnostic”, in which scientists were undecided towards the values associated with and perceived threats from NNS as well as the opportunities for their conservation. Their prevalence may be symptomatic of a transition process, suggesting a possible shift in opinions on NNS issues in the future. This observed trend of the debate is reflexive of the middle–ground approach on the perception and management of NNS proposed by Shackelford et al. (2013), in which the authors suggested a stepwise consideration when analyzing the situation of NNS. Although a cautious approach is recommended especially in the initial stages of the introduction and establishment of NNS (Shackelford et al. 2013), our results indicate that the net impact and the desirability of a species, not its origin, were what mattered most to scientists in our sample (see also Lindemann-Matthies 2016; Van Der Wal et al. 2015).

This study also aimed to describe and explain the views of scientists on the importance and contribution of NNS to biodiversity. We speculate that the above-mentioned diverse views indicated an evolution of the views of biodiversity and the values associated with NNS. This diversity of views may reflect a transitionary period during which the primary motives for protecting nature are called into question (Mace 2014; Sandbrook et al. 2019). In this sense, instead of the simplifying and leveling of values to match the views of a single group of biologists, the policy commitments and indicators used to measure progress in conservation may need to account for this diversity of views. Because NNS are also of importance and interest to the public, their conservation value should be captured by indicators of progress in biodiversity conservation.

A second important implication of this study derives from the observed “leading opinions groups” and points to a need to question the use of an apparent consensus to define policy instruments and biodiversity indicators. While being consistent with the traditional positions of invasion biologists, our results revealed the emergence of two other groups of opinions, reflecting a lack of consensus on the concepts, opportunities, and risks associated with NNS. This lack of consensus in values observed both among experts and non-experts (Humair et al. 2014; Young and Larson 2011) clearly showed that positions on the debate were not just a matter of diverging conceptual understandings and knowledge gaps but also reflected underlying personal values and/or alignments to social norms and group ideologies (Essl et al. 2017; Humair et al. 2014). Although divergence in concepts could be fixed and knowledge gaps bridged with more education, convergence of personal values, motivations, and emotions would be a much greater task.

This study was based on the opinions of 314 scientists, whom we presume were located primarily in Western countries (Europe, USA, and Canada) based on participation patterns in other studies. We do not know to what extent our sample was representative of the global community. Thus, as in any study of this nature, the precise percentage of respondents that we reported should be treated with caution and should not be extrapolated to represent the wider scientific community. Nevertheless, our study provides the best estimate to date of the relative importance of different value systems found within the scientific community and our findings clearly revealed a diversity of views regarding NNS. As such, results from our study call for a shift in how we collectively think about and define priorities for the conservation of biodiversity in its broadest sense. Here, we addressed a small subset of the interdisciplinary challenges that are linked to NNS and we look forward to other researchers exploring these questions in alternative ways and from different angles (importance of ethics, different types of intrinsic values, a regional focus, etc.). We believe that such efforts will be both timely and welcome in the context of the preparation for the post-2020 Strategic Plan for the Convention on Biological Diversity.

We thank the respondents who took the time to participate in this poll. We also thank Juliet Fall, Arian Wallach, and James Russell for their comments on an earlier version of this manuscript. We thank the University of Geneva for financial support.