All alien species will cause changes, i.e. impacts, to some attributes of their recipient systems (Ricciardi et al. 2013; Jeschke et al. 2014). These attributes may describe different aspects of the recipient ecosystem, such as species diversity, total biomass, carbon sequestration capacity, fire intensity, pollination frequency, etc. Impacted attributes may also be associated with both human well-being and socio-economic aspects, such as the number of people employed in forestry or fishing, food security, livelihood and human connection to nature. Basic scientific arguments advocate that changes in attributes should be investigated independently from ethical values in order to be objective (Slobodkin 2001). Authors strictly supporting these arguments state that value judgements cannot be empirically tested and that some ecologists fallaciously confuse these judgements with descriptions of environmental changes (Brown and Sax 2005; Sagoff 2018). In other words, one should measure the increase of a given attribute (positive impact) and the decrease of the same attribute (negative impact) along the full spectrum of changes, without any specific focus on one of the two directions (Jeschke et al. 2014, Fig. 1). Value-laden terms such as “beneficial” or “deleterious” should be avoided whereas terms such as “positive” or “negative” should be only used from a numerical standpoint, as in the increase or decrease in the value of a property (Brown and Sax 2005). Furthermore, this argument posits that invasion scientists should act similarly to astronomers or particle physicists, who analyse scientific phenomena without considering moral values or practical consequences of their scientific research (Slobodkin 2001; Brown and Sax 2004).

Under a value-free perspective, value judgement should not interfere with the study of impacts; it is theoretically expected that studies targeting alien species assess their impacts on the recipient system independently and unbiasedly from impact directions (e.g. meta-analyses which use effect size, such as in Castro-Díez et al. 2019). However, biases towards negative impacts on native biota have been reported, i.e. predominantly reporting on native biota suffering from aliens and ignoring native biota that profit from the presence of alien species (Goodenough 2010; Schlaepfer et al. 2011; Fig. 2). Furthermore, it is difficult to judge how large this alleged bias is because it is unknown if alien species more often cause a decrease (i.e. generate negative impacts), rather than an increase (i.e. generate positive impacts), to the attributes of their recipient systems (Charles and Dukes 2007; Vitule et al. 2012). An example of a negative impact may be the decrease of species diversity caused by alien populations of rodents introduced to islands (see also example 1, Fig. 1), whereas an example of a positive impact may be the increase of local species diversity caused by the establishment of an alien invertebrate that acts as ecosystem engineer (Castilla et al. 2004).

Figure 2.

Plot reporting the number of articles and fitted linear regression obtained using the following search strings in Google Scholar at the end of October 2019: In red: “negative * of alien species “ OR “negative * of non-native species “ OR “negative * of exotic species” OR “costs of alien species “ OR “costs of non-native species “ OR “costs of exotic species”; In green: “positive * of alien species “ OR “positive * of non-native species “ OR “positive * of exotic species” OR “benefits of alien species “ OR “benefits of non-native species “ OR “benefits of exotic species”.

Alternatively, there may be a bias toward studying and reporting negative impacts (Guerin et al. 2018). Multiple negative impacts of alien species (e.g. decrease in native population size) were considered as deleterious based on ethical and societal values (Jeschke et al. 2014; Bartz and Kowarik 2019). The urgency to investigate the conspicuous deleterious impacts that some aliens cause to native communities and human activities (Richardson et al. 2000; Pyšek et al. 2008; Simberloff et al. 2012) might have contributed to this bias even among natural scientists. Such urgency was, for instance, emphasised during the Fourth Meeting of the Conference of the Parties to the Convention on Biological Diversity held in Slovakia in 1998, which first considered “including the subject of alien invasive species in its longer-term programme of work”. The report of the meeting specifically noted “the significant adverse ecological and economic effects of certain alien species on biological diversity and human health” and “the importance of taking a precautionary and ecosystem approach when dealing with issues related to alien species” (UNEP 1998). The following editions of the conference considered “alien species that threaten ecosystems, habitat or species” as a cross-cutting and priority issue relevant to biological diversity, and advocated for the prevention and mitigation of their deleterious impacts, which has become a major cornerstone of invasion science. In addition to this, since many alien species were deliberately introduced to provide benefits to humans, such benefits might have seemed obvious, thereby preventing their systematic study. Many invasion scientists might also have investigated the unwanted deleterious consequences of alien taxa introductions in order to counterbalance a favourable attitude from many stakeholders towards alien taxa intentionally introduced for agriculture and forestry (Simberloff and Stiling 1996; Louda et al. 2003; Pyšek et al. 2008).

As most research assessing the impacts of alien species has been directed toward negative impacts, the magnitude of positive impacts has been rarely systematically assessed and quantified by using statistical or semi-quantitative tools (Goodenough 2010). Instead, the literature record of positive impacts seems rather anecdotal, with impacts usually defined according to human values (Vilà et al 2010; Schlaepfer et al. 2011). Thus, there are not only fewer studies that report positive impacts, but these studies often lack a systematic and evidence-based approach to classify and compare these impacts (Vilà et al 2010). Consequently, detailed descriptions of the mechanisms by which alien species can benefit their recipient ecological and the socio-economic systems are also scarce. Some mechanisms by which aliens positively affect the diversity and abundance of native taxa by providing food and refuge have been identified by Robinson et al. (2007), Goodenough (2010), Schlaepfer et al. (2011), McQuaid and Griffiths (2014) and Tassin and Kull (2015). Additionally, Kumschick et al. (2012) described mechanisms such as herbivory, competition or predation by which aliens may affect species that are degrading the ecosystem and thereby restore its historical functional state. Further studies on these underlying mechanisms may provide eco-evolutionary insights around alien-native coevolution, rapid adaptation, biotic resistance and niche vacancy. Greater knowledge has probably been gained around socio-economic benefits to human-well-being, as multiple authors identified mechanisms by which aliens increase ecosystem services and decrease ecosystem disservices (Katsanevakis et al. 2014; Vaz et al. 2017; Knapp et al. 2019; Shackleton et al. 2019a; Milanović et al. 2020). Despite these efforts, unified systematic approaches to capture the diversity of positive and beneficial impacts of aliens across taxa and geographic regions are still lacking.

Although impacts cannot be defined as deleterious or beneficial in an absolute way, changes caused by alien species may still be perceived as deleterious or beneficial according to societal and ethical values (Fig. 1, Vilà et al. 2010; Kumschick et al. 2012; Jeschke et al. 2014; Bartz and Kowarik 2019). Alien species can alter the demography of endangered populations and permanently modify native communities (Doherty et al. 2008, Gurevitch and Padilla 2004). Since native populations and communities have high conservation value, their decrease (i.e. negative impact) can be considered deleterious from a value-laden perspective (example 1, Fig. 1). This nature conservation perspective guided the development of some impact assessment frameworks frequently used (Vilà et al. 2019), such as the GISS framework (Nentwig et al. 2016) and the EICAT framework (Blackburn et al. 2014; Hawkins et al 2015; IUCN 2020), which both assess the deleterious impacts of alien species on native taxa. Alien species may also be perceived as deleterious to socio-economic systems and human well-being. For example, when alien species impede human activities such as fishing and farming or impair human health (Mazza et al. 2014; Rai and Singh 2020), personal safety or material and immaterial assets (Bacher et. al. 2018). Deleterious impacts on the social and economic sectors have been captured in the SEICAT framework (Socio-Economic Impact Classification of Alien Taxa), which adopts a scoring system analogous to EICAT to assess how human activities are affected by alien species (Bacher et al. 2018). Some impact assessment frameworks such as GISS (Nentwig et al. 2016) and INSEAT (Martinez-Cillero 2019), and many risk assessment frameworks, evaluate deleterious socio-economic impacts (for a review of impact assessment frameworks see Strubbe et al. 2019 and Vilà et al. 2019; for a review of risk assessment frameworks see Leung et al. 2012 and Kumschick and Richardson 2013).

Analogously to negative impacts that are perceived as deleterious to native communities and humans, many positive impacts can be considered beneficial according to values associated with nature conservation and human well-being. For example, some alien species may moderately increase fire frequency in their introduced range, thus providing benefits to native pyrophytes which require fire for germination (example 3 in Fig. 1). Alien plants can also increase the biomass of a recipient ecosystem, thus being beneficial to global carbon sequestration (example 4 in Fig. 1). Additionally, many alien species increase attributes that are relevant to societal values and human well-being. In other words, they increase existent, or provide additional, ecosystem services or beneficial contributions to people’s quality of life (Díaz et al. 2018) such as food and water provision, soil and sand stabilisation and nitrogen fixation (Vaz et al. 2017; Milanović et al. 2019; Shackleton et al. 2019a). In a world of increasing environmental issues, aliens can also help to reduce the impact of other stressors. Examples include alien plants which mitigate the effects of climate change by facilitating coastal protection from erosion and favouring carbon sequestration (example 4 in Fig. 1, Essl. et al. 2017, in Castro-Díez et al. 2019). However, not all environmental and socio-economic beneficial impacts coincide with positive impacts; for example, in the impact scoring framework proposed by Kumschick et al. (2012), beneficial impacts of alien animals are quantified by measuring to what extent they reduce the population density of species degrading the ecosystem (e.g. pest species). In other words, a negative impact (e.g. decrease of pest species abundance), may thus be considered beneficial from a nature conservation standpoint or according to other values and interests (example 2, Fig. 1). An alien bio-control agent (e.g. a parasitoid wasp) that reduces the abundance of an agricultural pest can be similarly considered beneficial to farmers and other stakeholders. Such species can thus provide additional benefits to humans by reducing ecosystem disservices (Vaz et al. 2017; Knapp et al. 2019; Milanović et al. 2019).

Human values and interests associated with the impacts of alien species affect whether and how these species can be managed. Some alien species have been intentionally introduced because of the benefits they can provide to people (Castro-Díez et al. 2019). Additionally, many aliens cause low or insignificant impacts to their recipient systems and can be simply considered inconsequential for ecosystems and society (Zengeya et al. 2017). Beneficial and inconsequential species do not generally require management interventions, and their prompt identification facilitates the allocation of management resources elsewhere (van Wilgen and Richardson 2004; Zengeya et al. 2017). Aliens that provide beneficial impacts to human well-being might, however, decrease the demography of native populations, thus being deleterious from a nature conservation standpoint (Doherty et al. 2008). More generally, stakeholders may have such disparate values and interests that their perception toward alien species can be simultaneously favourable and unfavourable (Novoa et al. 2018; Shackleton et al 2019b). Such disparate values (examples 2,3 and 4, Fig. 1) may cause a conflict of interests among different stakeholders and hamper management implementation (Jeschke et al. 2014; Crowley et al. 2017; Essl et al. 2017; Zengeya et al. 2017). For instance, van Wilgen and Wilson (2018) showed that control and regulation of a few alien taxa such as pine trees (Pinus spp.) and the rainbow trout (Oncorhynchus mykiss) were extremely controversial in South Africa, given these species cause both beneficial and deleterious impacts on different sectors of society. Analogously, the control of Paterson’s curse (Echium plantagineum), an alien plant that is highly toxic to livestock, has generated conflicts between Australian farmers and beekeepers, with the latter benefiting from the nectar produced by the plant (Messing 2000). Transparent and evidence-based descriptions of beneficial and deleterious impacts of alien species may thus help to support prioritisation, clarify and motivate values underlying management, identify conflicts of interests and advance dialogue among stakeholders.

We show that arguments from different perspectives invoke a greater consideration of positive and beneficial impacts in invasion science. The development of assessment frameworks that classify deleterious and negative impacts through a standardised and evidence-based approach (e.g. EICAT and SEICAT) has improved our understanding of such impacts. These frameworks describe the different ways in which alien taxa deleteriously interact with native taxa (impact mechanisms), and quantify the severity of such interactions (impact magnitude) (Blackburn et al. 2014; Nentwig et al. 2016; Bacher et al. 2018). The application of these frameworks to different taxa and ecosystems has allowed for the investigation of factors driving impact magnitude (e.g., Kumschick et al. 2013; Measey et al. 2016; Novoa et al. 2016; Evans et al. 2018) and the ranking of hundreds of alien species based on their deleterious impacts (e.g. Kumschick et al. 2015; Nentwig et al. 2018). Given the above considerations, some of these frameworks might be adapted to assess beneficial impacts. Detailed descriptions provided by these frameworks around mechanisms by which alien species cause deleterious impacts can be extended to capture mechanisms linked to beneficial impacts (i.e. Blackburn et al. 2014; Nentwig et al. 2016). Approaches adopted by existing frameworks to evaluate assessment uncertainty can also be followed because they might help to overcome methodological limitations associated with transparency, clarity and reproducibility (Vilà et al. 2019; Probert et al. 2020). However, some conceptual and methodological aspects should be considered when developing frameworks that assess positive and beneficial impacts.

Impact assessment frameworks classify deleterious impacts according to their magnitudes, i.e. by measuring to what extent alien taxa affect reference attributes. This facilitates comparison among taxonomically distant alien species and across spatial scale and habitats. However, several different strategies have been adopted to measure impact magnitudes. Frameworks such as those proposed by Sandvik et al. (2013) and Martinez-Cillero et al. (2019) use ranking scales which distinguish between low (or noticeable), substantial (or medium), and high (intense) impacts. The scales may be associated with parameters that can be numerically quantified such as the spatial extent of the non-native range, genetic diversity, fitness and abundance of native individuals or provision of ecosystem services (Bartz and Kowarik 2019; Crystal-Ornelas and Lockwood 2020). As a consequence, such scoring scales could be easily adapted to assess bidirectional changes (Martinez-Cillero et al. 2019). Although general scaling approaches may allow the assessment of many alien species and adopt a fully symmetrical bidirectional approach (Zengeya et al. 2017), they may still be prone to subjectivity, especially when the distinction between the magnitude levels is not accurately described. The EICAT framework (Blackburn et al. 2014), on the contrary, clarifies differences between magnitude levels by assuming that with each level of impact magnitude (from minimal concern to massive), a different level of organisation is affected (from native individuals to native communities). Clarity in describing distinct levels of impact magnitude might have contributed to the increasing use of EICAT among scientists and practitioners (Kumschick et al. 2020a). An analogous scoring approach that assesses ecological impacts based on organisation level has been also developed by Olenin et al. (2007). Such approaches, however, define the highest levels of impact magnitude according to the capacity of alien species to cause the extinction of a native species (Olenin et al. 2007; Blackburn et al. 2014). As extinction cannot be exactly mirrored by any other positive ecological phenomenon, the development of a perfectly symmetrical bidirectional adaptation of these schemes might be difficult to achieve. As a consequence, not all impact assessment frameworks can, or need to, adopt a fully symmetric bidirectional scoring scale to assess impact magnitudes. This limitation should be recognised in any conceptual attempt to adapt existing frameworks in order to assess the benefits of alien species.

Impact assessment frameworks are generally developed based on different values that should be recognised and explicitly stated. Values and perspectives influence how we select the attributes of ecosystems or human activities that will be assessed (Bartz and Kowarik 2019; Strubbe et al. 2019) and must be considered when making management recommendations and in final decision making (Probert et al. 2020). However, values and perspectives also define the aims and the intrinsic limitations of each framework. For example, when evaluating the changes caused by an alien species to the community of the recipient environment, we should choose and specify which taxa are taken into consideration. Scientists embracing a conservation standpoint might consider only native, or even endangered taxa, as they aim to quantify alien impacts on species of conservation interest. Scientists who follow a more basic science approach, however, could consider all taxa independently of their origin, as their aim is to measure the negative or positive impacts of aliens from a value-free perspective. The development of a framework that assesses deleterious and beneficial impacts should thus disclose which values underlie the framework and whether the framework distinguishes between deleterious/beneficial impacts and negative/positive impacts. Such a disclosure of values can also be facilitated by the development, and adoption, of a more neutral and transparent terminology in invasion science. The distinction we have drawn in our manuscript between positive/negative and beneficial/deleterious impacts, for example, has been instrumental in defining impacts regardless of whether they were associated with human values. Both terms “positive” and “negative”, however, have in general an intrinsic value connotation and are often used as synonyms of “beneficial” or “favourable” and “detrimental” or “deleterious” in invasion science and other scientific disciplines. Given this lack of linguistic consistency, there might be the necessity to develop a more neutral and transparent terminology in invasion science that unequivocally clarifies whether an impact is defined in accordance to human values or only from a mathematical and value-free perspective.