Biological invasions, driven by human-mediated species movements, pose significant threats to global ecosystems and economies. The classification of non-native species is a complex issue intertwining ecological considerations and ethical concerns. The need for nuanced and less ambiguous terminology is emphasised, considering biogeographic, evolutionary, and ecological principles. In-country translocations of native species into ecosystems in which they do not naturally occur, are often overlooked and are the least regulated among species movements, despite being increasingly common in conservation. Our case studies, spanning various ecosystems and taxa, illustrate the diverse impacts of translocations on native species and ecosystems. The challenges associated with translocated species underscore the urgency for robust risk management strategies and rigorous monitoring. A comprehensive and adaptable management framework that considers translocated species for evidence-based management decisions is critical for navigating the complexities of translocations effectively, ensuring the conservation of biodiversity and ecosystem sustainability.

Biological invasions, conservation, invasive species management, translocations

Biological invasions occur when species are moved by human activities from their native range to new areas where they have no evolutionary history and are a major global economic and ecological concern (Simberloff et al. 2005). Biological invasions are recognized as a pervasive threat to biodiversity and human well-being, especially in aquatic ecosystems (Cuthbert et al. 2021). Numerous pathways can facilitate the spread of aquatic non-native species (Ruiz et al. 2011), with human-mediated pathways involving global trade and transportation (Avila et al. 2020). Once established in new habitats, aquatic non-native species can disrupt local ecosystems by competing with native species for resources, modifying habitats, and altering nutrient cycles, often with severe consequences for human well-being (Hald-Mortensen 2023). Economically, these species can damage sectors including fisheries and, among others, public welfare, leading to substantial financial costs (Cuthbert et al. 2021). Yet not all species that are introduced originate from distant, biogeographically distinct regions. Rather, it often occurs that species are translocated within biogeographic regions from ecosystems they are native to, into others they are non-native to, causing a terminological tempest (Soto et al. 2024) and in essence creating an unregulated status. Here, we discuss the status of these species in the context of biological invasions using several key examples.

The classification of non-native species is a complex issue intertwined with both ecological considerations and ethical concerns (Richardson et al. 2011) and was recently reviewed and discussed by Soto et al. (2024). Ethically, the language used in invasion science to describe non-native species often mirrors societal views on foreignness, with terms for non-native species such as “alien” reflecting and even reinforcing xenophobic attitudes (Soto et al. 2024), paralleling language used against human immigrants. From an ecological perspective, the current classification systems used for ‘invasiveness’ can differ substantially, where the focus is the ecological impact of the species or its ability to establish and spread, but rarely both (Soto et al. 2024).

In general, policies and management of non-native species rely strongly on national boundaries (Piria et al. 2021). This can be highly problematic as these boundaries fail to consider biogeographic principles concerning evolutionary history and intricate ecological interactions of the species being moved (Soto et al. 2024). This is especially true for freshwater ecosystems, where nativeness and non-nativeness may even differ between adjacent river basins within the same biogeographic realm (e.g Warren et al. 2024). This is then exacerbated by ambiguous terminology with, for example, the EU Invasive Alien Species Regulation (Regulation (EU) 1143/2014) definition of an “invasive alien species” being ‘animals and plants that are introduced accidentally or deliberately into a natural environment where they are not normally found, with serious negative consequences for their new environment’, with the aim of the associated legislation to ‘prevent, minimise and mitigate the adverse impacts posed by these species on native biodiversity and ecosystem services’. Accordingly, there remains considerable ambiguity as to what constitutes the ‘natural environment’ and ‘native biodiversity’ (Chew and Hamilton 2011). We thus propose the requirement for a more nuanced and less ambiguous terminology for native species translocated within their native region (Essl et al. 2021) that combines the issues of the species’ biogeographic region of origin and ecosystems in which it naturally occurs, as well as its evolutionary history and ecological role(s), rather than just relying on geo-political (often national) boundaries and one that is distinctive from flawed concepts like neonativeness (Essl et al. 2019, 2021). More explicitly, we suggest that current terminology, such as “native biodiversity” and “non-native species” when considered in the assessment of biological invasions is too associated with national boundaries and their use must instead be based on sound biogeographical, evolutionary and ecological principles (Nehring and Klingenstein 2008; Wolter and Röhr 2010).

The abuse of terminology in this regard is well illustrated by native species that can become pests in their native region (previously named “native invasions”, Simberloff and Rejmánek 2011; see also Soto et al. 2024) through human activities. This can occur through anthropogenic driven environmental changes that result in the abundance of native species rapidly increasing through shifts in life history traits, and through human-induced habitat changes that create novel environments where some native species can form highly abundant populations to the detriment of others (e.g. the transformation of lotic to lentic environments through impoundment; Šmejkal et al. 2023). It can also occur as a direct result of human actions, including intentional stocking, where a species considered native (according to national boundaries) is moved into a new area within either their original biogeographical range or into a new biogeographic range within the country (Carey et al. 2012). We argue this activity, despite often practised by conservationists to e.g. protect a highly endangered species (Ricciardi and Simberloff 2009; Bradley et al. 2022), represents the release of a non-native species and potentially results in a biological invasion whose harm on the receiving ecosystem is likely to be underestimated (Gilroy et al. 2017). It is these releases of species within national boundaries, but between biogeographic areas, that ignore evolutionary histories and ecological roles, which we consider as being highly problematic for invasion management (Usher 2000; Soto et al. 2024).

Efforts to conserve biodiversity and the aim of invasion scientists to understand and mitigate biological invasions are often perceived as a philosophical paradox due to synergistic overlaps concomitant to differing priorities (i.e. species native in one region but invasive in another; Marchetti and Engstrom 2016). In recent years, the importance of in-country translocations of native species has increased in conservation worldwide (Vitule et al. 2019) yet are still largely overlooked by invasion scientists (see Glamuzina et al. 2017), despite being particularly common in certain countries (Tarkan et al. 2017). However, while conservation related translocations are often pre-planned and strictly regulated, movements of species for use in fisheries, aquaculture and the ornamental trade are less regulated, leading to widespread secondary spread (Vander Zanden and Olden 2008). Translocated species (sensu stricto) thus can pose a considerable threat to native species and ecosystems, especially where the translocation has been poorly regulated (e.g. Hodder and Bullock 1997; Glamuzina et al. 2017), which we demonstrate in the following case studies.

These case studies indicate that the translocation of species between river basins may exhibit diverse reactions based on the specific environmental conditions in which they are introduced (Tarkan et al. 2017). This inherent variability emphasises the need for a nuanced understanding of ecological dependencies, as not all translocated species respond uniformly to their native (and respectively non-native) counterparts (Vitule et al. 2019). The underlying factor driving such varied responses lies in the ecological dependency of species, whereby their behaviour is intricately influenced by the environmental context (Strona et al. 2021).

Accordingly, we argue that the issue of translocated species within national boundaries demands a re-evaluation of the concept of ‘native area’ (Guichón et al. 2015) and associated terminology (Soto et al. 2024), particularly in the contexts of fisheries, aquaculture, and the ornamental trade. Traditional classification systems based on national boundaries are insufficient for addressing the ecological complexities of species translocations (Pyšek et al. 2004). A bio-geographically informed approach, recognizing the ecological and evolutionary contexts of species, is imperative. These could, among others, include river basin district type or – if possible – even more granular approaches as implemented in the Water Framework Directive (Nilsson et al. 2004) and thus, we emphasise that our primary concern lies with the movements for fisheries, aquaculture, and ornamental trade, areas where risk screening and regulatory measures are more strictly adopted. The implementation of such an approach would involve considering the historical distribution, ecological interactions, and evolutionary relationships of species to define their nativity more accurately. This shift in perspective would enable conservationists and policymakers to develop more effective and ecologically sound strategies for managing non-native species and allow a more accurate risk screening and assessment process (Copp et al. 2016).

The evident importance of species translocated within their native region in the context of biological invasions, equivalent to that of non-native species, highlights the need for a flexible management framework designed to fully incorporate and address the nuances of species propagated for commercial sale. Such a framework should consider both the native species natural ecosystems, biogeographic distribution, and evolutionary history when outlining its natural occurrences. One such framework could be the Dispersal-Origin-Status-Impact (DOSI) assessment scheme, introduced by (Soto et al. 2024). DOSI classifies populations of non-native species at the population level. For this, it assesses non-native species based on their dispersal methods (assisted or independent), origin (allochthonous or autochthonous), current status (expanding, stationary, or shrinking), and impact (ecological, economic, health, or cultural). DOSI’s flexible and comprehensive approach supports objective, data-driven decision-making for managing biological invasions, allowing for prioritisation of interventions at various scales. This method represents an improvement over previous strategies by addressing the needs of managers and stakeholders with limited resources. DOSI could be expanded to include introduced species (i.e., species translocated within their native range to ecosystems where they do not occur naturally) or native pests whose inclusion might refine the management strategies under DOSI. DOSI only considers negative impacts (i.e., potential threats), acknowledging that negative impacts considerably outweigh and are distinct from any potential benefits. However, the aim of DOSI is to prioritise populations of non-native species for management interventions based on local risks, disregarding the feasibility or existence of adequate approaches, and the species’ ability to spread beyond current confinements. While this is one possibility, the intricate challenges associated with translocated species stress the urgency for robust risk management strategies, complemented by meticulous monitoring and centralised databases, to navigate the complexities of translocations in more effective ways.

The use of convoluted terminology can hinder effective science communication and eventually limit the effectiveness of native species conservation and non-native species management. The incidence of species translocated within their native region from ecosystems in which they are considered as native into ecosystems to which they are non-native creates another important ambiguity that requires terminological clarification.

We dedicate this work to the memory of Prof Gordon H Copp who passed away on 8 July 2023 for his inspirational work and enthusiasm on invasion biology and risk management.