Our understanding and management of biological invasions relies on our ability to classify and conceptualise the phenomenon. This need has stimulated the development of a plethora of frameworks, ranging in nature from conceptual to applied. However, most of these frameworks have not been widely tested and their general applicability is unknown. In order to critically evaluate frameworks in invasion science, we held a workshop on ‘Frameworks used in Invasion Science’ hosted by the DSI-NRF Centre of Excellence for Invasion Biology in Stellenbosch, South Africa, in November 2019, which led to this special issue. For the purpose of the workshop we defined a framework as “a way of organising things that can be easily communicated to allow for shared understanding or that can be implemented to allow for generalisations useful for research, policy or management”. Further, we developed the Stellenbosch Challenge for Invasion Science: “Can invasion science develop and improve frameworks that are useful for research, policy or management, and that are clear as to the contexts in which the frameworks do and do not apply?”. Particular considerations identified among meeting participants included the need to identify the limitations of a framework, specify how frameworks link to each other and broader issues, and to improve how frameworks can facilitate communication. We believe that the 24 papers in this special issue do much to meet this challenge. The papers apply existing frameworks to new data and contexts, review how the frameworks have been adopted and used, develop useable protocols and guidelines for applying frameworks to different contexts, refine the frameworks in light of experience, integrate frameworks for new purposes, identify gaps, and develop new frameworks to address issues that are currently not adequately dealt with. Frameworks in invasion science must continue to be developed, tested as broadly as possible, revised, and retired as contexts and needs change. However, frameworks dealing with pathways of introduction, progress along the introduction-naturalisation-invasion continuum, and the assessment of impacts are being increasingly formalised and set as standards. This, we argue, is an important step as invasion science starts to mature as a discipline.

CBD introduction pathway classification framework, Environmental Impact Classification for Alien Taxa (EICAT), invasive alien species, invasive species, Socio-Economic Impact Classification of Alien Taxa (SEICAT), Unified Framework for Biological Invasions

The study of biological invasions has a rich history of developing and refining hypotheses, frameworks, theories, and other conceptual constructs with the aim of assisting with resolving particular problems and in some cases moving beyond case studies (observations of a small number of invasive taxa, invaded habitats or invasion events) to arrive at generalisations or principles that apply more widely. These conceptual constructs often link insights from fundamental research to policy and management responses. Frameworks, in particular, are an important way to communicate concepts and ideas between people. As humans, we like to structure the world around us; to some extent frameworks are scientific models of how we think the world works that allow us to test our ideas, debate edge cases, and build new hypotheses. Just like any scaffolding, frameworks are intended to be built upon. Reviewing developments in “implementation science”, Nilsen (2015) posits that a framework usually denotes “a structure, overview, outline, system or plan consisting of various descriptive categories, e.g., concepts, constructs or variables, and the relations between them that are presumed to account for a phenomenon”. In this typology, frameworks are not explanatory but “only describe empirical phenomena by fitting them into a set of categories”. Frameworks are especially useful when they are used to collate, organise, combine, simplify, and synthesise a large volume of new information; to classify and integrate insights from various perspectives and disciplines; to bridge gaps between science and policy and between disciplines; and to provide roadmaps to guide further research inquiries. These endeavours all rely on frameworks to circumscribe and classify the problem.

However, as invasion science originates from various discipline-specific questions and problems, attempts at circumscription and classification have arisen from multiple different origins. These differences in origin largely align with traditional disciplinary boundaries (zoology, botany, marine biology) and debates (e.g., utilisation vs. protection or humans as a part of nature vs. humans as a threat to nature). In consequence, there are a plethora of terminologies, differences in emphasis, and similar ideas are expressed in slightly different formats. Taking the development of hypotheses as an analogy, many hypotheses used to explain aspects of biological invasions overlap, some are vague, and some can be collapsed to general ecological theories that need not be related to biological invasions at all (Catford et al. 2009; Enders et al. 2020). Furthermore, as only a few hypotheses in invasion science have attracted sustained attention, few hypotheses have consistent and strong evidentiary support (Jeschke et al. 2012; Ricciardi et al. 2013; Traveset and Richardson 2020). As such, it is difficult to differentiate between hypotheses that provide insights into the processes at play and those that should not be the basis for developing scientific models and management recommendations as they are misleading (Crystal-Ornelas and Lockwood 2020).

The challenge for those working on invasion frameworks is similar–that of demonstrating the utility of frameworks, being clear as to the contexts under which particular frameworks apply, and adapting (or abandoning) frameworks in response to new evidence or needs. Frameworks are needed both to address particular specific problems, to improve general understanding, and ideally to facilitate the transfer of lessons learnt from the general to specific and vice versa (Lawton 1996). This tension between generalisation and utility is crucial [cf. invasion syndromes for one practical approach of addressing it (Novoa et al. 2020)]. The context-dependency of the biological invasions phenomenon means there is substantial value in taking an idiographic approach, i.e., studying case by case to uncover mechanisms and consequences (Simberloff 2004); tailored frameworks can be very valuable in such cases. However, a major goal of some frameworks has been to facilitate generalisations and comparisons across scales, taxa, and biological realms, and more broadly to formalise frameworks as standards that are intended to be used by all stakeholders involved (Box 1). For example, the so called Unified Framework for Biological Invasions aimed to link frameworks developed by botanists and zoologists (Blackburn et al. 2011); the EICAT impact classification framework (Blackburn et al. 2014), which has been recently adopted as a standard of the IUCN, aims to facilitate the measurement and reporting of invasive species impacts in a consistent manner (IUCN 2020) (Box 2); and the CBD has proposed an introduction pathway classification framework that bridges decades of debate on how invasive species are transported out of their native range (CBD 2014) (Box 3). These frameworks have been proposed to be incorporated into biodiversity standards (Groom et al. 2019) with a view to developing a standardised system for monitoring and reporting on biological invasions that can be applied across scales from local to global, across habitats from coral reefs to mountain tops, across taxa from fungi to ferns to frogs, and across pathways from hitchhikers on plastic debris to seeds sent through e-commerce (McGeoch and Jetz 2020). There is thus some evidence that invasion science is coalescing around a few frameworks and formalising them as standards (Boxes 1–3). However, the frameworks are still rarely explicitly used in practice (Wilson et al. 2020, this issue), and our experience when applying the most commonly cited frameworks to real data and situations has been that they are very useful but that there are a number of practical challenges to be resolved, some of which are fundamental to the field (see the section on ‘Putting frameworks to the test’ below).

So, are current frameworks fit for purpose? How do they perform in practice? Can they be adapted to deal with new contexts? Do they need to be revised and adapted to deal with new information? Can frameworks be linked together to facilitate the transfer of lessons learnt from the general to the particular? What gaps are there that need addressing?

In light of rapid developments in the field, we decided it was important to take stock and assess the current state of frameworks used in invasion science. We invited a wide range of researchers focussing on biological invasions to a workshop to discuss, develop, and revise ideas. In particular, we asked prospective attendees to develop draft manuscripts before the meeting with the aim of formalising their thoughts and sharing them in advance of the discussions. The workshop itself, ‘Frameworks used in Invasion Science’, was held 11–13 November 2019 in Stellenbosch, South Africa, and was hosted by the DSI-NRF Centre of Excellence for Invasion Biology (for details of the workshop and how the special issue developed see Suppl. material 1).

One of the main areas of discussion at the workshop was to define what is meant by a framework and to clarify the overall aim of such a framework. It was felt that frameworks should be useful, and the broader, the better. However, generalisations are only worthwhile if they do not come at the cost of the utility of the framework for its original purpose. Frameworks are often used for purposes for which they were not initially intended and in some cases for which they are not suited (see examples in Wilson et al. 2020, this issue). Frameworks created in one context and naively used in other contexts might mean important details are missed by those applying the frameworks or that the problem is made much more complicated than it actually is. In other words, the sensitivity and specificity of frameworks are not always clear.

Over the course of the workshop, the question ‘What is a framework?’ was repeatedly debated, with such debate providing a valuable anchor for our discussions. We eventually settled on the following working definition:

A framework is a way of organising things that can be easily communicated to allow for shared understanding or that can be implemented to allow for generalisations useful for research, policy, or management.

Building on this, we developed an overall goal of the workshop, dubbed ‘the Stellenbosch Challenge for Invasion Science’:

Can invasion science develop and improve frameworks that are useful for research, policy or management, and that are clear as to the contexts in which the frameworks do and do not apply?

A major goal of the workshop was for participants to formalise their thoughts in manuscripts, and to ‘stress-test’ the frameworks–indeed a survey conducted as part of this special issue found that while invasion scientists feel some of the major frameworks are very influential, the frameworks still lack serious critical examination (Wilson et al. 2020, this issue). The 24 papers in this special issue revisit many of the philosophical underpinnings and practical challenges associated with attempts to integrate, reconcile, and synthesise thoughts and concepts in invasion science (Appendix 1).

In achieving these aims, this special issue, we argue, addresses the Stellenbosch Challenge. The papers address the utility of frameworks for research, policy, and management; they clarify the contexts in which the frameworks do and do not apply; and they discuss how the frameworks need to be developed and improved to facilitate shared understanding. In particular, the special issue addresses all these above issues with respect to the rapidly developing field of impact assessment.

A framework is a way of organising things that can…allow for shared understanding

A notable emerging feature of this special issue is that while the papers cover a wide range of topics, taxa, habitats, and environments, there is some evidence of a growing consensus. Together, the 24 papers of the special issue cite well over a thousand different publications, but many of the papers cite the same handful of frameworks (Fig. 1, Appendix 1). The authors of this special issue are certainly not divided into distinct camps that use different frameworks. The leading frameworks are widely cited and highly influential (Wilson et al. 2020, this issue). Moving forward, we posit that it is critical to ensure frameworks in invasions’ science are designed to also respond to the multitude of growing, changing, and interacting global change drivers under which biological invasions are playing out. For example, Robinson et al. (2020, this issue) highlight how climate change will have dramatic and varied impacts on biological invasions that will require new ways of thinking, emphasising the imperative of collecting foundational data and monitoring change. And, as outlined by Sinclair et al. (2020, this issue), frameworks should be explicit in how humans affect biological invasions, and how biological invasions affect humans.

Figure 1.

A citation network of papers within this special issue. Each node represents an article, with the node radius proportional to the number of citations. Citations between papers within the special issue have been excluded, and this editorial was not included at all. Numbered nodes are papers in the special issue (Appendix 1) and lettered nodes are the 15 articles that were cited six or more times in the network. The colours represent different modularity classes of the network (the light green one appears to be related to impact assessments). Of the 1520 papers cited 87.2% were only cited by one paper in special issue, and less than 1% were cited by four papers. This network can thus be seen as indicative of a wide-ranging field linked by a few key frameworks, though the nature of the special issue and the authors involved means there are some significant biases and self-selections occurring (which, we expected, would have biased the network towards being more connected than it would otherwise be). The network was built in Gephi (0.9.2). A Bacher et al. (2018), Socio-economic impact classification of alien taxa (SEICAT); https://doi.org/10.1111/2041-210X.12844 B Blackburn et al. (2011); A proposed unified framework for biological invasions; https://doi.org/10.1016/j.tree.2011.03.023 C Blackburn et al. (2014); A Unified Classification of Alien Species Based on the Magnitude of their Environmental Impacts; https://doi.org/10.1371/journal.pbio.1001850 D Evans et al. (2016); Application of the Environmental Impact Classification for Alien Taxa (EICAT) to a global assessment of alien bird impacts; https://doi.org/10.1111/ddi.12464 ental Impact Classification for Alien Taxa (EICAT); https://doi.org/10.1111/ddi.12379 F Hulme et al. (2008); Grasping at the routes of biological invasions: a framework for integrating pathways into policy; https://doi.org/10.1111/j.1365-2664.2007.01442.x G Latombe et al. (2017); A vision for global monitoring of biological invasions; https://doi.org/10.1016/j.biocon.2016.06.013 H McGeoch et al. (2016); Prioritizing species, pathways, and sites to achieve conservation targets for biological invasion; https://doi.org/10.1007/s10530-015-1013-1 I Nentwig et al. (2016); The generic impact scoring system (GISS): a standardized tool to quantify the impacts of alien species; https://doi.org/10.1007/s10661-016-5321-4 J Richardson et al. (2000); Naturalization and invasion of alien plants: concepts and definitions; https://doi.org/10.1046/j.1472-4642.2000.00083.x K Seebens et al. (2017); No saturation in the accumulation of alien species worldwide; https://doi.org/10.1038/ncomms14435 L Wilson et al. (2009); Something in the way you move: dispersal pathways affect invasion success; https://doi.org/10.1016/j.tree.2008.10.007 M Pyšek et al. (2008); Geographical and taxonomic biases in invasion ecology; https://doi.org/10.1016/j.tree.2008.02.002 N Kesner and Kumschick (2018); Gastropods alien to South Africa cause severe environmental harm in their global alien ranges across habitats; https://doi.org/10.1002/ece3.4385 O Harrower et al. (2017); Guidance for interpretation of CBD categories on introduction pathways.

While each paper in the special issue tackles specific parts of the Stellenbosch Challenge and draws from particular frameworks (Appendix 1), the frameworks themselves are not explicitly linked. At the workshop there was substantial discussion on whether the Stellenbosch Challenge could be satisfied by the creation of a single all-encompassing framework. However, there was general consensus that the frameworks do not always align, nor should they be forced to do so (cf. the comparison of EICAT and the Red List by Van der Colff et al. 2020, this issue). There was agreement that it is more realistic to aim for a hierarchy of frameworks where important contextual detail is nested within the overarching ideas, rather than aiming for an ‘über-framework’ that tries to embrace all contexts. The analogy with hypotheses in invasion science is again pertinent. Jeschke and Heger (2018) very elegantly demonstrate the value of the hierarchy of hypotheses approach to organise ideas within invasion science. A similar approach to frameworks in invasion science would help clarify how frameworks constructed to respond to particular needs or contexts are related to each other, how they can share ideas and approaches, and to identify gaps where new frameworks might be valuable. We present a tentative sketch of the inter-connection of existing frameworks in Figure 2.

Figure 2.

A tentative hierarchical structure linking frameworks in invasion science. Three levels are proposed here, though details at the finest level are only shown for one component—the part of the Unified Framework that addresses transport across the geographic barrier. Numbers represent papers in the special issue as per Appendix 1 and are placed according to how they fit in with the existing frameworks. Papers touch on multiple aspects and different hierarchical levels. However, each paper is only indicated once on the diagram at the place we feel it contributes the most; except for 19. Sinclair et al., which, to highlight how some studies are cross-cutting, is plotted on levels A and B, and, as the paper also discusses how the transport process should be viewed as a coupled-human natural system (CHANS), it could arguably have been plotted on level C as presented here as well. At level B (the core of invasion science), there are well established frameworks for the impacts of species and the introduction-naturalisation-invasion continuum, but there are no equivalent well recognised frameworks for interventions (or the impacts on sites). The intervention activities shown are based on the categories used by the Cambridge Conservation Forum framework for evaluating projects with the addition of a pathway management activity (Kapos et al. 2008).

At a broad scale (Fig. 2A), frameworks in invasion science should link to other drivers of global change (Robinson et al. 2020, this issue, #18 on Fig. 2A), other areas of biological research (7. Hulme et al. 2020, this issue; 13. Paap et al. 2020, this issue), and to societal issues more generally (15. Potgieter and Cadotte 2020, this issue). These linkages can be made within the understanding that biological invasions can be viewed through the prism of pathways, species or sites (4. Essl et al. 2020b, this issue). These linkages can also be made recognising that the phenomenon involves bio-geographical and ecological processes (e.g., the introduction-naturalisation-invasion continuum), that there are environmental and societal impacts (e.g., the ICAT frameworks), and that biological invasions pose societal challenges that require political and management responses. These three issues (biogeography, impacts, and interventions) arguably form the core of invasion science (Fig. 2B). However, these issues are not always congruent. For example, while impact, abundance, and geographical distribution are often correlated, alien species can have massive negative impacts without forming a naturalised or invasive population (Ricciardi et al. 2013), and widespread, abundant invaders [‘successful’ as per 10. Latombe et al. (2020, this issue)] might have negative impacts scored as Minor or Minimal Concern under the ICAT frameworks (Ricciardi and Cohen 2007). Also, while pathways of introduction represent an important elucidation of the first stage of the invasion process, they do not necessarily map neatly on to pathways of spread within a region (6. Faulkner et al. 2020, this issue; 14. Pergl et al. 2020, this issue, 11. Liebhold et al. 2020, this issue). It is important, therefore, to ensure that the domain of applicability and relevance of each framework is clear, and that if linkages are made these are done without compromising the original purpose for which the framework was constructed. As an example, a recent effort to link interventions to the introduction-naturalisation-invasion continuum forces management terms on to invasion stages and barriers, conflates activities, goals, and objectives, and in so doing neglects the primary purposes of the framework, which is to facilitate interventions (Robertson et al. 2020).

Finally, it is important to zoom into on-the-ground management needs to ensure that there are frameworks that can be used to improve our understanding and management of particular issues (one example is shown in Fig. 2C). Context-specific frameworks are needed: for management prioritisation in Italy and Brazil (1. Bertolino et al. 2020, this issue; 24. Ziller et al. 2020, this issue); to support decisions regarding the listing of alien species under South African regulations (9. Kumschick et al. 2020b, this issue); to provide clarity regarding the risks and appropriateness of regulating horticultural cultivars (3. Datta et al. 2020, this issue); and to ensure that the monitoring of alien plant species in Hawai’i is relevant to management (2. Brock and Daehler 2020, this issue). It might be possible to extend such frameworks to similar contexts, but ultimately if those frameworks are not well suited to the problem they were designed to address, then they need to be adapted or abandoned.

The need to zoom in and out to different spatial or thematic scales is currently being developed further by workshop participants with a view to producing a hierarchy of frameworks. Parallel to this work, workshop participants are reviewing the history of frameworks in invasion science and developing a typology to classify them. Finally, participants felt a natural conclusion of the workshop would be to refine and recast some existing frameworks. Specifically, participants suggested that the frameworks used to classify populations according to their stage along the introduction-naturalisation-invasion continuum (Blackburn et al. 2011) and the CBD’s introduction pathway classification framework (Box 3) deserved renewed attention. We believe that this special issue provides a necessary precursor to these important products.

We could, of course, not address all issues related to invasion frameworks at the workshop or in this special issue. There are notable gaps in the implementation of existing frameworks that deserve much more attention, for example the need for: frameworks to be modified so that they are relevant to different ecological contexts (e.g., freshwater, marine, micro-organisms); a way to incorporate expert opinion in transparent and standardised ways; and methods to apply frameworks when biogeographic and administrative boundaries do not align. There is also a need to consider if existing frameworks can be applied to address broader issues such as invasions at the gene level and range shifts resulting from climate change or other human modifications of the environment (e.g., managed relocation, assisted migrations). These gaps in the ability of frameworks to deal with different contexts impact our ability to monitor and report on invasions [e.g., see Zengeya and Wilson (in press) for South Africa].

Biological invasions are a central factor in global environmental change as they impact, and are impacted by, climate, ecosystem functions and services, and species extinction (Vitousek et al. 1997; Ricciardi et al. 2017). One potential avenue for further work is to try to link frameworks in invasion science more clearly to broader issues within conservation biology, community ecology, evolutionary biology, and global environmental change. Similarly, alien species are now regularly incorporated into foundational ecological and evolutionary science as ‘probes’ that can effectively test core tenets in these fields (e.g, Strauss et al. 2006; Vellend et al. 2007; Guisan et al. 2014) and ‘biological assays’ to test dominant paradigms in biogeography (Rouget et al. 2015). If invasion frameworks are to facilitate interdisciplinarity they must be clearly articulated by invasion scientists in collaboration with researchers in other fields to ensure that the definitions and processes that these frameworks capture can be understood and adopted across the various disciplines. However, of the ~110 authors of this special issue, we estimate around 70% would describe themselves as having biological invasions as a main interest or responsibility and about two-thirds are primarily based at a university. All but a handful are ecologists, highlighting the ongoing need for better integration with other disciplines and the social sciences in particular. When invasion frameworks do not successfully bridge disciplines and provide a link between research and implementation, then it seems inevitable that there will be points of confusion and tension (Richardson and Ricciardi 2013), leading to the reinvention or 'creative' use of terminology within allied fields (Essl et al. 2020a; Wilson 2020), and lost opportunities for reciprocal advancement in knowledge (Hulme 2014). We see this articulation between disciplines using invasion frameworks as particularly pertinent given the expected massive shift in species ranges due to climate change and the increasing frequency with which we are confronted with emerging infectious diseases in human and non-human populations (Ogden et al. 2019; Nunez et al. 2020).

Finally, if, as per our working definition, a framework should “…be easily communicated to allow for shared understanding…” then at least some frameworks should also be valuable aids for communicating between invasion scientists and the people and industries that are impacted (negatively and positively) by alien species. Arguably, one of the most effective communication tools in invasion science is the invasion curve [The invasion curve is a roughly logistic shaped curve of ‘area infested’ or something similar plotted against time. It is split into different stages with different management actions highlighted. It is perhaps best exemplified by the version of the Department of Primary Industries (2010)]. However, this simplifies the issue and so is not useful as a framework in practice. Similarly, the impact equation of Parker et al. (1999) captures the essence of the problem—impact is the product of the range size of a species, its abundance per unit area, and the effect per individual or per biomass unit–—but is also not easy to implement in practice (Blanchard et al. 2011). Many frameworks within the field of invasion science might be primarily about facilitating communication between invasion scientists, however it will be valuable to also have frameworks that clarify key aspects of biological invasions in a way that links the ‘nuts and bolts’ of invasions with societal priorities, and to create or modify frameworks in invasion science so they are easily understood not just by scientists, policy makers, and managers, but also by broader stakeholders.

It appears from the set of articles in this special issue that invasion science is maturing as a distinct discipline. The process of developing, refining, and increasingly implementing frameworks suggests the field is moving from ‘storming’ to ‘norming’ [to paraphrase a framework from psychology (Tuckman 1965)]. If we assume that trends in globalisation will continue, as seems almost certain, there is now strong evidence that the taxonomic variety and number of species that have the opportunity to establish as aliens somewhere on Earth will continue to rise (Seebens et al. in press). In this context, frameworks in invasion science should be viewed as tools that are worthwhile only if they are used and are useful. It is important that frameworks are increasingly tested (be it within policy, research or management settings) and any limitations clearly shared with others in and outside the field. We suspect that the overriding importance of context in invasion science will continue to be the rule rather than the exception, and that frameworks will need to adapt to these contingencies to remain useful. We consider this approach to be encompassed by the ‘Stellenbosch Challenge for Invasion Science’. We believe the articles within this special issue (Appendix 1) show how responding to this challenge can improve our understanding of, and responses to, biological invasions.

The urgent need to reduce carbon emissions meant that several people decided not to attend the workshop in person. Moreover, the review and revision of the articles published in this special issue happened against the backdrop of the COVID-19 pandemic (Nunez et al. 2020). While face-to-face workshops like the one described here might be less common in future, we hope that they will soon be possible again. A beautiful venue, good food, and stimulating company will not resolve biological invasions, but they make the process that bit more enjoyable (Figure 3).

Figure 3.

Workshop attendees benefitting from the shade of an alien Eucalyptus sp. on a transformed lawn.

This special issue emerged from the workshop ‘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. We thank all the workshop participants, those who contributed but could not attend, and all those who reviewed manuscripts for their ideas, inspiration, and hard work. Tumeka Mbobo and the staff of the CIB, especially Jean Lategan, are thanked for their help in planning and running the workshop and the special issue. We would also like to thank the teams at Pensoft for their tireless work. DMR acknowledges support from the Oppenheimer Memorial Trust (grant 18576/03). JRUW, SK, and TAZ thank the South African Department of Forestry, Fisheries, and the Environment (DFFtE) for funding, noting that this publication does not necessarily represent the views or opinions of DFFtE or its employees. QG thanks the Belgian Science Policy Office's Brain program (BelSPO BR/165/A1/TrIAS) and the visiting fellowship programme of the DSI-NRF Centre of Excellence for Invasion Biology for funding. SB thanks the University of Fribourg for funding. Dan Simberloff, Ingolf Kühn, and Phil Hulme provided valuable comments on a draft of this editorial.