Transdisciplinarity, and allied approaches, can be contrasted with basic, applied, disciplinary, multi-disciplinary, and interdisciplinary research. We developed a decision tree to help researchers and funders distinguish TDR from other research modes (Figure 1).

Basic or pure science is concerned with “conceptual or methodological problems for a discipline to describe and explain processes by means of models and methods, which are taken to be universally valid under controlled conditions” (Hirsch Hadorn et al., 2006) [pp. 124]. While it is not directly concerned with societal goals such as informing environmental policy and practice, basic science can support the objectives of applied research (Hirsch Hadorn et al., 2006). Disciplinary and multidisciplinary applied research do focus on solving problems, differing mainly on the number and variety of disciplines included. Non-academics can also be strongly involved in these research modes, making them closely linked to a “participatory action research” approach, for example.

Multidisciplinary research tends to retain disciplinary autonomy over methodologies, with results being collated with limited integrating synthesis (Max-Neef, 2005; Wickson et al., 2006). Conversely, interdisciplinary research generally involves multiple disciplines sharing problem formulation and a methodological approach (Wickson et al., 2006). Interdisciplinarity is often considered as a feature of TDR (Wickson et al., 2006; Jahn et al., 2012; Lang et al., 2012; von Wehrden et al., 2019). Nevertheless, they differ in the prominent inclusion of non-academic actors in the latter (von Wehrden et al., 2019). The commitment between participants from research institutions and other stakeholders in TDR involves collaboration in most research stages, including problem definition and goal setting (Thompson Klein, 2004). TDR may also require: the group of participants to be representative of a particular community; the project process to involve a sufficient level of interaction between researchers and stakeholders, often resulting in the co-production of knowledge (Norström et al., 2020); and stakeholder inputs to be incorporated transparently (Wiek et al., 2014). These TDR features (explicit societal goal, interdisciplinary, and participatory) can encourage policy makers and managers to take up the results of research.

Setting the research scope and defining the research problem are particularly important stages, since they will influence, for example, research funders’ ability to judge whether a project has been developed in collaboration with users, and whether a TDR approach would be beneficial in addressing a problem. For example, the research scope may be defined very narrowly (e.g., fish biology), implying that a certain problem does not require a TDR approach, which is often true. However, setting the scope in collaboration with research users and participants may reveal a broader research problem that could benefit from a TDR approach. Therefore, when judging whether a project’s scope (and thus the corresponding research approach) was set appropriately, funders may use the “problem solving” feature of TDR as a criterion, critically enquiring what the core problem is, and whether the intended project outputs effectively contribute to a solution. This requires the assessor to know the research context to be able to judge such matter.

Projects using a non-TDR approach may be important components of a larger TDR program (e.g., Campbell et al., 2015). Thus, if funders are interested in supporting TDR, they may need to distinguish independent research projects from broader programs. For example, an “umbrella” environmental TDR research program on Integrated Water Management may include non-TDR projects to address basic knowledge gaps (e.g., on fish ecology). A research question such as “what does this fish eat?” can be addressed by pure disciplinary approaches. Nevertheless, it may be contributing to addressing broader questions such as “what are the consequences of water management in a river for people who rely on that fish?” In such a case, project assessments may consider whether there are clear links at program level that allow individual projects to contribute to broader research questions and ultimately address broader research problems.

We now turn to the features of the research problem and context where a TDR approach could be more appropriate than the alternative research modes.

TDR is often adopted in response to complexity (Hirsch Hadorn et al., 2006; Simon and Schiemer, 2015; Fritz and Meinherz, 2020; Verwoerd et al., 2020). A complex issue likely involves multiple interacting variables, with multiple feedback loops, which could be either positive or negative. This results in difficulty discerning cause-and-effect relationships, and high uncertainty about the consequences of certain actions. Complex systems include those defined based on interactions between social, economic, ecological and political systems (Simon and Schiemer, 2015), or where there is high uncertainty about the “drivers of change” of the system (Peterson et al., 2003). When issues emerge in such complex systems that require changes in societal practices, they may involve disputed values, high stakes and urgent decisions (Hirsch Hadorn et al., 2006).

The benefits of TDR for addressing complex problems with socio-political dimensions arise in part from its emphasis on participation and inclusiveness: “research on complex sustainability problems requires the constructive input from various communities of knowledge to ensure that the essential knowledge from all relevant disciplines and actor groups related to the problem is incorporated” (Lang et al., 2012 pp. 26). Secondly, the involvement of a mix of disciplines, including social and natural sciences, is often needed to address complex socio-environmental issues. Some empirical problems, such as certain ecological models, can be highly complex but do not encompass societal problem solving, and so may not require an interdisciplinary approach. For example, biological research on the population dynamics of a single species could be addressed by disciplinary or multi-disciplinary research (Hirsch Hadorn et al., 2006).

In addition, most environmental management contexts are characterized by polycentric governance systems, with decision-making power distributed across local-state-national governments, and among both private and public enterprises across water, energy, food and other domains (Morrison et al., 2019; Pahl-Wostl et al., 2020). However, environmental decision support systems and tools often assume single types of decision makers, and hence are poorly suited for decentralized and polycentric decision-making contexts (Zulkafli et al., 2017). TDR can support the complex processes of negotiation, deliberation and social learning through which decisions arise in such contexts and build ongoing networks and adaptive capacity (Roux et al., 2017).

Environmental research often includes participants whose knowledge systems involve different worldviews, identities, practices and ethics. This may include, for example, biophysical scientists, social scientists, environmental practitioners, farmers, and the holders of Indigenous knowledge and other local knowledge (Tengö et al., 2017). Bridging or integrating such diverse knowledge types can be challenging (Reed and Abernethy, 2018). TDR, due to its participatory and inter-disciplinary features, can facilitate the co-production of knowledge between science and other knowledge systems. Drawing from a wider knowledge pool may achieve not only a more legitimate solution across multiple groups, but a more efficient and effective one. It can also give greater voice to those with non-dominant paradigm perspectives.

Researchers using TDR can employ diverse approaches to bridge diverse knowledge systems in the co-production of knowledge. Reed and Albernethy (2018) show how a professional facilitator can support research partnerships between scientists, policy advisors, and practitioners from diverse cultural backgrounds. Tengö et al. (2017) describe a Multiple Evidence Approach used within the Convention on Biological Diversity (CBD) and the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) to connect Indigenous and local knowledge systems with scientific knowledge systems. Their approach focuses on validation within, rather than across, those systems. Others similarly call for the inclusion of Indigenous and local knowledge as scientific paradigms in their own rights (e.g., Milgin et al., 2020), an idea that could be supported through the interdisciplinary feature of TDR. This would require de-colonising TDR (i.e., challenging the colonial practices that have influenced research in the past, and which are still present today) by including concepts such as Indigenous TDR, and Indigenous research methodology frameworks (Chilisa, 2017).

TDR can help researchers to navigate contested issues by bringing research participants together and providing multiple perspectives on the research problem (Schneider et al., 2019). Contestation is related to complexity, as it reflects a plurality of values and norms, and divergent perspectives on problem definition and acceptable solutions. It could also be present in research contexts with diverse knowledge systems such as colonial states. Developing research under such conditions can be challenging, especially when they include power imbalances between different social groups.

Power imbalances between different social groups associated with a research problem require the careful consideration of the ethical dimensions of research. Power imbalance does not necessarily relate to contestation, since lower power groups may be submissive or even unable to articulate their interests in a context of negotiation (Fritz and Meinherz, 2020). In situations of power imbalance, TDR may give greater voice to disadvantaged groups by enhancing the capabilities of research participants (what Fritz and Meinherz (2020) refer to as “power to”). It can also stimulate social learning to support research participants in using collective action to disrupt discourses and structures holding power (through what Fritz and Meinherz (2020) refer as “power with”). However, if not carefully managed, TDR can further entrench disadvantage by, for example, proposing changes to resource uses that ignore vulnerable or marginalised social groups benefiting from such practices (Blythe et al., 2018).

Participation is a key TDR feature supporting research in contested and power-imbalanced contexts. Indeed, Jahn et al. (2012) suggest, in their typology of problems according to the strength of agreement on knowledge and values, that the higher the contestation, the more stages of research participants need to be involved in (i.e., research scoping, design, data collection, etc.). In such cases, additional time and resources may be required, potentially including approaches such as stakeholder analysis, working with lower capacity groups, hiring professional facilitators or mediators, supporting reflexive meetings, and open discussion forums (Lang et al., 2012).

Contestation and power imbalance may also increase the risk that even a TDR approach will fail to deliver research or societal outcomes (e.g., a solution to the problem at hand). However, using TDR could still be a better option than alternative approaches since the mutual learning, even from failure, can be beneficial for future attempts of solving the problem (Fam and O'Rourke, 2020). Moreover, non-participatory environmental management research approaches might succeed at delivering research outputs in a highly contested context, all the while reinforcing power imbalances (Blythe et al., 2018).

TDR is particularly suitable to achieving solutions that require transformative changes, meaning substantial or fundamental changes producing a novel state rather than moderate adjustments to the existing system (e.g., adaptation) (Blythe et al., 2018). However, desired sustainable futures can be strongly contested by groups with asymmetric power (Fritz and Binder, 2020). In such cases, achieving consensus may not be possible or even desirable, because consensus “can mask plural notions about what the problem is exactly, what constitutes relevant evidence, and what, therefore, are considered appropriate solutions” (Blythe et al., 2018, pp. 1214).

In a review of 31 TDR projects, Schneider et al. (2019) identified three general mechanisms that allow TDR to contribute to transformations towards sustainability. First, by promoting different types of knowledge for more informed and equitable decision-making. Access to information can help, for example, stakeholders to understand and prioritize certain problems such as irreversible changes to socio-ecological systems. Second, TDR can foster social learning for collective action; and third, it can enhance research participants’ competences for reflective leadership. Thus, in projects aiming at transformations towards sustainability, an investment in TDR is more likely to be justifiable because it can link scientists and stakeholders into equitable partnerships that can mobilise the co-productive capacities (across cognitive, social, material and normative domains) required to generate the societal changes needed. Outcomes can potentially include overcoming conflict and an eventual shift in power (Hill et al., 2017).

Fritz and Binder (2020) analysed different types of power exerted by researchers, practitioners and funding bodies in five projects aiming to contribute to transformations towards sustainability. They showed how these actors can foster the second and third mechanisms described by Schneider and others above. Researchers often play a powerful role in initiating and controlling TDR by, for example, selecting participants and setting the rules of participation. Practitioners influenced the process by contesting the research rules, withdrawing from projects, and controlling researchers’ access to the field. Funding bodies influenced participation practices in the projects by stipulating the topical foci, the types of project leadership desired, and resource allocation. Through the funding regulations, funding bodies could foster the realisation of TDR ideals, including shared ownership, responsibility and co-leadership. Conversely, funding bodies could play ambiguous or partly incoherent roles by, for instance, calling for participation without adapting funding mechanisms accordingly (Fritz and Binder, 2020).

This project used scientific knowledge from the ecological and hydrological disciplines to determine the water requirements for the significant aquatic fauna and riparian plants (e.g., Beesley et al., 2020; Burrows et al., 2020; Beesley et al., 2021; Canham et al., 2021). Two main groups of non-academic actors were involved in the project: government scientists and planners, and Traditional Owners and Indigenous rangers. The project was designed with government planners to address key ecological knowledge gaps to inform a water allocation plan. The research was then undertaken by a collaborative team of university and government ecologists, hydrologists, statisticians, government policy-advisors, Traditional Owners and Indigenous rangers. Traditional Owners and Indigenous rangers advised on field sites and species of interest and jointly collected field data, and government staff also collaborated on the interpretation and publication of scientific findings. To improve its potential to address the current contestation around water allocation planning in the region, the outcomes of this ecological research were delivered in formats appropriate for both the government and Traditional Owners to ensure the different groups could use this information in discussions about water allocation in the catchment and the potential consequences for the environment.

This study built on the need, identified by the study “Socio-hydrological model, principles and key considerations”, to support better consideration of Traditional Owners’ ways of knowing and managing water. The study did so by collaboratively articulating Aboriginal water governance principles and identifying gaps in current settler colonial water policy for the consideration of these principles. A collaborative team of researchers (social scientists) and Aboriginal custodians of the Martuwarra/Fitzroy River waters worked over 3 years via iterative action research to produce a conceptual model of custodial relationships with Living waters. The same researchers also consulted with government water policy staff to produce a conceptual model of the water resource paradigm that is the basis of water policy in Australia (Laborde and Jackson, 2022). There were two main user groups involved in this study: government staff and Aboriginal custodians of the Martuwarra/Fitzroy River.

This study used published literature to 1) develop a conceptual model that represented the potential impacts of water resource development on the hydrological, social and ecological relationships of water regimes in the Fitzroy River, and 2) use the model to derive a set of principles and considerations to protect aquatic ecosystems and their dependent human cultures and livelihoods (Douglas et al., 2019). The study drew on disciplinary evidence from hydrological, ecological and social sciences to account for the complex interactions between cultural, ecological, socio-economic and governance systems involved. It was developed collaboratively by university researchers, government scientists and water policy staff, the latter being the primary non-academic actors involved in this desktop study as they required knowledge on the available evidence and knowledge gaps for planning consultation and decision-making processes for water allocation planning. Native Title has been determined over the near-entirety of the Martuwarra/Fitzroy River catchment. Thus, the study identified that building legitimacy in e-flows and water planning requires recognizing the diversity of knowledge systems existing in the region, thus including both the Indigenous and non-Indigenous governance and management systems operating at multiple scales.

This project aimed to support Traditional Owners to bring together both scientific and Indigenous knowledge to improve adaptive environmental management and decision-making in their traditional territories (Country) (Hill et al., 2021). Participatory construction of a 3D model of a river catchment, augmented with projected digital data, and diverse maps of power relations among social groups in the catchment, supported showing and sharing of different types of knowledge relevant to the governance and planning goals of the Traditional Owners. The project involved diverse knowledge systems, providing an opportunity for multiple Traditional Owner groups in the catchment to come together with one another, and with social and natural scientists, to strengthen their relationships and build trust. By sharing their knowledge and learning scientific and political knowledge together, Traditional Owners reported feeling empowered to use these knowledges to inform management and development decisions.

This project aimed to demonstrate how to operationalize participatory, multi-objective catchment planning. It included conservation planners, economists, ecologists and social scientists collaborating closely with a team of research users from Traditional Owner groups, government, the agricultural, mining and tourism industries, and environmental organizations (Kiatkoski Kim et al., 2021a; Álvarez-Romero et al., 2021). This group collaboratively constructed and assessed the outcomes of alternative development and management scenarios. Differences in perspectives, interests (i.e., contestation) and power were navigated by following a participatory scenario planning approach and investing significant time and resources on stakeholder engagement, a detailed understanding of the context, and by hiring a professional facilitation team knowledgeable of the region to assist in those tasks.

The studies were situated in the same socio-environmental system and operated concurrently. The project teams collaborated around the explicit aim to undertake a TDR approach to research, and such collaboration, including its detailed outcomes, are described elsewhere (Kiatkoski Kim et al., 2021b). All projects involved strong engagement with research users. However, their different aims and societal goals resulted in a varied number of disciplines, research users and knowledge systems being involved in each study. Thus, the location of different studies in the continuum between lower-and higher-TDR (Figure 2) generally depends on the scope of the study and on the strategies adopted to deal with the contextual features that are most relevant to its objectives. The outcomes of such studies also depended on their scope and the strength of their links to certain stakeholders. Studies 1–3 focused on water management, held close links with government (a key decision-maker), and thus have directly contributed to the Fitzroy catchment water allocation framework and to stakeholders’ feedback to government on this plan. Studies 4 and 5 had much wider scopes and focused on discussions broader than the water allocation plan. They contributed with outcomes such as fostering discussions about desirable and undesirable futures, and strengthening relationships and Indigenous institutions (Kiatkoski Kim et al., 2021b). The latter set of outputs might be less tangible than the former (and thus more difficult to measure objectively), but they can be equally as important for planning and decision-making processes. Last, it is also important to note that, in the studies portrayed, costs tended to increase (in proportion to study size) towards higher TDR approaches due to investments in addressing the features described in Section 3.