EICAT categories are distinct from each other, as they target different levels of organisation in the native species affected (e.g. individuals and populations, see IUCN 2020a, b; Box 1). However, within an impact category, impacts can vary in severity: for example, two alien species classified as having Major (MR) impacts are not necessarily equally as harmful to native species and the environment as a whole. To have MR impacts, both of these alien species would have, at least, caused the local disappearance of a sub-population of a native species (but this disappearance would be reversible if the alien species were no longer present in the affected site, as per IUCN 2020a, b). However, EICAT does not specify, for example: (i) how abundant the native species was before the arrival of the alien species, (ii) what function the lost native species had in the environment and (iii) to what extent this function can be replaced by the alien species or other natives. Furthermore, EICAT classifications do not directly make it obvious how many native species are affected – sometimes only one native species declines due to an alien species, in other cases whole communities change their structure. Data on which, and consequently how many, native species are affected can be extracted from the EICAT data (at least, if records for these impacts are available). On that note, we would like to encourage authors of primary impact studies to publish the list of affected species to aid this process.

The EICAT protocol has been applied to a broad variety of taxonomic groups, mainly for research purposes. This includes the following: birds (e.g. Evans et al. 2016), amphibians (Kumschick et al. 2017), reptiles (Kraus 2015), some gastropods (Kesner and Kumschick 2018), some marine fish (Galanidi et al. 2018), bamboos (Canavan et al. 2019) and some mammals (Hagen and Kumschick 2018). These studies demonstrate that EICAT is applicable across a wide range of taxa and habitats. To date, none of these assessments has been reviewed and approved by the EICAT Authority (the body established to govern EICAT assessments; IUCN 2020a, b), as these formal processes are still being developed. This step will be a necessary requirement for EICAT assessments to be accepted and published by IUCN and to ensure consistent application of the protocol. Furthermore, many taxa have not yet been formally assessed with EICAT at a global level.

EICAT assessments for all alien taxa, not only taxa with harmful impacts [Moderate (MO) or higher], will be made available through an online database linked with the IUCN Global Invasive Species Database (GISD). This means that assessments of alien taxa, for which there are no impact data (DD; Box 1), taxa for which the evidence available shows they do not affect the performance of native individuals (MC) and taxa with very low impacts (MN), will also all be made available. Taxa classified as having No Alien Population (NA) and Not Evaluated (NE) will not be included in the EICAT database. Despite the intended use for alien species and populations, the categories and criteria can, with small adaptations, also be applied to native taxa in their native ranges (see, for example, Canavan et al. 2019). Even though assessments on native species impacts will not be accepted by IUCN as part of EICAT, they can nonetheless provide important information for conservation managers and policy-makers.

A taxon’s EICAT classification may differ depending on the geographic scale under consideration. Sub-global (e.g. regional, national or habitat-specific) EICAT assessments only include information on impacts from a specific region or habitat, as opposed to global EICAT assessments which should include information from anywhere in the global alien range. Examples of completed sub-global assessments include alien grasses (Visser et al. 2017) and alien fish (Marr et al. 2017) in South Africa and mammals, amphibians and reptiles on Cuba (Borroto-Paez et al. 2015; Borroto-Paez and Manica 2017). Assessments can also be undertaken at smaller scales if data availability allows, as for terrestrial invasions on Marion and Prince Edward Islands (Greve et al. 2017) and for the black locust tree (Robinia pseudoacacia) in Europe, where habitat-specific EICAT assessments were produced (Branquart et al. 2016). For example, such classifications can be useful to provide information for local management actions or reporting on the status of an invasion in a region (e.g. van Wilgen and Wilson 2018 for South Africa), but will not be reviewed by IUCN or displayed on the EICAT website. However, country-specific data on impacts and their associated EICAT classifications are planned to be included in the Global Register for Introduced and Invasive Species (GRIIS; http://www.griis.org/about.php; Pagad et al. 2018). These data can complement global assessments as country assessors might have access to local grey literature like reports and databases which are often written in local languages. Such assessments might unlock these data which can subsequently be added to the global EICAT data on the IUCN EICAT platform.

One of the responses to threats by alien species globally has been the development of local, national and international policies (Turbelin et al. 2017). The development of regulations on alien species, in most cases, includes lists of species to be regulated, for example, to implement control actions or trade restrictions (e.g. Essl et al. 2011; García-de-Lomas and Vilà 2015). Such lists need to be developed in a manner that adheres to international agreements, such as the International Plant Protection Convention (IPPC; FAO 1996), the World Organisation for Animal Health (OIE 2011) and the World Trade Organisation (Shine et al. 2000) and need to follow best practices including, for example, transparency and evidence-based decision-making (Roy et al. 2018). EICAT has been used to contribute to processes leading up to policy-making and the development of lists and regulations as a transparent and evidence-based tool (Pergl et al. 2016; Roy et al. 2019). It is also an integral part of the risk analysis framework used to underpin the listing of alien taxa under South African national regulations (DEA 2014; Kumschick et al. 2020a, b). Furthermore, it is used in Europe as a reference to select species that will undergo full risk assessment for inclusion in the list of Invasive Alien Species of Union Concern (Regulation 1143/2014) and related national lists (Bertolino et al. 2020).

However, it must be stressed that EICAT, in itself, does not constitute a risk assessment and should not be used as one. Impact assessment tools like EICAT provide useful information on environmental impacts which may provide information for the process of risk assessment. However, risk assessments require additional information on the likelihood of the hazard occurring, such as the probability that the alien taxon will enter, establish and spread at a certain site. For policy-making and legislative decisions, additional information on benefits, costs and efficiency of management options are required (Kumschick et al. 2020; Vimercati et al. 2020; Bertolino et al. 2020).

Many of the processes feeding into policy-making and the regulation of alien taxa also apply to conservation planning and the setting of management priorities, i.e. there is a large overlap between the two as priority setting is also needed for policy-making (e.g. McGeoch et al. 2016; Booy et al. 2017; Kumschick et al. 2020a). In each case, impact is an important factor to be taken into account for decision-making and EICAT has been put forward as a suitable protocol for this purpose (McGeoch et al. 2016). For example, Katsanevakis et al. (2016) adapted and used EICAT to aid prioritisation of conservation actions in the Mediterranean Sea. The principles of EICAT – adapted to a questionnaire – have also been used as part of a decision-support scheme for alien species management in cities and been applied in Cape Town, South Africa (Gaertner et al. 2017).

Ideally, EICAT should not be used in isolation to set management priorities (as also outlined in Branquart et al. 2016), especially for already established alien taxa. However, EICAT can be useful as an early warning system for taxa which have been shown to cause harmful impacts in other areas to which they were introduced and so could be considered a “rapid response” tool. When resources are limited and decisions need to be taken without delay, EICAT can provide useful information on potentially harmful taxa, allowing prioritisation of their interception at borders or rapid management actions for new incursions. EICAT shows the maximum impact ever recorded for certain populations of an alien taxon and collates all records of impact for the taxon. It is, therefore, a useful reference to select the most harmful taxa, providing an overview of how they impact native species and ecosystems. To set management priorities for alien taxa most effectively, their current and potential future local impact should be considered and EICAT can clearly contribute to assessing the former, but could also be used for prioritising species for the latter. Nevertheless, where possible, additional information should be taken into account, including, for example, the distribution and the invasion potential of the taxon in the region to be managed, management options available and their costs and effectiveness. The same applies when setting priorities for site and pathway management. EICAT data can be useful to assess which pathways facilitate the movement of more harmful taxa (Pergl et al. 2017) or which regions are more at risk due to high-impacting aliens being present there (Nentwig et al. 2010). However, managing a pathway is not necessarily needed just because it contains harmful taxa (MO, MR, MV) and a taxon with a high impact elsewhere does not necessarily have the same impact in the region of interest. Nevertheless, EICAT provides important information feeding into prioritisation processes for alien taxa, especially considering that the main justification for management, in many cases, is their negative impacts (e.g. McGeoch et al. 2016; Roy et al. 2019).

Simple and understandable indicators can be produced from EICAT to track changes in the magnitude of environmental impacts from alien species over time. This can provide information for policy processes at the global level, such as the CBD Strategic Plan, UN Sustainable Development Goals and the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) or at the national level to support the monitoring of management measures. For example, EICAT has been recommended as a tool to monitor biological invasions globally and is listed as one of three essential variables to report on when monitoring biological invasions at country level (Latombe et al. 2017). The number and identity of taxa in each impact category, for example, can give an indication of how many (potentially) harmful taxa are present in the country and, if collected over time, can be used to track trends in the impacts of these taxa. Furthermore, impact is one of the main indicators used to provide information for reports on the status of alien taxa in a country (Wilson et al. 2018).

Aside from the availability (or lack) of data on impacts, the sky is the limit for the uses of EICAT for research. The questions which EICAT can help to answer are diverse, but due to the nature of the system are focussed around topics related to the magnitude and type of alien species’ environmental impacts and the availability of impact data.

The most basic application of EICAT enables alien species with severe impacts to be distinguished from those which are less harmful, at least on the basis of available data. This allows comparisons of species with different reported levels of impact, which have been made within taxonomic groups, including birds (Evans et al. 2016), amphibians (Kumschick et al. 2017), fish (Galanidi et al. 2018) and bamboos (Canavan et al. 2019). EICAT data can also be used to compare impact magnitudes between taxonomic groups (similarly to Kumschick et al. 2015). Comparisons need to be interpreted with caution, as the absence of evidence of impacts does not mean that no impacts are occurring (see, for example, Hulme et al. 2013); however, empirical studies can only ever proceed on the basis of what data are available (Blackburn et al. 2015).

The ways in which alien taxa affect native species can differ between taxonomic groups or habitats. EICAT has been used to show that the importance of mechanisms can differ between groups of insects (McGeoch et al. 2015), amphibians and reptiles (Kraus 2015), birds (Evans et al. 2016) and gastropods (Kesner and Kumschick 2018). Some impact mechanisms are associated with more harmful impacts (Evans et al. 2016; Rockwell-Postel et al. 2020). Furthermore, the available data suggest that certain regions are more highly impacted by alien species than others. Assessing which regions are more highly impacted and improving our ability to understand why, can aid in setting priorities for management and resource allocation (e.g. Evans et al. 2016), albeit that spatial biases in the literature (e.g. Evans and Blackburn 2020) should be considered when interpreting such findings. Similarly, certain habitats could be more vulnerable to impacts by certain taxa, as in the case of Robinia pseudoacacia, which affects open habitats like grasslands more severely than closed, forested habitats (Branquart et al. 2016).

Theory suggests that species traits should play a role in their invasiveness and impacts. EICAT can be useful to study patterns related to the traits of alien species with and without harmful impacts (e.g. Evans et al. 2018a). It has also been used to test general invasion hypotheses related to the severity of impacts of alien taxa, such as the island susceptibility hypothesis (Hagen and Kumschick 2018). Additionally, studies indicate that we lack information on the environmental impacts of many alien species (Simberloff et al. 2013; Kumschick et al. 2015; Evans et al. 2016). EICAT provides a structured way to identify data gaps and can, therefore, help to set priorities for future impact research (Evans et al. 2018b; Evans and Blackburn 2020).

EICAT provides a solid base for communicating the breadth and diversity of impacts which alien taxa can cause on the environment and the range in magnitude of those impacts. Other communication tools have been put forward to achieve the same goal, including the “100 of the world’s worst invaders” list compiled by experts of the IUCN Invasive Species Specialist Group (Lowe et al. 2004) or the “100 worst list” of the European Union (Vila et al. 2009) and more recently using a similar impact scoring approach for European aliens (Nentwig et al. 2018). One of the main advantages of the EICAT approach compared to a “100 worst list” is that it showcases not only species with high impacts, but also gives the range in known magnitudes of impact caused by different alien taxa.

One particular communication value of EICAT is that it translates a complex issue – the magnitude of environmental impacts from alien taxa – into a simple but well-defined set of categories. It has been repeatedly used for this purpose in scientific papers studying certain organisms, for example, giant African land snails (Achatina fulica) on Christmas Island (O’Loughlin and Green 2017), crayfish spp. in parts of the USA (DiStefano et al. 2015), the anole lizard (Anolis porcatus) in Brazil (Prates et al. 2016) and red king crab (Paralithodes camtschaticus) on the Euro-Arctic shelf (Christiansen et al. 2015). It is also a standardised tool that can be used to assess alien taxa from any taxonomic group, making it applicable across the globe. This lends itself to becoming a powerful communication tool, in much the same vein as the IUCN Red List, which shares the same characteristics, but categorises species based on their risk of extinction. As with the IUCN Red List, EICAT can be used to communicate and engage with a wide variety of audiences for different purposes.

EICAT classifications can be converted into info-graphics, or even art, to help communicate information on impacts from alien species (e.g. as done for the IUCN Red List data, https://www.sharktrust.org/shark-science or https://infowetrust.com/endangeredsafari/), making the messages and results more attractive to print, online and on social media. EICAT can also be used by civil society organisations in supporting them in funding applications and communicating and engaging with stakeholders to raise awareness and change behaviour or support them in undertaking action on the ground. In addition, as it is an IUCN ‘global standard’, it is also more likely to be a known and ‘trusted’ information source for policy-makers, supporting scientists in bridging the science-policy interface at a national and global level.