Alien species are introduced to new regions in many different ways and for different purposes. A number of frameworks have been developed to group such pathways of introduction into discrete categories in order to improve our understanding of biological invasions, provide information for interventions that aim to prevent introductions, enable reporting to national and international organisations and facilitate the prediction of threats. The introduction pathway classification framework proposed by the Convention on Biological Diversity (CBD) as a global standard is comprised of six main categories and 44 sub-categories. However, issues have arisen with its implementation. In this position paper, we outline five desirable properties of an introduction pathway classification framework – it should be compatible (i.e. the level of detail of the categories is similar to that of the available data), actionable (i.e. categories link to specific interventions), general (i.e. categories are applicable across the contexts that are of interest (e.g. taxa, habitats and regions)), equivalent (i.e. categories are equivalent in their level of detail) and distinct (i.e. categories are discrete and easily distinguished) – termed the CAGED properties. The six main categories of the CBD framework have all of the CAGED properties, but the detailed sub-categories have few. Therefore, while the framework has been proposed by the CBD as a global standard and efforts have been made to put it into practice, we argue that there is room for improvement. We conclude by presenting scenarios for how the issues identified could be addressed, noting that a hybrid model might be most appropriate.

biological invasions, biosecurity, Convention on Biological Diversity, framework, introduction effort, invasion biology, mode of introduction, propagule pressure

Information on how and why alien species are introduced to new regions provides the foundation for pre- and at-border management strategies that aim to prevent the introduction of harmful species (Hulme et al. 2008; Hulme 2015). However, alien species can be introduced through a vast number of introduction pathways and, as there are limited resources available to manage introductions, important pathways must be identified and prioritised (Hulme et al. 2008; Essl et al. 2015; Essl et al. 2020). A number of frameworks have been developed to aggregate the immense number of introduction pathways into discrete categories (Essl et al. 2015). These introduction pathway classification frameworks (hereafter simply frameworks) are used in analyses that improve our understanding of biological invasions, provide information for interventions that aim to prevent introductions, enable reporting to national and international organisations and facilitate the prediction of threats. The different frameworks were developed for use in different contexts and they differ with respect to the way in which they were developed and in their level of detail. For example, there are six main categories and 20 sub-categories in the framework used by the European Alien Species Information Network (EASIN), which supports European states by providing information for policy and management (Tsiamis et al. 2017); the framework developed by Wilson et al. (2009) comprises six categories and aims to improve understanding of the underlying properties of pathways and the consequences for invasion success; and the framework developed by Hulme et al. (2008) has six categories and was developed to provide information for decisions on existing regulatory instruments.

Based on the framework of Hulme et al. (2008), a hierarchical framework was developed to assist countries to achieve Aichi Biodiversity Target 9 of the Convention on Biological Diversity (CBD), specifically the requirement to identify and manage pathways of introduction (https://www.cbd.int/sp/targets/, CBD 2014; Essl et al. 2015; Scalera et al. 2016; Harrower et al. 2018). This framework (hereafter referred to as the CBD framework) was proposed by the CBD as a global standard (CBD 2014). With six main categories and 44 sub-categories, the CBD framework is very detailed (Fig. 1). The six main categories of the CBD framework, which were adopted from the framework of Hulme et al. (2008), were developed by classifying pathways, based on three shared attributes: the degree of human involvement, the means of transport and the means of subsequent introduction (Hulme et al. 2008). In contrast, the 44 sub-categories of the CBD framework were developed by comparing and incorporating existing frameworks [including those used by the Global Invasive Species Database (GISD), the Centre for Agriculture and Bioscience International’s (CABI) Invasive Species Compendium and the Delivering Alien Invasive Species Inventories for Europe (DAISIE) consortium].

Figure 1.

The introduction pathway classification framework proposed by the Convention on Biological Diversity (CBD 2014). The nomenclature proposed in Harrower et al. (2018) has been implemented. The mechanisms of introduction and main categories were adopted from the framework developed by Hulme et al. (2008).

Assessments that have attempted to apply the CBD framework have highlighted implementation issues (Saul et al. 2017; Tsiamis et al. 2017; Zenetos 2017; van Wilgen and Wilson 2018; Pergl et al. 2020). For instance, the sub-categories cannot accommodate all data and some sub-categories overlap and are indistinguishable. Subsequent to the launch of the CBD framework, guidelines were produced which clarified the definitions of the framework’s main categories and sub-categories and proposed small adjustments to the framework’s nomenclature and structure to address some of the areas of confusion and uncertainty (Harrower et al. 2018, for details see Appendix I: Fig. A1). To date, the guidelines and the proposed changes do not appear to have been officially recognised by the CBD.

Despite these issues, the development of the framework and its recognition by the CBD was an important step towards the global implementation of a shared terminology and classification framework for pathways (Rabitsch et al. 2016, but see Paap et al. 2020). Parties to the CBD have been encouraged to make use of the framework (Essl et al. 2015) and efforts have been made to implement it. Data in existing databases have been re-classified using the CBD framework (Saul et al. 2017; Tsiamis et al. 2017; van Wilgen and Wilson 2018; Pergl et al. 2020), its terminologies have been included in the vocabularies of global data standards (Groom et al. 2019), it has been used in national level reporting (Wilson et al. 2018) and is employed in the system (NOTSYS) used by member states of the European Commission to report new observations of invasive species that are of Union concern (https://easin.jrc.ec.europa.eu/notsys). However, some countries still make use of their own frameworks. For example, New Zealand’s biosecurity surveillance system uses a framework that comprises six pathway categories (Fig. 2). This framework, unlike others, does not consider the degree of human involvement when classifying introductions into pathway categories, but introductions are instead classified based on the location of biosecurity interventions. This approach means that intentional and accidental introductions can be classified into the same pathway category.

Figure 2.

The six pathway categories recognised by New Zealand’s biosecurity surveillance system. These categories are linked to the location of biosecurity pressures and interventions (see Suppl. material 1: Fig. S1). Figure redrawn from Ministry of Agriculture and Forestry (MAF) Biosecurity New Zealand (2008).

In this position paper, we discuss why introduction pathway classification frameworks are needed and identify the desirable properties that these frameworks should have in order to achieve their stated purpose. Based on these properties, we assess the main categories and sub-categories of the CBD framework and the framework used by New Zealand’s biosecurity surveillance system. We conclude by presenting scenarios for how frameworks might be improved in future.

An ideal framework should aim to: improve our understanding of biological invasions, provide information for policy and management interventions that aim to prevent introductions, enable reporting to national and international organisations and facilitate the prediction of threats. Unfortunately, existing frameworks differ in terms of their structure and in the context for which they were developed and, thus, rarely address all four of these aims.

Frameworks have been used in retrospective analyses, whereby historical introduction data are categorised and the frequency of introductions through each of the pathways is assessed. These analyses improve our understanding of how and why alien species have been introduced in the past. Additionally, when information is included on invasion success and impacts, these analyses can be used to explore the link between the pathway through which an organism was introduced and its likelihood of becoming invasive and causing harm (Wilson et al. 2009; Pyšek et al. 2011; Faulkner et al. 2016; Pergl et al. 2017). Many of these retrospective analyses have been performed and they have answered a wide range of questions, including how the pathways vary in their importance geographically and over time, as well as across taxonomic groups, habitats and spatial scales (Hulme et al. 2008; Pyšek et al. 2011; Katsanevakis et al. 2013; Faulkner et al. 2016; Zieritz et al. 2017). A variety of frameworks have been used for this purpose; however, frameworks with broad, inclusive categories are most useful for cross-cutting analyses, as introductions that occur in a variety of contexts can be classified within the same categories. Although such frameworks are particularly suitable for studies at a global scale (e.g. Hulme et al. 2008; Saul et al. 2017), they have also been used successfully in cross-cutting analyses at regional- (Pergl et al. 2017) and national-scales (e.g. Pyšek et al. 2011; Faulkner et al. 2016).

Frameworks are also used to direct or provide information for policies, legislation and pre- and at-border management strategies that focus on either preventing the introduction of specific priority species or managing specific pathways or vectors of concern (to reduce propagule and colonisation pressure (Lockwood et al. 2005, 2009)). Examples of these interventions include inspections at ports of entry that aim to prevent the introduction of potentially harmful species (Liebhold et al. 2006; McCullough et al. 2006) and policies that require foreign vessels entering a region to exchange and/or flush their ballast tanks with mid-ocean saltwater (Bailey et al. 2011). As it would be impossible to create different legislative instruments for the vast number of pathways that exist, frameworks with broad, inclusive categories are most useful for legislation and policy (Hulme et al. 2008). However, to provide information for direct interventions, more detailed categories that are explicitly constructed with interventions in mind may be required.

Another important role of frameworks is to assist with the standardised monitoring and reporting of biological invasions at different scales (Latombe et al. 2017; Wilson et al. 2018; Groom et al. 2019). Indeed, the CBD framework has been proposed as an explicit component of global monitoring for biological invasions and its vocabulary has been proposed as part of the Darwin Core biodiversity standards (Latombe et al. 2017; Groom et al. 2019). As pathways of introduction vary across regions and countries (Essl et al. 2015), it would be difficult to create a detailed framework that comprises the introduction pathways that are important for all regions and, therefore, frameworks with broad, inclusive categories are likely to be most useful for monitoring and reporting, particularly at the global scale.

Finally, frameworks can be incorporated in risk analyses (Kumschick et al. 2020) and horizon-scanning exercises (Matthews et al. 2014, 2017; Tsiamis et al. 2020) to make predictions concerning future invaders and their pathways of introduction and to determine what can be done to prevent future introductions. Frameworks with detailed categories are likely to be most useful for this purpose and, indeed, the detailed sub-categories of the CBD framework have been used in horizon-scanning exercises (e.g. Tsiamis et al. 2020).

In order for a framework to achieve all of these purposes, we suggest that it should have five properties, that we have termed the CAGED properties. Frameworks must be: Compatible, Actionable, General, Equivalent and Distinct (CAGED) (Table 1). We have not attempted to rank or weight these properties as their relative importance will vary depending on the context for which the framework is developed, but we argue that all are required for a framework to be effective. Whether a framework possesses the CAGED properties could also vary, based on the context of interest; for instance, a framework developed for regional use could possess the CAGED properties within that context, but not at a global level. Note these properties are, we believe, discrete. For example, if introductions are described by more than one category, then the framework is too detailed and is not compatible with the available data. Nonetheless, the categories might still be applicable across different taxonomic groups, habitats, regions and time periods and so the framework has generality. In contrast, a category could be compatible with the available data, but will not be general if it is only applicable to one type of organism, habitat or region.

Based on the CAGED properties, we evaluated the framework used by New Zealand’s biosecurity surveillance system, the six main categories of the CBD framework and the 44 sub-categories of the CBD framework (Table 2 and for details on each framework, see Figs 1, 2). We aimed to assess whether a diverse set of frameworks exhibited the CAGED properties at a global level and so these frameworks were selected as they vary in their level of detail, in the approach followed for their development and in the political level for which they were developed (Table 2). The main categories and sub-categories of the CBD framework were assessed separately as they were developed separately, using different methods. Furthermore, assessing the levels separately meant that the usefulness of each level could be determined, and issues that are specific to each level of the framework could be identified.

An introduction pathway classification framework will likely be an important tool in efforts to track progress towards meeting the Convention on Biological Diversity’s post-2020 target on invasive alien species (Essl et al. 2020). However, the current CBD framework, or at least the framework’s sub-categories, has few of the properties that such a framework should have (i.e. the CAGED properties). The relative importance of the CAGED properties will vary depending on the context for which a framework is developed. However, all the CAGED properties are required if frameworks that are developed in a global context (like that proposed by the CBD) are to be effective. We present four scenarios for how the identified issues could be addressed, with the aim of ensuring the development of a global level framework that has categories that are compatible, actionable, general, equivalent and distinct (i.e. CAGED).

1 Refine the current CBD framework: make adjustments as required and create a process for updating and adapting the framework so that it can better respond to the needs of the users. As the framework was developed within the last six years, there may not have been sufficient time for the framework to be adequately tested and for wrinkles to be identified and ironed out. The framework has already been put into practice and so, this would be the simplest way to move forward. As a start, the adjustments and recommendations proposed by Harrower et al. (2018) and Pergl et al. (2020) could be widely tested and adopted and the terminologies could be better aligned with those used in related fields (see Paap et al. 2020). The effort that would be required to implement any changes (i.e. the re-assignment of data) would also need to be considered. Furthermore, before being put into effect, the changes would need to be tested and widely accepted, ideally published in a peer-reviewed journal, recognised by the CBD and maintained as a standard (Wilson et al. 2020-a).

2 Develop a new framework: design a new framework that has categories that are CAGED at a global level. The development of a new framework should ideally be based on first principles and there would have to be a process to obtain consensus from the global community on interpretations of categories and their definitions (Tsiamis et al. 2017). Before adoption, the framework would need to be thoroughly tested using data from a wide range of regions, taxonomic groups and habitats. This test should involve a number of individuals from all groups of stakeholders that would apply the framework (i.e. more than one person should classify pathways using the framework) and the consistency of their classifications should be assessed. This would require a considerable amount of work and records in existing databases that have been classified using the CBD framework would have to be re-classified. This new framework would ideally be backwardly compatible with the CBD framework, though this would be undesirable if it came at the expense of the CAGED properties.

3 Develop context-specific frameworks: biological invasions are not managed at a global scale and so a single global framework may not be appropriate. For example, in South Africa, most known introductions for which a pathway was recorded have been assigned to only four of the CBD framework's sub-categories (‘horticulture’, ‘biocontrol’, ‘agriculture’ and ‘other escape’) and there were no introductions through ten of the 44 sub-categories (Suppl. material 1: Fig. S3). Therefore, in the South African context, it is inappropriate to manage many of the pathways in the CBD framework and reporting on them could be misleading. For example, there has only been one known introduction to South Africa for fur farming and so, while technically one can report that this pathway is managed, this is misleading (Table 1). The CBD framework may also not be appropriate in the context of secondary dispersal (i.e. dispersal of an alien species after introduction; but see Pergl et al. 2020) and so, a separate framework may be required to monitor and report on the movement of alien species post-border. Therefore, context-specific frameworks could be more valuable. As context-specific frameworks would make data exchanges and cross-cutting research more difficult and labour intensive, they should ideally align.

4 Use a hybrid model: Use the six main categories of the CBD framework (possibly with the recommendations of Pergl et al. (2020) and Paap et al. (2020) incorporated following testing), but encourage countries to develop their own sub-categories (e.g. that are explicitly relevant for local management) or adapt the sub-categories so that they meet their needs. Countries could report to international organisations at the main category level, but context-specific variation would be accommodated at lower levels of the framework. This would additionally allow countries or regions (e.g. Europe) that have adopted the CBD framework to continue to use it in its entirety.

In conclusion, the main categories of the CBD framework have all of the desirable properties of an introduction pathway classification framework, but the sub-categories have few and so there is a need for improvement. Whether one of the four scenarios listed above is the best way to move forward or whether a different approach is preferable, will require further discussion. Even in the absence of most CAGED properties, all current frameworks can help to improve our understanding of biological invasions. However, to facilitate cross-cutting analyses, provide information for policy and enable reporting to national and international organisations, a classification at a higher level using a few, inclusive categories that fulfil the CAGED properties appears most appropriate. While higher level categorisation can also provide information for management interventions that aim to prevent introductions and facilitate the prediction of threats, detailed, context-specific categories may be more effective in these instances. Thus, our view is that, while it is possible to refine or fundamentally recast the CBD framework, a universal framework may simply be too general to ever be useful in specific applied contexts. As such, we believe a hybrid model – a few general categories at the global scale and context-specific sub-categories driven by local needs at a regional level – may be the most appropriate.

This paper emerged from the workshop on ‘Frameworks used in Invasion Science’ hosted by the DSI-NRF Centre of Excellence for Invasion Biology in Stellenbosch, South Africa, 11–13 November 2019, that was supported by the National Research Foundation of South Africa and Stellenbosch University. The South African Department of Forestry, Fisheries and the Environment (DFFtE) are thanked for funding, noting that this publication does not necessarily represent the views or opinions of the DFFtE or its employees.