1. Ecosystem reference levels: how do we know when EBM has succeeded?

Ocean stewardship is not simple. Rather than maintaining piecemeal efforts, scientists, managers, conservationists, and policymakers have agreed that restoration and protection of the oceans will require a more integrated approach (National Marine Fisheries Service 2008, Karr 1992, Pauly 1995). A unified appeal for marine ecosystem-based management (EBM) has made the task of developing concrete methods for implementation quite urgent (Sainsbury et al. 2000, Castilla 1999, Dayton et al. 2000). Indeed, if the goal is maintenance and sustainable use of a healthy ecosystem (McLeod et al. 2005), it follows that those responsible for achieving this objective require a means to track the progress of their efforts. As discussed above, indicators allow the tracking of progress and change.


Terminology and Concepts


Reference level derived from time periods or locations free from human pressures


Indicator value suggestive of progress toward targets


Reference level pegged to an extreme value beyond which undesired change occurs


Sudden change in a response variable resulting from smooth and gradual change in a causal factor

Normative reference level

Reference level defined based on what is socially acceptable, i.e., according to norms


Define what is generally accepted within a cultural context, and may serve as societal standards to evaluate ecosystem conditions, human activities, or management strategies

Reference direction

Which specifies how the trend in an indicator relates to the desired state of the ecosystem

Reference level

Point value or direction of change used to provide context so that changes in indicator values can be interpreted relative to desired ecosystem states

Reference point

Precise values of indicators used to provide context for the current status of an indicator


Reference level that signals a desired state

Many authors have considered ecosystem health to be the structure and function of the ecosystem desired by stakeholders in a specific management context (Pitcher 2001, Jackson et al. 2001, Aronson et al. 2005, Butler and Campbell 2004). Thus, as we have previously emphasized, many attributes of ecosystem health, such as resilience, are difficult to measure directly. Proponents of using human health as an analog to ecosystem health note that just as cholesterol, stress, and income levels can serve as indicators for gauging human health (a state of physical, mental, and social well-being; World Health Organization 1948), the status of an ecosystem’s health can be measured via proxy using a suite of ecosystem indicators. For example, it is widely appreciated that the abundances of certain species of jellyfish and top predators provide information about the status of marine ecosystems because they reflect underlying changes in important ecosystem functions (e.g. Purcell et al. 2007, Heithaus et al. 2008).

To be useful from a policy and management perspective, ecosystem indicators must be linked to reference levels. Reference levels provide context so that changes in indicator values can be interpreted relative to desired ecosystem states (Rice 2003, Pitcher 2001, Aronson et al. 2005, Nichols 2003). Following with the human health analogy, one reference level for household income, a social well-being indicator, might be the poverty line (Hagenaars and Praag 1985). In single-species and single-sector management, reference levels are also fairly well established. Examples include target population sizes for recovery of endangered species (Gerber and Hatch 2002), the harvest rate corresponding to maximum sustainable yield in a fishery (Walters and Martell 2004), the critical level of nutrient input beyond which a clear freshwater lake becomes turbid (Schindler 1974), and, acceptable concentrations of toxic contaminants in water bodies (Suter 2007). While existing reference levels such as these provide a useful starting point (Tallis et al. 2010), EBM requires the consideration of how interactions among species and management sectors affect overall ecosystem state and potential trade-offs among indicator values (Cheung and Sumaila 2008). Reference levels set to guide management of species, habitats, and water quality individually may need to be modified or supplemented with additional indicators, and corresponding reference levels, in order to steward multiple ecosystem components simultaneously. We believe that many of these challenges can be met by adopting successful approaches from other management contexts for use on the ecosystem level. Here we describe several approaches for linking indicator values and trends to reference levels related to ecosystem health, and provide some examples for how they might be applied in Puget Sound. A summary of existing targets and/or reference levels for Puget Sound follows.