The National Wildlife Refuge System protects extensive salt marsh acreage in the northeastern United States. Much of this habitat has been degraded by a succession of human activities since the time of European settlement (Gedan and others, 2009), and accelerated rates of sea-level rise exacerbate these effects (Gedan and others, 2011; Kirwan and Megonigal, 2013). Therefore, strategies to restore and enhance the ecological integrity of national wildlife refuge (NWR) salt marshes are regularly considered. Management may include such activities as reestablishing natural hydrology, augmenting or excavating sediments to restore marsh elevation, controlling invasive species, planting native vegetation, minimizing shoreline erosion, and remediating contaminant problems. Uncertainty stemming from incomplete knowledge of system status and imperfect understanding of ecosystem dynamics commonly hinders management predictions and selection of the most effective management options. Consequently, tools for identifying appropriate assessment variables and evaluating tradeoffs among management objectives are valuable to inform marsh management decisions.

Structured decision making is a systematic approach to improving the quality of complex decisions that integrates assessment metrics into the decision process (Gregory and Keeney, 2002). This approach involves identifying measurable management objectives and potential management actions, predicting management outcomes, and evaluating tradeoffs to choose a preferred alternative. From 2008 to 2012, the U.S. Geological Survey (USGS) and U.S. Fish and Wildlife Service (FWS) used structured decision making to develop a framework for optimizing management decisions for NWR salt marshes in the FWS Northeast Region (that is, salt marshes in the coastal region from Maine through Virginia). The structured decision-making steps were applied through successive “rapid prototyping” workshops, an iterative process in which relatively short periods of time are invested to continually improve the decision structure (Blomquist and others, 2010; Garrard and others, 2017). The decision framework includes regional management objectives addressing critical components of salt marsh ecosystems, and associated performance metrics for determining whether objectives are achieved (Neckles and others, 2015). The regional objectives structure served as the foundation for a consistent protocol for monitoring salt marsh integrity at these northeastern coastal refuges, in which the monitoring variables are linked explicitly to management goals (Neckles and others, 2013). From 2012 to 2016, this protocol was used to conduct a baseline assessment of salt marsh integrity at all 17 refuges or refuge complexes in the FWS Northeast Region with salt marsh habitat (fig. 1).

The Eastern Shore of Virginia National Wildlife Refuge protects 180 hectares (ha) of salt marsh at the southern end of the Delmarva Peninsula, in Northampton County, Virginia, between the Atlantic Ocean and Chesapeake Bay (fig. 2). The Fisherman Island National Wildlife Refuge protects an additional 393 ha of salt marsh on a barrier island separated from the Delmarva Peninsula by Fishermans Inlet (fig. 2). The two refuges are administered jointly and are considered together for descriptive and management purposes. The salt marsh on the refuges provides critical breeding, migratory, or wintering habitat for bird species of highest conservation priority, including Anas rubripes (American black duck), Branta bernicla (Atlantic brant), and Ammodramus caudacutus (saltmarsh sparrow) in the New England and mid-Atlantic coast bird conservation region of the U.S. North American Bird Conservation Initiative (Steinkamp, 2008; Denmon, 2018; U.S. North American Bird Conservation Initiative, 2020). The salt marsh also provides important foraging habitat for wading birds, such as Ardea alba (great egret) and Egretta thula (snowy egret), during the breeding season. The primary threats to this habitat are marsh submergence associated with rising sea level, spread of the invasive reed Phragmites australis (hereafter referred to as Phragmites), and overabundance of Odocoileus virginianus (white-tailed deer; Denmon, 2018). Salt-marsh management goals set by the FWS for the refuges include maintaining the long-term productivity, integrity, and function of this habitat for marsh-dependent birds (FWS, 2004; Denmon, 2018). In this study, the regional structured decision-making framework was used to help prioritize salt marsh management options for the refuges.

A regional framework for assessing and managing salt marsh integrity at northeastern NWRs was developed through collaborative efforts of FWS regional and refuge managers and biologists, salt marsh research scientists, and structured decision-making experts. This process followed the discrete steps outlined by Hammond and others (1999) and Gregory and Keeney (2002):

This sequence of steps was applied through successive workshops to refine the decision structure and incorporate newly available information. Initial development of the structured decision-making framework occurred during a week-long workshop in 2008 to define the decision problem, specify management objectives, and explore potential strategies available to restore and enhance salt marsh integrity. During 2008 and 2009, workshop results were used to guide field tests of salt marsh monitoring variables (Neckles and others, 2013). Subsequently, in 2012, data and insights gained from these field tests were used in a two-part workshop to refine management objectives and develop the means for evaluating management outcomes (Neckles and others, 2015).

From the outset, FWS goals included development of an approach for consistent assessment of salt marsh integrity across all northeastern NWRs (fig. 1). Within this regional context, staff at a given refuge must periodically determine the best approaches for managing salt marshes to maximize habitat value while considering financial and other constraints. The salt marsh decision problem was thus defined as applying to individual NWRs over a 5-year planning horizon. The objectives for complex decisions can be organized into a hierarchy to help clarify what is most important to decision makers (Gregory and others, 2012). The hierarchy of objectives for salt marsh management decisions (table 1) was based explicitly on the conservation mission of the National Wildlife Refuge System, which is upheld through FWS management to “ensure that the biological integrity, diversity, and environmental health of the System are maintained for the benefit of present and future generations of Americans,” as mandated in the National Wildlife Refuge System Improvement Act of 1997 (16 U.S.C. §668dd note). Two fundamental objectives, or the overall goals for salt marsh management decisions, were drawn from this policy to maximize (1) biological integrity and diversity, and (2) environmental health, of salt marsh ecosystems. Participants in the prototyping workshops deconstructed these overall goals into lower-level objectives relating to salt marsh structure and function and identified performance metrics to evaluate whether objectives are achieved (table 1). In addition, performance metrics were weighted to reflect the relative importance of each objective (Neckles and others, 2015).

The hierarchy of objectives for salt marsh management (table 1) provides the foundation for identifying possible management actions at individual NWRs and predicting management outcomes. Workshop participants developed preliminary influence diagrams (app. 1), or conceptual models relating management actions to responses by each performance metric (Conroy and Peterson, 2013), to guide this process. To allow metric responses to be aggregated into a single, overall performance score, participants also defined value functions relating salt marsh integrity metric scores to perceived management benefit on a common, unitless “utility” scale (Keeney and Raiffa, 1993). Stakeholder elicitation was used to determine the form of each value function relating the original metric scale to the utility scale, ranging from 0, representing the lowest management benefit, to 1, representing the highest benefit (app. 2). Neckles and others (2015) provided details regarding development of the structured decision-making framework and a case-study application to Prime Hook National Wildlife Refuge in Delaware.

In February 2018, FWS regional biologists, biologists and managers from four northeastern NWR administrative units, and USGS research scientists (table 2) participated in a 1.5-day rapid-prototyping workshop to apply the regional structured decision-making framework to the Eastern Shore of Virginia, Fisherman Island, and Plum Tree Island National Wildlife Refuges and the Long Island National Wildlife Refuge Complex. Participants worked within refuge-specific small groups to focus on management issues at individual refuges. Plenary discussions of common patterns of salt marsh degradation, potential management strategies, and mechanisms of ecosystem response offered additional insights to enhance refuge-specific discussions.

Participants identified a range of possible management actions for achieving objectives within each marsh management unit at the Eastern Shore of Virginia and Fisherman Island National Wildlife Refuges and estimated the total cost of implementation over a 5-year period; the specific years of implementation were not identified in this prototype. Potential actions to enhance salt marsh integrity ranged from targeted efforts that restore hydrologic connections, increase surface-water drainage, or control predators, deer, or spread of Phragmites, to large-scale projects that alter marsh elevation (table 3). Participants predicted the outcomes of each management action 5 years after initial implementation in terms of salt marsh integrity performance metrics. For most metrics, baseline conditions within each unit measured during the 2012–16 salt marsh integrity assessment (FWS, 2016) were used to predict the outcomes of a “no-action” alternative. Baseline conditions were estimated by using expert judgement for three metrics that lacked assessment data (abundance of American black ducks, density of spiders, change in marsh surface elevation relative to sea-level rise). Regional influence diagrams relating management strategies to outcomes aided in predicting consequences of management actions (app. 1). Although the influence diagrams incorporated the potential effects of stochastic processes, including weather, sea-level rise, herbivory, contaminant inputs, and disease, on management outcomes, no attempt was made to quantify these sources of uncertainty during rapid prototyping. Management predictions also inherently included considerable uncertainty surrounding the complex interactions among controlling factors and salt marsh ecosystem components.

Following the workshop, the potential management benefit of each salt marsh integrity performance metric was calculated by converting salt marsh integrity metric scores (table 3, workshop output) to weighted utilities (table 4) using regional value functions (app. 2). Weighted utilities were summed across all salt marsh integrity metrics for each action; this overall utility therefore represented the total management benefit, across all objectives, expected to accrue from a given management action (table 4). Constrained optimization (Conroy and Peterson, 2013) was used to find the management portfolio (the combination of actions, one action per marsh management unit) that maximizes the total management benefit across all units under varying cost scenarios for the combined refuges. Constrained optimization using integer linear programming was implemented in the Solver tool in Microsoft Excel (Kirkwood, 1997). Budget constraints were increased in $25,000 increments up to $50,000; in $50,000 increments up to $300,000; in $100,000 increments up to $1 million; in $500,000 increments up to $3 million; and in $1 million increments thereafter. The upper limit to potential costs was not determined in advance; rather, it reflected the total estimated costs of the proposed management actions. A cost-benefit plot of the portfolios identified through the optimization analysis was used to identify the efficient frontier for resource allocation (Keeney and Raiffa, 1993), which is the set of portfolios that are not dominated by other portfolios at similar costs (or the set of portfolios with maximum total benefit for a similar cost). The cost-benefit plot also revealed the cost above which further expenditures would yield diminishing returns on investment. To exemplify use of the decision-making framework to understand how a given portfolio could affect specific management objectives, the refuge-scale management benefits for individual performance metrics were compared between one optimal portfolio and those predicted with no management action taken.

Potential management actions identified to improve marsh integrity at the Eastern Shore of Virginia and Fisherman Island National Wildlife Refuges included trapping mammalian predators to increase the nesting success of tidal marsh birds; controlling Phragmites at the edges of the marsh management units; digging runnels, or shallow creeks, to enhance surface-water drainage; and restoring natural hydrology through breaching roads or installing culverts (table 3). For costs ranging from $0 to $6.03 million, the estimated management benefits for individual actions across all metrics, measured as weighted utilities, ranged from 0.579 (for implementing no action in the Raccoon management unit) to 0.956 (for digging runnels by hand in the ESV Marsh and Fisherman Island West Marsh management units), out of a maximum possible total management benefit of 1.0 (table 3, table 4). In two marsh management units (Raccoon, Fisherman Island West Marsh), the alternative with both the lowest management benefit and lowest cost was the “no action” alternative (management action A). However, in four marsh management units, implementing certain management actions (using herbicides to control invasive plants in Bull Marsh, Skidmore S, ESV Marsh, and Fisherman Island East Marsh; creating more pools and runnels in Bull Marsh) was predicted to yield a lower total management benefit than implementing no management action.

Constrained optimization was applied to identify the optimal management portfolios over 5 years for a range of total costs to the refuges’ administration. As total cost increased from $0 (no action in any unit) to approximately $3 million, the total management benefit at the refuge scale increased from 4.746 to 5.379 (a 13-percent increase; table 5) out of a possible maximum of 6.0 (the maximum possible management benefit of 1.0 for any management action, summed across the six marsh management units). Further increases in the budget constraint yielded the same management portfolio (that is, portfolio 8, table 5). Graphical analysis showed a fairly consistent increase in management benefit as costs increased to $143,000 (fig. 3, portfolio 5). Portfolio 5 represented the turning point in the cost-benefit plot. As expenditures increased beyond the cost of portfolio 5, total management benefit continued to increase but at a lower rate, yielding diminishing returns on investment (fig. 3).

The potential management actions selected within the set of portfolios that yielded the greatest total management benefit per unit cost (table 5, portfolios 2 through 5) largely differed across the marsh management units. In two marsh management units (ESV Marsh and Fisherman Island West Marsh), these portfolios always included hand-dug runnels as the optimum management action. In the other marsh management units these portfolios included breaching a road to restore natural hydrology (Bull Marsh); creating islands for marsh-nesting birds or spreading sediment to raise marsh elevation (Skidmore S); trapping predators to enhance nest success of tidal marsh obligate birds (Raccoon); and reducing the abundance of deer to minimize their effects on marsh vegetation (Fisherman Island East Marsh). Some potential management actions were never included in the portfolios yielding the greatest benefit per cost. For example, although applying herbicide to remove Phragmites was identified to improve the integrity of all marsh management units, this action was never selected. Similarly, reducing disturbance to the marsh by adding restrictions to refuge permits was suggested as a possible action in four marsh management units, but predicted benefits (increases in numbers of tidal marsh obligate birds) were low and this action was never included in an optimal portfolio.

Examination of the refuge-scale metric responses to actions included in portfolio 5, which is the turning point in the cost-benefit plot (fig. 3), revealed how implementation could affect specific management objectives. The actions included were predicted to achieve gains in the overall management benefits derived from density of spiders (as an indicator of trophic health), number of tidal marsh obligate breeding birds, species richness of nekton, and reduced flooding duration (fig. 4). Ecologically, the combination of actions in portfolio 5 may result in the following average changes (derived as the relative difference between the predicted metric scores for the actions implemented in portfolio 5 and the “no-action” alternative within each marsh management unit, averaged across all units; table 3): a 77-percent increase in tidal marsh obligate bird counts, a 39-percent decrease in the deviation of surface flooding from the ideal reference condition, an 18-percent increase in nekton species richness, and a 50-percent increase in spider density. The management benefits predicted for portfolios 2 through 4, at total costs up to $83,000, were derived primarily from expected improvements in surface-water drainage, and presumed increases in densities of spiders and numbers of tidal marsh obligate birds (table 3, table 4).

A regional structured decision-making framework for salt marshes on NWRs in the northeastern United States was applied by the USGS, in cooperation with the FWS, to develop a tool for optimizing management decisions at the Eastern Shore of Virginia and Fisherman Island National Wildlife Refuges. Use of the existing regional framework and a rapid-prototyping approach permitted NWR biologists and managers, FWS regional authorities, and research scientists to construct a decision model for the refuges within the confines of a 1.5-day workshop. This preliminary prototype provides a local framework for decision making while revealing information needs for future iterations. Insights from this process may also be useful to inform future habitat management planning at the refuges.

The suite of potential management actions and predicted outcomes included in this prototype (table 3) were based on current understanding of the Eastern Shore of Virginia and Fisherman Island National Wildlife Refuge salt marshes and hypothesized process-response pathways (app. 1). Tidal flooding is the predominant physical control on the structure and function of salt marsh ecosystems (Pennings and Bertness, 2001), and there is widespread scientific effort to elucidate how salt marshes may respond to accelerating rates of sea-level rise and management strategies to enhance their sustainability (Kirwan and Megonigal, 2013; Roman, 2017). Management interventions frequently proposed to mitigate effects of increased inundation on coastal marsh soils include increasing marsh drainage and raising marsh elevation (Wigand and others, 2017). In this prototype, digging runnels by hand to improve drainage of the marsh platform (in ESV Marsh and Fisherman Island West Marsh management units) and broad-scale deposition of sediments to raise marsh elevation (Skidmore S management unit) were predicted to yield similar increases in total management benefit, but sediment deposition was an order of magnitude more costly (that is, $90,000 compared to $9,000; table 3, table 4). Multiple, interacting factors influence the long-term success of restoration actions in prolonging marsh integrity and improving marsh resilience, and ecosystem responses may depend on site-specific factors (Roman, 2017; Raposa and others, 2018; Perry and others, in press). Future iterations of this decision model can incorporate improved understanding of both implementation costs and marsh responses to management actions. In addition, during construction of the regional decision model, lack of widely available data on rates of vertical marsh growth led to the adoption of a very coarse scale of measurement for change in marsh surface elevation relative to sea-level rise (table 1). In 2012, surface elevation tables (Lynch and others, 2015) were installed in each marsh management unit to obtain high-resolution measurements of change in marsh surface elevation. Incorporating this information into subsequent iterations of this structured decision-making framework would likely improve predictions related to the potential for marsh surface elevation to keep pace with sea-level rise.

Results of constrained optimizations (table 5) based on the objectives, management actions, and predicted outcomes included in this prototype identified four areas in which to improve the utility of the prototype for refuge decision making. First, although minimizing the spread of Phragmites is a management concern at the Eastern Shore of Virginia and Fisherman Island National Wildlife Refuges, removing Phragmites through use of herbicides was not selected for any optimal portfolio. The transparency of the structured decision-making framework reveals the tradeoffs associated with applying herbicide to reduce the spread of invasive plants. In most instances, controlling invasive plants was predicted to increase the percent cover of native vegetation and the abundance of tidal marsh obligate birds (table 3) and the management benefits associated with achieving these specific objectives (table 4). However, spraying herbicide, which is a potential environmental contaminant, had direct negative consequences on the objective to minimize herbicide use and often resulted in a net decrease in the total management benefit (table 4). Thus, the benefits associated with use of herbicide to reduce invasive plants may not offset the negative value of applying environmental contaminants. These results emphasize the importance that refuge managers have placed on reducing spread of invasive plants through various methods, including mechanical control and prescribed burning (Denmon, 2018). This prototype could be adapted to allow managers to evaluate the relative expected benefits and detriments of chemical and other control methods.

Second, the habitat management plan for the Eastern Shore of Virginia and Fisherman Island National Wildlife Refuges identifies seaside sparrows as a priority resource of concern in salt marsh habitat (Denmon, 2018). These birds nest primarily in smooth cordgrass cover characteristic of the low marsh zone, where the tall vegetation allows relatively high nest placement that minimizes risks of tidal inundation (Gjerdrum and others, 2005). However, nest predation increases with nest height, and recent studies suggest that as sea-level rise increases the vulnerability of nests to tidal flooding, predator control may be required to maintain viability of seaside sparrow populations (Hunter, 2017; Roberts and others, 2019). In this prototype, trapping predators to enhance nest success of tidal marsh birds was predicted to improve the total management benefit in all marsh management units; however, this action was included in an optimal management portfolio for the Raccoon management unit only, where the estimated cost of implementation was lowest. Therefore, managers may want to explore alternative approaches to reduce nest predation, such as fencing around individual nests (Post and Greenlaw, 1989), in future prototypes.

Third, although loss of marsh area through shoreline erosion is a concern in the Raccoon and Fisherman Island West Marsh management units, reducing wave action through construction of living shorelines (plants or other natural elements for shoreline stabilization) was excluded from all but the costliest optimal portfolio. Living shorelines may be cost prohibitive at these refuges. Additionally, deconstructing the objective of maintaining the extent of the marsh platform into subordinate objectives and performance metrics related to both horizontal and vertical gains and losses of marsh substrate may help focus decision making on erosion of marsh edges as a driver of marsh area.

Finally, the constrained optimizations analyzed in this report were based on estimations of management costs. As salt marsh management is undertaken around the northeast region, a detailed list of actual expenses can be compiled, including staff time for project planning as well as materials, equipment, contracts, and staff time for implementation. For example, Iacona and others (2018) recommended a standardized approach for summarizing and reporting financial costs of management interventions. This could allow future iterations of this decision model to include more accurate cost estimates, while contributing information to help others identify cost-effective salt marsh management techniques.

The prototype model for the Eastern Shore of Virginia and Fisherman Island National Wildlife Refuges provides a useful tool for decision making that can be updated in the future with new data and information. The spatial and temporal variability inherent in parameter estimates were not quantified during rapid prototyping. Previously, preliminary sensitivity analysis revealed little effect of incorporating ecological variation in abundance of marsh-obligate breeding birds on the optimal solutions for Prime Hook National Wildlife Refuge (Neckles and others, 2015). This lends confidence to use of this framework for decision making; however, including probability distributions for each performance metric in the decision model could be a high priority for future prototypes. Future monitoring of salt marsh integrity performance metrics will be useful to refine baseline parameter estimates and to determine the background rate of change in the absence of management actions; feedback from measured responses to management actions around the region will help reduce uncertainties surrounding management predictions. The structured decision-making framework applied here to the Eastern Shore of Virginia and Fisherman Island National Wildlife Refuges is based on a hierarchy of regional objectives and regional value functions relating performance metrics to perceived management benefits. It will be important to ensure that subsequent iterations reflect evolving management objectives and desired outcomes. Elements of the decision model could be further adapted, for example, through differential weighting of objectives or altered value functions, to reflect specific, local management goals and mandates. Future optimization analyses that use this framework could also incorporate additional constraints on action selection, such as ensuring that particular actions within individual marsh management units are included in optimal management portfolios, to further tailor the model to refuge-specific needs.