Coastal Services Center

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Irreversibility, Sustainability, and Safe Minimum Standard

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Human activities and their associated pressures on the natural world have resulted in the development of a wide array of management strategies that are aimed at restoring ecosystems to a close approximation of their former condition (NRC 1992). However, the planning and implementation process of restoration must consider a series of important questions that go hand-in-hand with uncertainty and natural variability. Has an ecosystem been altered so drastically that its current state is essentially irreversible? If not, is the ultimate desired condition practically achievable and sustainable? Adequately answering these questions is typically a difficult task; often there is a high degree of uncertainty in estimating ecosystem response to restoration activities. This uncertainty prevails because ecosystems are especially complex, possess variations in stability and resilience, and have response thresholds that are hard to perceive or understand fully. Also, our understanding of ecosystem functions is limited.

Natural resource economic theory posits that society’s valuation of ecosystems (and individual species) is rooted to the services that they provide (Pearce and Turner 1990). When ecosystems are degraded or otherwise altered to the point at which their services decline, society may choose to invest in restoring the system to regain these services. However, high levels of complexity and uncertainty can make restoration activities prohibitively risky and resource-intensive (e.g., large wetlands, rainforest, and highly degraded complex systems in general). Thus, there must be a balance between desired ecological conditions (or services), the costs of actions required to sustain or restore these services, and the uncertainty (or risk) related to successful achievement of restoration, that must be maintained throughout the process of restoration.

Irreversibility, sustainability, and safe minimum standard (SMS) are three terms associated with how restoration planning efforts need to approach and understand the relative complexity of ecosystems. Irreversibility is one significant risk to restoration practices for obvious reasons: if an altered ecosystem is in an irreversible state, then it is unlikely be restored. However, there may be different degrees of irreversibility, depending on the magnitude of restoration effort. For example, an ecological state may be, in effect, irreversible at low levels of restoration effort but completely reversible at higher levels (e.g., small reductions in fisheries harvest versus moratoriums). In the context of environmental restoration, sustainability means that the restored ecosystem can continue to provide ecological services and functions on its own, without continued intervention. The restored ecosystem needs to be sustainable at some minimal level, both practically and in an economic sense. The concept of sustainability underlies the impetus for many restoration plans. The desire for restored systems to be sustainable is typically high. To achieve sustainability across many scales, safeguards are often proposed (and practiced) to protect ecosystem integrity, to prevent damage, or to ensure sustainability. The development of plans to prevent ecosystem damage or restorative ecosystem function is often challenged by the difficulty in measuring or understanding natural variability and uncertainty. Concepts such as “safe minimum standard” and the “precautionary principle” have evolved to help manage some of the resultant risk and uncertainty. This article will explore how irreversibility, sustainability, and the concept of SMS can influence the goals and objectives of ecosystem restoration. Issues that currently face restoration planning and implementation are also discussed.
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Irreversibility

Ecosystems are dynamic and are always poised to adapt and change in response to their surroundings. Their interconnected components respond directly to outside influences, such as solar energy, temperature, and many additional physical forces, but they also respond indirectly to secondary, internal processes that are controlled by the interactions of their components (individuals, species, trophic communities). Ecosystem stability is often defined as a steady-state condition that can rapidly recover from perturbation and return to an intact structure. This steady-state condition, or equilibrium, can oscillate over time and space; however, it remains cohesive and does not shift into new, different states. Most ecosystems can be characterized by some degree of stability when disturbed, particularly at the scale of time and space in which we typically observe them. However, some ecosystems can exist in nonequilibrium and be in a persistent state of evolution to different states. Many ecosystems will experience multiple stages of stability, where perturbations (changes) induce shifts between them. This is illustrated in Figure 1, the Holling 4-Box Model (Holling 1992), which represents four basic states common to all complex systems, and some key features within each of those states:

Exploitation. In this state, opportunistic species are able to colonize in places that were previously inhospitable or unattainable. This state usually follows some sort of perturbation, or threshold (ecological or physical), that allows for the introduction of new species and communities.

Conservation. In this ecological state, the communities that have established footholds following perturbation, have matured and exhibit climax ecological conditions. Ecological networks (or webs) have been well-established and tend to function efficiently. Small perturbations and oscillations between states can occur; however, primary functions and communities are relatively stable.

Release. Significant physical or ecological events result in an often catastrophic system alteration. These events include fires, storms, disease, or invasion by pests or competing species.

Reorganization. Finally, as a consequence of these significant perturbations, the ecosystem reorganizes. Reorganization can result in ecosystems that are similar to their previous states or they can end up being quite different (Costanza and others, 1993).

Figure 1. Holling 4-Box Model of ecosystem succession with respect to connectedness (x-axis) and stored capital (y-axis). This general concept model illustrates the phase shifts following significant perturbation (disturbance) to reorganization (microbial scale), exploitation (pioneer/opportunistic species), and conservation (climax). From Holling (1992).

Figure 2. External conditions affect the resilience of multi-stable ecosystems to perturbation. The bottom plane shows the equilibrium curve. The stability landscapes depict the equilibriums and their basins of attraction at five different conditions. Stable equilibriums correspond to valleys; the unstable middle section of the folded equilibrium curve corresponds to a hill. If the size of the troughs shown in this figure (called "attraction basins") is small, resilience is small, and even a moderate perturbation may bring the system into the alternative basin of attraction. In essence, the relative amount of energy it would take to move the marble in this figure from one side of the surface to the other represents stability and resilience. Taken from Scheffer and others (2001).

Figure 3. Alternative equilibrium turbidities caused by disappearance of submerged vegetation when a critical turbidity is exceeded. The arrows indicate the direction of change when the system is not in one of the two alternative stable states. These response curves are based on three assumptions: 1) turbidity of the water increases with the nutrient level, 2) SAV reduces turbidity, and 3) vegetation disappears when a critical turbidity is exceeded. Taken from Scheffer (2001).

Figure 4. "Marble-in-a-cup" representation of the stability properties of lakes at five different levels of nutrient loading. Taken from Scheffer (2001).

Figure 5. Conceptual model of hysteresis effect (see lake example, Figure 3) that approaches irreversibility. In this case, the lowest turbidity that can be achieved through nutrient reduction alone remains above the established critical level. Adapted from Scheffer (2001).

The degree to which an ecosystem can retain stability in the face of extreme perturbation is called resilience. Ecosystems possessing high resilience can be pushed to extremes without reorganizing into a different form of stable state. Systems lacking resilience can be “pushed” into an alternative stable state, of which there may be more than one. Perturbations need not be sudden events, such as hurricanes and fire. They are often gradual and cumulative, such as the processes of eutrophication, grazing pressure, or climate change. In these cases, the system is slowly pushed to the capacity to retain its stability and goes through a release phase, as shown in Figure 1. However, in reorganizing, the key components of the original ecosystem are either removed or substituted with alternative components. Keystone species are those on which a large number of other species in the ecosystem depend. During release and reorganization, keystone species that formed the basis for the original ecosystem structure are replaced by others (opportunists). The new configuration provides a new organizational trajectory and thus a new stable state. In some cases, this shift in stable state can be irreversible. Alternatively, if the perturbation is removed or reversed (e.g., by a reduction in nutrient load), the system may follow a nonlinear response and ultimately may only approximate the original state. The new state can have a significantly different response relationship to the original perturbation. This process of nonlinearity in stressor-response behavior is called hysteresis. Hysteresis describes the potential difference between an ecosystem’s response to a perturbation and its removal, or reversal, over time (Figure 2).

A good conceptual model of hysteresis in aquatic systems is provided by Scheffer (2001). Shallow lakes and ponds (and estuaries) possess benthic (bottom) and pelagic (water column) interactions that are strongly coupled. For instance, the growth of submerged aquatic vegetation (SAV), an important aquatic ecosystem property, is primarily dependent upon light availability. If the water column becomes clouded because of suspended matter, the growth and survival of SAV will decline as a result of a decrease in light availability. In aquatic systems that are being over-enriched by nutrients, the process can result in an increase in the growth of phytoplankton and an overall increase in the amount of organic matter present within the water column. This increases water-column turbidity and reduces the intensity and duration of light reaching SAV, causing it to die off. Without SAV present, bottom sediment is more easily resuspended by wave energy and fish foraging. Thus, there is a reinforcing feedback occurring: a decrease in light availability reduces the coverage of SAV, which further decreases light availability. In addition, increased nutrient load promotes the growth of benthic macroalgae, which compete with SAV for habitat and can significantly exacerbate the decline of SAV. A restoration plan that involves nutrient reduction would be a logical course of action to reverse the degradation that this ecosystem has experienced. However, the mechanisms responsible for maintaining high turbidity in the water column are quite strong and may not respond directly to reductions in nutrients.
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Case Study: Hysteresis in an SAV Community

Shifts between alternative stable states have been studied in lake ecosystems. The sudden transition between clear, SAV-dominated lake waters to turbid SAV-degraded systems have been associated with increased nutrient supply. Upon increased rates of nutrient delivery, a threshold was passed at which the effects of eutrophication resulted in a widespread decline in SAV habitat. However, the response of SAV to the same magnitude of nutrient reduction was not favorable. The reason was found to be two-fold: 1) the lost capacity of SAV to trap suspended sediments created a persistent turbid state, and 2) loss of SAV habitat increased the availability of small zooplankton, which then resulted in an increase in the population of the fish that prey on them. These fish also forage for food in the mud, increasing the degree of resuspended sediment and, therefore, decreased water clarity. This resistance of ecosystem restoration to nutrient reduction in a linear manner is illustrated in Figures 3 and 4. In these figures, response curves associated with nutrient loads and reductions have different slopes and intercepts. The thresholds that determine critical turbidity are different, particularly once SAV has been removed from the ecosystem. Scheffer (2001) illustrated how the restoration of SAV was expedited by temporarily limiting the populations of foraging fish, resulting in a more rapid response to nutrient reduction. In addition, whole-lake experiments on responses to nutrient inputs showed that sudden, significant reductions in fish populations in addition to nutrient reductions resulted in restored permanent clear states (reductions in turbidity) (Meijer and others, 1994).

The nonlinear response nature of hysteresis and critical thresholds can make the result of restoration activities difficult to assess. Sometimes the expected ecosystem functional outcomes lag behind estimated recovery times and require additional intervention to “push” the state back to its former condition. However, instances can also occur in which expected recovery is never quite achieved, and therefore a perpetual irreversible state is attained (Figure 5).
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Sustainability

Sustainability has many meanings and definitions that are relevant to different applications of the word. The description, below, is offered by Columbia University’s Biosphere 2 Center and summarizes the Webster’s dictionary definition (Webster’s 1996):

Sustainability is a noun referring to a state or condition that can be maintained over an indefinite period of time. The verb sustain is rich with meaning and in various contexts can connote to keep up or keep going; to endure without giving way; to support, hold, or bear up; to provide for; to supply with necessities of life; to uphold as valid, just, or correct….

Many principles are associated with sustainability, including environmental health or condition, social equity, and economic development. These are not exclusive from one another, but embody separate complex systems. In the context of this article, the principle of environmental health and condition is perhaps most relevant. However, human-imposed influences on natural systems are often driven by actions relevant to the other principles of sustainability. Thus, sustainability is associated with the continued function, or desired condition, of an ecosystem over some selected period of time without intervention (self-renewing). Likewise, sustainability refers to the continued services provided by an ecosystem (e.g., the capacity to attenuate or absorb pollutants, or to provide the basis for the production of economically valuable commodities).

Natural capital is a term used to describe the collective services provided by the world’s ecosystems. Natural capital consists of both renewable and nonrenewable resources. Renewable resources include things such as trees and fish that can be replenished. They are also the wide variety of services that ecosystems provide (e.g., energy, materials, food, and waste assimilation). Nonrenewable resources include those that are finite and once consumed are lost forever (fossil fuels, minerals). Human capital, on the other hand, is the ability to transform and provide use of natural capital in terms of refined goods and services (e.g., machinery, labor, technological advances). Natural and human capital is substitutable to some degree — people can make up for declining resource quality with better methods for extraction, alternative resources, or more efficient use of resources. According to this argument, natural capital will probably not limit a path of sustainable development, as evidenced, for example, by declining real resource costs for many varieties of raw materials going back over 100 years. Other economists argue that human and natural capital are complementary (Daly 1996; Costanza and Daly 1992). The complementary argument states that sustainable economic activity requires both the use of human capital and a sustaining capacity of ecosystems to provide natural resources and waste assimilation. Once natural capital is exhausted or degraded to a critical level, either resources become scarce, or our waste stream becomes detrimental. Economic throughput, it is argued, requires a source of natural capital regardless of the size and efficiency of human capital. Because throughput in economic systems originates from a stock of natural capital and ultimately requires ecosystem functions to “recycle” or renew exhausted throughput, the economy becomes a subset of the world’s ecosystem (Daly 1996). Herman Daly proposed two views of the world's natural capital supply and economy: empty world and full world (Figure 6). The empty world view assumes a near limitless reserve of natural capital, because the rate of consumption (i.e., the size of the economy) is relatively small compared with the rate of resource renewal or the total supply of nonrenewables. In the full world view, resources are scarce or consumed at rates nearing the natural capacity to renew. These views illustrate how as growth occurs, the economic system eventually becomes limited by the supply of natural capital. Under this model, the global carrying capacity is limited by the ability of natural capital to support a minimum level of sustainable resources. One factor that may affect this difference in perspective is that, historically, extraction and materials utilization costs are directly paid by the firms engaged in the activity; thus, there has been unremitting economic pressure to reduce extraction costs and minimize material use. In contrast, disposal and recycling of wastes has been a “free” service of the environment, so it is not surprising that it has often met or gone beyond its true biological limit.

Figure 6. Comparison between "empty world" and "full world" models developed by Herman Daly (1996). Since the ecosystem remains constant in scale as the economy grows, it is inevitable that over time, the economy becomes larger relative to the containing ecosystem. The real world contains elements of both the "empty world" (declining resource costs, which are paid for) and the "full world" (declining assimilative capacity because markets may not exist or are highly imperfect).

Restoring ecosystems is one of many ways to invest in the sustainability of the services provided by natural capital. Restoration, by definition, means that a desired condition once existed and that degradation has resulted in some loss of natural capital. The likely (and desired) result of ecosystem restoration is an increased ability to produce “goods” such as energy sources for higher trophic levels, or materials valued by the economy. Restoration may increase the assimilative capacity of an ecosystem and, therefore, provide “services” associated with renewing waste. Many restored ecosystems provide both goods and services; however, the value of these goods and services is not immediately quantifiable, nor are they completely understood. Resource valuation and protection (or conservation) can be applied to avoid these losses and the added resource burden of having to invest in restoration activities.
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Resource Protection: The Safe Minimum Standard (SMS) and the Precautionary Principle

Protecting natural resources usually involves the consideration of relative resource value and opportunity costs to society (e.g., the economic cost of not developing). Determining the economic value of the services provided by ecosystems is shrouded by statistical uncertainty (or risk) and true uncertainty (unknown probability or “indeterminacy”). Uncertainty imposes a risk to decision-makers who are responsible for the preservation or restoration of ecosystems (stock of natural capital). Although risk assessments are now a common feature of government regulation activities, true uncertainty is still a challenge to most environmental protection and restoration strategies (Costanza and Cornwell 1992).

Much attention in resource protection and restoration is focused on the relative value of the resource at risk. Many attempts have been made to quantify the economic value of a range of important ecosystems and their services (Fisher and others, 1986; Lipton and others, 1995; Daly 1996; NRC 1992; Costanza 2000). These studies have been focused on specific collections of resources and whole-system processes. Nevertheless, the practice of maximizing the value and stock of natural capital, as well as ensuring a sustainable future stock, requires some form of control over resource exploitation.

Benefit-cost assessment (BCA) is one conventional approach used to evaluate production and investment decisions associated with natural resource use and restoration. Attempts to expand this approach have resulted in establishing evaluations based on resource importance and reversibility (King 1994). However, as a result of high levels of uncertainty and conflicting scientific reporting, a series of approaches that attempt to find safe solutions (minimize risk and opportunity costs) have evolved. The application of a SMS (Ciriacy-Wantrup 1952) has been proposed with the goal to preserve and maintain a renewable resource at a level that would preclude extinction, unless social costs were prohibitive or “immoderate.” This approach is not necessarily a conservation measure, but a framework to avoid natural disasters. Similarly, the precautionary principle (Perrings 1991; Costanza and Cornwell 1992) has been proposed as added “insurance premiums” associated with preventing irreversible, long-term degradation in the face of high uncertainty (both ecological and economic) (Ehrlich 1994). Costanza and Cornwell (1992) proposed a combined precautionary principle/polluter pays (“precautionary polluter pays principle”) in which strong economic incentives would be provided through a bonding system to pay for ecosystem damages up front.
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Concluding Remarks

In most cases, restoration activities are intended to increase an ecosystem’s ability to provide beneficial services. Restoring ecosystems to their exact former condition is rarely experienced; however, close approximation is possible and becomes, therefore, the goal of restoration planning (NRC 1992). Also, in some cases, the term “restoration” is better substituted with “enhancement” when improvement, but not restoration occurs. Nevertheless, restoration activities face several challenges associated with uncertainty. Some ecosystems have been perturbed to the point at which limits of stability and resilience have been exceeded and the resulting condition may represent alternative stable states. These new stable states do not necessarily possess the same response characteristics to the forces that resulted in change. For practical reasons, some state shifts may be considered irreversible due to the impracticality of necessary restoration efforts. Others must be understood to an adequate degree such that projected responses to restoration activities are not undermined by the effects of hysteresis.

Sustainability within ecosystems is necessary in supporting sustainable economies, according to ecological economists. This interdependency is due to the direct relationship between the economy and the capacity of the world’s ecosystems to provide resources and waste assimilation. Ecosystem restoration can play a significant role in providing greater, sustainable stocks of natural capital. Mechanisms, such as the concepts of SMS and the precautionary principle, have been studied and developed to provide guidance to minimize risk of resource loss in the face of complexity and uncertainty.
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References

Ciriacy-Wantrup, S.V. 1952. Resource Conservation. University of California Press. Berkeley, CA.

Costanza, R. 2000. "Social goals and the valuation of ecosystem services." Ecosystems Volume 3. Pages 4 to 10.

Costanza, R., and L. Cornwell. 1992. "The 4P approach to dealing with scientific uncertainty." Environment. Volume 34, Number 9. Pages 12 to 20 and 42.

Costanza, R., and H. Daly. 1992. "Natural capital and sustainable development." Conservation Biology. Volume 6. Pages 37 to 46.

Costanza, R., W.M. Kemp, and W.R. Boynton. 1993. "Predictability, scale, and biodiversity in coastal and estuarine ecosystems: implications for management." Ambio. Volume 22. Pages 88 to 96.

Daly, H.E. 1996. Beyond Growth. Beacon Press. Boston, MA.

Ehrlich, P.R. 1994. "Ecological economics and the carrying capacity of Earth." In Investing in Natural Capital. Jansson, A., and others eds. Washington, D.C. Island Press

Fisher, A., and others. 1986. (University of California, Berkeley). Economic Valuation of Aquatic Ecosystems. 1986 Oct; Final Report to U.S. Environmental Protection Agency, Cooperative Agreement No. 811847.

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King, D.M. 1994. "Can we justify sustainability? New challenges facing ecological economics." In Investing in Natural Capital. Jansson, A., and others eds. Washington, D.C., Island Press

Lipton, D., and others. 1995. Economic valuation of natural resources -- A handbook for coastal resource policymakers. NOAA Coastal Ocean Office, Silver Spring, MD.

Meijer, M.L., and others. 1994. "Long-term responses to fish-stock reduction in small shallow lakes: Interpretation of five-year results of four biomanipulation cases in the Netherlands and Denmark." Hydrobiologia. Volume 276. Pages 457 to 466.

NRC. 1992. Restoration of Aquatic Systems. 1st ed. NRC, National Academy Press. Washington, D.C.

Pearce, D.W., and R.K. Turner. 1990. Economics of Natural Resources and the Environment. 2nd Ed. The Johns Hopkins University Press. Baltimore, MD.

Perrings, C. 1991. "Reserved rationality and the precautionary principle: technological change, time and uncertainty in environmental decision making." In Ecological Economics: The Science and Management of Sustainability. 3rd ed. Columbia University Press. New York, NY.

Scheffer, M. 2001. "Alternative attractors of shallow lakes." The Scientific World. Volume 1. Pages 254 to 263.

Scheffer, M., and others. 2001. "Catastrophic shifts in ecosystems." Nature. Volume 413. Pages 591 to 596.

Webster's Encyclopedic Unabridged Dictionary of the English Language. 1996. Gramercy Books / Random House, New York, NY.
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