A decision framework for the management of established biological invasions

When invasive aquatic species are recognized as having negative impacts on ecosystems where they have colonized or been introduced, our workshop participants agreed that the default assumption is that managers or, more broadly, society should aim to eradicate them (e.g., Aichi Target 9, Convention on Biological Diversity 2011). Yet eradication can be cost-prohibitive, damaging to managed ecosystems, and oftentimes impossible with the available tools and organizational capacity (Hazelton et al. 2014). If eradication is not perceived as achievable and the invasive species is deemed established (Fig. 1), how can land managers decide whether a particular aquatic invasive species warrants active management? Our working group identified three key dimensions to this question: (1) efficacy, (2) the socio-ecological context, and (3) net benefits.

“Efficacy” encompassed aspects of available management options, as some tools or actions might have superior outcomes in terms of both the suppression of the invasive species and the recovery of the ecosystem post-treatment. Workshop members acknowledged that this aspect of efficacy might vary with the geographic extent of invasion, the time frame under consideration, and the accessibility of the site. In evaluating efficacy, workshop members agreed that it is essential to define realistic management goals, which members also acknowledged rarely aspire to eradication when aquatic invasive species have already become established (Fig. 1). For example, a goal to slow invasive species’ spread may prioritize resourcing public education, vector management (i.e., regulations and enforcement of clean equipment protocols), and dispersal pathways or suppression of marginal populations. In contrast, the goal of protecting a particular high-value conservation asset, such as a provincial park or critical habitat for a sensitive species at risk, may achieve greater success through routine population suppression within clearly demarcated boundaries.

Secondly, the “socio-ecological context” was recognized by workshop members as influential both in defining management goals and in enabling managers to achieve those goals effectively. Members extended this concept to incorporate multiple factors, including the available capacity in terms of resources, funding, knowledge base, personnel, and complementarity with ongoing management activities. Workshop members recognized that these contextual questions must be continuously addressed in both the immediate term and the long term, as management of established invasions requires sustained effort (Larson et al. 2011), while resource availability can change with political mandates and annual budget processes. As identified by Redpath et al. (2013), the management context for invasive species extends to consider the public support, political will, community engagement, agency niche, and regulatory framework necessary for management success as these factors often influence resourcing decisions. In both the academic literature and in practice, it is well recognized that community engagement is a key component of conservation success (e.g., Stokes et al. 2006; Cooke et al. 2013; Reed et al. 2018). These considerations go beyond “acceptability” analyses of various management strategies and interventions to recognize the multilayered, nuanced, and often-changing relationships between people and invasive species. For Indigenous communities, in particular, the invasive species designation is linked to colonial timelines, the privileging of a settler-colonial idealization of pre-contact areas as natural or pristine wilderness, and a world view premised on the subjugation of nature (Subramaniam 2014). Interventions to eradicate invasive species may have direct, negative impacts on traditional livelihoods or land tenure claims for Indigenous or marginalized groups and may be seen as part of a suite of “top-down, problem-focused land and resource management interventions” that disrupt the complex and dynamic relationships among people and environments (Reo and Ogden 2018).

Workshop members recognized that without this socio-ecological context for success, the risk of failure may be too high. Members emphasized that programmatic failure could carry severe consequences in terms of jeopardizing management agency appetite or the capacity to tackle invasive species in the future or by creating social conflict that could endanger other management goals (Mackenzie and Larson 2010). For example, while community members may understand the impacts of invasive species, their trust and support for management actions can be complicated by distrust of the management agency, personal experience with the invasive species, and confusion over roles, responsibilities, and mandates of government and scientists (Wald et al. 2019). Some working group members acknowledged the vulnerability inherent in invasive species management and that outside factors can hinder progress and result in the loss of community trust and support. Moreover, community perceptions and behaviours are never static; community views of land and resource management interventions may draw from other social or political views and prior experiences, which may not correlate neatly with specific demographic groups (Stinchcomb et al. 2022). Adequate public and stakeholder engagement in invasive species management may lead to more democratic and less conflict-prone interventions (Crowley et al. 2017; Shackleton et al. 2019).

All workshop participants acknowledged that invasive species can exert both desirable and undesirable influences on invaded ecosystems. The “net benefits” dimension entailed considering the costs and benefits of taking a given management action, which members emphasized must be weighed against the null scenario of taking no management action. For example, if the social, environmental, or economic costs of allowing the invasive alien species to spread and establish are very severe—for example, extirpation or extinction of an endangered species—the tolerance for potential environmental damages associated with an effective suppression strategy or the willingness to pay for a costly management action likely would be high. Members sought a comprehensive costs and benefits assessment that would also consider the persistence or reversibility of impacts from the aquatic invasive species and from the treatment of that invasive species. This net benefits assessment also considers that invasive species can contribute valuable ecosystem services (Hershner and Havens 2008) and that their removal can initiate secondary invasions by other non-native species taking advantage of the ecological disturbance (e.g., Pearson et al. 2016; Robichaud and Rooney 2021). Similarly, management actions were recognized as variable not only in their efficacy but also in their environmental and economic costs. For example, the use of chemicals is likely to achieve greater population suppression than mechanical means of controlling invasive plant species in large areas (Beric and MacIsaac 2015). But there are significant risks: chemicals may have non-target effects (e.g., Beaulieu et al. 2021; Beecraft and Rooney 2021) and risk residue accumulation (e.g., Sesin et al. 2021), particularly where established invasive species require repeated treatments. Thus, the costs and benefits analysis should be tailored to the specific management option being considered.

Working group members used these three dimensions—efficacy, socio-ecological context, and net benefits—to structure a draft consensus decision tree that further postulated three key prerequisites for active management of established invasive species, which we pose as questions in Fig. 2.

In the decision tree (Fig. 2, question 1), the effects of the invader on the invaded ecosystem are considered comprehensively, including both negative and positive impacts. To encourage a thorough consideration, the working group members developed a checklist tool (Appendix B). This assessment is scoped to a particular location, given site-specific natural heritage features and landscape context. A manager could consider the available evidence on the effect of the invasive species on a range of environmental (e.g., water quality and floral diversity), economic (e.g., infrastructure maintenance and tourism), and social (e.g., public health and safety and stakeholder interest) factors. The manager could then evaluate whether the net effect of the invasive species on each factor was positive, negative, or neutral given their system knowledge. Factors lacking an adequate basis for assessment could be scored as unknown to highlight knowledge gaps and potential research priorities. The manager could then weigh the prevalence of factors judged to be positive or negative to reach a decision.

Our checklist supports a multi-criteria approach to decision-making (e.g., Liu et al. 2011), with environmental, economic, and social dimensions (Appendix B). We do not provide a prescription for how to weigh positive and negative factors because those choices will depend on specific management goals, management priorities, and the context for managing a given parcel of land. The checklist’s purpose is to help managers apply systems thinking to the question of whether to manage an invasive species actively. Individual managers can avail themselves of appropriate rubrics for weighing the relative importance of the factors they list using our checklist. This multi-criteria approach emphasizes an “on the balance” consideration of multiple and disparate criteria by focusing on each factor as net positive, net negative, or neutral rather than attempting to assign commensurable monetary values for factors for which no accepted method of market valuation exists. This emphasis yields a systematic approach to decision-making that evaluates options according to multiple and sometimes conflicting criteria and objectives that managers define a priori.

If the negative environmental, economic, and social effects at the local scale did not outweigh the benefits of the invasion, members agreed that the larger landscape context would provide additional insight. If the local parcel of concern could serve as a propagule source to neighbouring lands, then relevant adjacent jurisdictions are consulted and a multi-criteria decision analysis can be repeated between neighbouring landowners or within an integrated land-use management network. If the invasion is not deemed to have a net negative effect on the local or regional system, then members agreed that monitoring could be recommended with periodic re-evaluations of the checklist tool (Appendix B) to engage in adaptive management, whereby a cycle of action, monitoring, analysis, and communication could be repeated to learn about the system being managed (e.g., Núñez-Regueiro et al. 2020).

If the negative environmental, economic, or social effects at the local and(or) landscape scale are deemed to outweigh any benefits of the invasive species, then individual management options are defined and considered in a similar multi-criteria decision analysis (Fig. 2, question 2). This process draws on the dimension of efficacy described above, as the purpose is to identify the optimal management strategy for a given parcel of land among all possible management options.

Working group members defined the net benefits with the following equation:

In this equation, the net benefits of management strategy “i” (Net benefits_i) equal the change in ecological benefits to the invaded ecosystem due to management strategy “i” (ΔB_i), minus the change in damages to the invaded ecosystem imposed by management strategy “i” (ΔD_i) and the change in monetary costs of managing the land if strategy “i” were implemented (ΔC_i). Thus, the management action most effective at suppressing the invasive species population under the current site conditions is not necessarily the optimal choice if it presents a significant risk of non-target effects or imposes unjustified financial costs. While the change in management costs for a given strategy would likely be positive, a strategy could free up resources when compared with the current management approach. Therefore, the formula calls for consideration of the change in management costs rather than the simple cost of each candidate strategy. The challenge is combining the environmental benefits and damages with the financial costs of management, as these are not in equivalent units. Manager discretion will be needed to implement this formula, but the explicit representation of changes to environmental benefits, damages, and costs associated with each management strategy could support managers in ranking their options.

If no available management option has benefits that would outweigh the sum of its damages and costs, then managers could continue to monitor and support research into improved treatment technologies that are either lower cost or that present less risk to the treated ecosystem. If multiple management options exist with positive net benefits, then the option with the greatest net benefit could be favoured, though working group members highlighted that this optimal technique or action may only be undertaken if sufficient resources and socio-political support are in place to have a high probability of success in achieving the management goals for that property (Fig. 2, question 3). Management goals are set by the managers for a given parcel of land and likely reflect the stage of invasion (Fig. 1).

Among the alternative actions, managers are encouraged to consider the “no control” option, thus permitting the invasion to progress. We emphasize that this decision may be, in some cases, an appropriate course of action. “No control” is not necessarily an abdication of responsibility, especially if it is a considered decision, but under the workshop framework it is crucial to recognize that a decision not to control the invasion does not mean that managers are expected to take no action. As emphasized by its centrality in Fig. 2, monitoring is expected to accompany any management action and especially the “no control” action. Monitoring the invasive species may provide an experimental control to learn how an invasion progresses in the absence of active management. These monitoring data can advance our knowledge and be extremely valuable to inform subsequent decision-making. In this way, our decision framework (Fig. 2) lends itself well to an adaptive management framework, whereby the outcome of management decisions feed into future decision-making. For example, we only learned that Phragmites australis ssp. australis invasion can be benign at early invasion stages by monitoring before acting (Robichaud and Rooney 2017). However, we caution against managers defaulting to monitoring rather than control actions due to a perceived lack of specific, place-based knowledge as this can impede critical action required to save threatened species (e.g., Buxton et al. 2020).

When defining the socio-ecological context dimension, members emphasized that aquatic invasive species management can be conflict-prone (e.g., Stokes et al. 2006) and that meaningful, substantive stakeholder engagement and social impact assessment can lead to greater success in achieving management objectives (Crowley et al. 2017). Working group members asserted that the opposite is also true: the loss of political will and public support or the rise of social conflict can ensure management failure through resource reallocation and mandate redirection before eradication is achieved. Support and resources thus form a positive feedback loop (Fig. 3), whereby support is necessary to secure resources for management but is also dependent on successful outcomes and avoidance of non-target effects of management. Under-resourcing the management of an invasive species can lead to failure, which workshop participants noted further erodes support and can lead to continued resource withdrawal.

Working group members agreed that a comprehensive assessment of the resources necessary for sustained implementation of a management strategy needs to be considered carefully and that management should commence only once the resources necessary to sustain it are in place. Until then, managers can focus on gathering resources and fostering both the public and political support needed to ensure success (Fig. 2, question 3). Collective action can be a powerful approach to setting the stage for successful management through resource pooling and coordinated efforts (Braun et al. 2016). Managers can consult the agencies or groups charged with the management of neighbouring jurisdictions to build partnerships in support of their optimal management strategy (Fig. 2). To promote a comprehensive assessment of resource needs and socio-political support, the working group members devised a list of questions managers can ask themselves before undertaking their optimal management action (Appendix C).

When active management of an invasive species is deemed necessary, there may be numerous options that vary in effectiveness at suppressing the species, in their potential to damage the treated ecosystem, and in the resources needed to complete the management. We encourage managers to weigh the changes in costs, damages, and benefits comprehensively before selecting the management action that yields the greatest net benefit. Such an analysis may provide insight into whether there is a need for more fundraising, greater engagement with stakeholders, or further research into alternative management approaches.

Where feasible, eradication can be more environmentally and ethically sound than sustained suppression efforts, as eradication may not perpetually usurp management resources or foster reliance on tools like pesticides (Cloute and Veitch 2002). However, ongoing management efforts may be required where eradication is impossible due to widespread establishment and a high potential for reinvasion from neighbouring properties. The resource needs to sustain long-term goals such as asset protection and slowing the spread can be large (Hazelton et al. 2014), yet resource availability can ebb and flow with public and political support (Fig. 3). It is important to consider the long-term sustainability of management interventions to avoid failure resulting from inadequate resources. Managers may work to set aside resources to monitor the effect of the intervention adequately (e.g., both pre- and post-intervention), interpret the monitoring data, and disseminate the data to key stakeholders. If managers have not prepared in these ways, they may risk alienating stakeholders and souring community and political opinion in ways that may jeopardize subsequent control efforts. It is crucial to invest in social impact analysis and engage stakeholders to help nurture the conditions necessary to succeed (Crowley et al. 2017), leading to greater support in the future.