Introduction
Few decision support structures are available to help conservation practitioners and habitat managers determine whether an
established invasion should be controlled or simply monitored on a given land parcel within their jurisdiction. This is particularly challenging for those managing populations “at the late end of the invasion curve,” once the species has become established in an ecosystem (
Fig. 1A, stage IV). The term
established generally applies to invasive species capable of reproducing to maintain a self-sustaining population in the invaded region, but as recognized by
Kočovský et al. (2018), in practical terms it may be a code word that indicates whether an invasion is deemed manageable or not. As the extent of the invasion increases, the management costs expand while the likelihood of eradication shrinks (
Beric and MacIsaac 2015;
Green and Grosholz 2021). Management goals may then shift from the finite objective of eradication to sustained efforts towards population suppression, asset protection, and slow-the-spread strategies (
Forrest et al. 2009;
Larson et al. 2011;
Fig. 1). For our purposes, we adopt the pragmatic definition that an invasive species is
established once eradication is no longer the goal of management efforts (end of stage II in
Fig. 1). It does not necessarily follow that eradication is no longer possible, and eradication targets may be readopted because new tools, scientific understanding, resources, or even political will and social license are dynamic (
Kočovský et al. 2018).
Given limited resources and the realities of conservation triage (i.e., that limited resources should be allocated to achieve the greatest conservation benefit; sensu
Bottrill et al. 2008), we recognized the need for systematic guidance for when resource-intensive and potentially environmentally harmful treatment is warranted. Existing approaches to support decision-making and inform option prioritization in conservation planning fall short. Cost-benefit analysis, for example, is widely used in the risk assessments of aquatic invasive species (e.g., the global database InvaCost;
Leroy et al. 2022). Yet cost-benefit analysis has been criticized as both data-intensive and overly reductionist because it requires monetary values for all costs and benefits when it is recognized that ecological functions and processes are not easily or adequately monetized (
Wegner and Pascual 2011;
Hirsch Hadorn 2021). Alternative approaches, such as multi-criteria decision analysis (reviewed in
Liu et al. 2011), may better integrate non-monetary values and incorporate the multiple dimensions of complex socio-ecological systems (e.g.,
Saarikoski et al. 2016).
To have general applicability, a decision support framework must be flexible to suit diverse management goals and integrate social and political contexts. In contrast with classic conceptions of biological invasion as a purely biological phenomenon, socio-economic factors, including resource and landscape governance, community engagement, and behavioural psychology, also contribute (
Fig. 1), making the problem space multi-layered and multi-scaled (
Larson et al. 2011). To address this significant challenge requires “systems thinking” (
sensu Meadows and Wright 2008), which adopts a holistic view of how the elements of a system interrelate and interact (e.g.,
Fig. 1B). Given this neglected systems perspective, we formed an expert working group from multiple disciplines to tackle the question of “under what circumstances could we accept and thus adapt to the presence of an invasive species?” An interdisciplinary working group allowed us to think innovatively, incorporating expert elicitation methods to derive an adaptive decision model. Working group members completed two tasks: (1) they developed a framework to support decision-making and (2) they determined under what conditions active management of an established invasive species is warranted to achieve management objectives. To narrow our question further, we focused on aquatic invasive species and applied our expert-derived decision framework to a suite of management scenarios using invasive
Phragmites australis ssp.
australis (European common reed) as a case study.
Materials and methods
We brought together a binational, interdisciplinary group of practitioners from academic, government, and non-government organizations to synthesize their knowledge and experience from both the natural and social sciences. Working group members were selected based on their expertise related to invasive species and environmental decision-making, with a conscious effort made to include individuals from various professional, disciplinary contexts and career stages. We did not attempt to engage Tribal or First Nations communities. Thirteen experts from 10 institutions participated in the final working group: (1) the Great Lakes Science Center (U.S. Geological Survey), (2) the Canadian Wildlife Service (Environment and Climate Change Canada), (3) the Great Lakes Commission, (4) the Canadian Food Inspection Agency, (5) the Ontario Ministry of Northern Development, Mining, Natural Resources and Forestry, (6) Ontario Ministry of Environment, Conservation and Parks, (7) the University of Waterloo (Biology Department; School of Environment, Resources, and Sustainability), (8) McGill University (Biology Department), (9) Purdue University (Department of Anthropology), and (10) Ducks Unlimited Canada. Two early-career researchers from the University of Waterloo acted as rapporteurs throughout the workshop.
Working group members were invited to a two-day, “World Café” style workshop (
Brown 2010) in Fergus, ON, Canada, in October 2017, an evidence-based format widely adopted to facilitate innovative and collaborative discussion. The workshop began with introductions, setting the context, discussing pre-assigned readings that reviewed pertinent approaches to our research questions (i.e.,
Martin and Blossey 2013;
Gaertner et al. 2016;
Lodge et al. 2016;
Kopf et al. 2017), and setting the unifying objectives, which were to (1) develop a practical decision framework for whether to manage an invasive species once it has become regionally established and (2) apply this framework to the case of invasive
Phragmites australis ssp.
australis. Workshop participants agreed on a definition of “invasive species” as species that are introduced from outside a given geographical region that can create self-sustaining populations in a new environment that may have a detrimental impact on extant ecosystems, similar to the definition of
Richardson et al. (2000). The working group acknowledged that not all non-native species are invasive and that even invasive species may provide valuable ecosystem services.
Day One of the workshop consisted of identifying the key dimensions (i.e., characteristics and limitations) of aquatic invasive species and their management. After these dimensions were agreed upon (see Results), working group members were randomly assigned to one of the four groups to develop frameworks for considering these dimensions. Our rationale for assigning members to groups at random was to support our goal of interdisciplinarity without requiring us to label participants as belonging to a single group. Once each sub-group had completed their own draft framework, members gathered to share their results and synthesize an initial consensus framework that integrated aspects of all the four groups.
Day Two began with a brief overview of invasive
P. australis and its management in North America.
Phragmites australis ssp.
australis is a wetland grass that has been considered one of the greatest threats to North American wetlands because, if untreated, it will continue to spread (
Catling and Mitrow 2011;
Saltonstall and Meyerson 2016;
Jung et al. 2017). Despite large annual expenditures on
P. australis management—for example, the expenditure of $4.5 million USD annually in the USA towards extensive herbicide treatment—the management objectives of
P. australis eradication and ecosystem services recovery may not be fully achieved (
Martin and Blossey 2013;
Rohal et al. 2018). Given the expense, the risks to ecosystems and human health associated with herbicide application (e.g.,
Van Bruggen et al. 2018), and the general failure of management actions to extirpate invasive
P. australis fully from invaded properties (e.g.,
Hazelton et al. 2014), it remained unclear how to weigh the risks of intervention vs. non-intervention.
Our working group members were then randomly assigned to four new sub-groups where they reviewed the draft consensus framework developed on Day One. As discussion prompts, sub-groups were given six P. australis case scenarios designed by co-authors Rooney, Robichaud, and Larson to represent diverse contexts and management goals (Appendix A). For example, one scenario consisted of managing the linear expansion of P. australis along a roadway, a second dealt with small patches of P. australis near a busy boat launch, and a third scenario considered managing P. australis in areas with species at risk (Appendix A). Rather than report on the outcome of applying the framework to each scenario, the sub-groups used the scenarios as an instrument to test the framework. The discussions sparked by consideration of these scenarios identified strengths in the draft consensus framework, including the structured manner by which the framework encouraged managers to consider the connectivity of their management parcel to other communities and habitats. The sub-groups also identified weaknesses in the draft consensus framework, such as the difficulty of conducting a holistic assessment when it requires weighing financial costs against ecological and social costs that are not easily assigned a monetary value. Further, when a manager is responsible for multiple sites, the draft consensus framework did not provide a means to prioritize among sites.
A whole-group discussion of the six different scenarios revealed the need for formal tools to support the implementation of the framework. Three new breakout sub-groups were created to develop three tools to assess key prerequisites to any management intervention action: (1) assessment of the net negative effects of the invasive alien species at a given site, (2) assessment of the net environmental benefits of the available management activities at a given site for a specified management goal, and (3) assessment of whether the resources and socio-political context were compatible with achieving the site’s specified management goal. To address these three prerequisites, working group members self-identified and joined one of the three “tool” groups where they had the most expertise to contribute. The rationale for allowing group members to now self-select for these three new sub-groups was that each sub-group was focused on developing a different tool, and we desired that participants contribute where they had the most interest and expertise. After the sub-group discussions, members reconvened to share the tools developed by their sub-group with all the working group members. By the end of Day Two, members had agreed on a framework with three supporting tools, the workshop coordinators synthesized these results, and members were invited to review and co-author the subsequent manuscript.
Conclusion
The framework derived by our interdisciplinary working group highlights the challenge and complexity of managing invasive species that are well established and entrenched within a landscape (i.e., late in the invasion curve), when eradication is not perceived as a suitable management goal. Key advantages of our approach include that it engaged experts and practitioners from multiple disciplines and brought them to consensus on a decision support framework that promotes systems thinking.
The importance of systems thinking in decision-making about invasive species management was a consistent message emerging in our working group. Critically, applying a systems lens to the problem of invasive species management requires that decision makers move well beyond cost-benefit analysis, where all factors must be expressed in common monetary units to be incorporated in the decision-making process. Notably, members emphasized that the reversibility or persistence of costs and impacts from either the aquatic invasive species or suppression efforts should also be weighed. More flexible multi-criteria decision-making approaches can enable more holistic assessment of costs and consequences that are not easily assigned a monetary value.
Given the variability in site conditions and management approaches, eradication of non-native
Phragmites at the landscape scale is very difficult to achieve and(or) may easily be undone by reinvasion from uncontrolled neighbouring populations (
Hazelton et al. 2014). Evaluation of the common default position of widespread management is important because alien species, even ones that negatively affect native species and could qualify as “invasive” by our definition, may provide ecological, economic, or social benefits in some contexts. Failing to consider these potential benefits can lead to net ecological degradation and negative social impacts such as increased conflict among stakeholders. Leaving the invasion to progress unchecked also imposes a cost, so it is vital that such cost-consequence accounts consider the change in management costs rather than the simple cost of alternative management strategies.