2.2 Thresholds

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Managing resilience requires identifying and managing the critical thresholds that separate desirable states from undesirable states.

Example: Lakes, Agriculture, and ThresholdsKnowing the factors that push a system beyond a threshold may be more important than knowing the threshold per se. Once a threshold has been crossed it may be difficult or even impossible to return to a previous state. Thus it is important to understand what it is that moves a system closer to a threshold both if it is to be avoided but also if transformation of the system is a management objective.

Key Messages

• A threshold has been crossed when system feedback changes, and consequently the defining characteristics of the system change, leading to an alternate system state.

• One might consider different configurations or alternate states of a system based on the history of the system, the range of conditions seen in similar systems, or by way of different visions for the future of the system (see 2.2 Alternate States).

• Some changes in system state are difficult or impossible to reverse. It is not always possible to restore lake clarity for example, by simply reducing phosphorus levels to what they were before the threshold was crossed. The threshold that triggers a change from clear to cloudy lake may not be the same threshold that will trigger change from a cloudy to clear lake.

• Thresholds can also move. In general, we often find that it is the slowly-changing variables in a system that cause a shift in the position of thresholds. Phosphorus input levels that a clear lake could absorb in the past, may suddenly trigger change to a cloudy lake through a process of eutrophication. In this case, phosphorus bound up in lake sediments is released, changing the lake’s tolerance to the amount of external phosphorus inputs.

• Thresholds, sometimes referred to as a tipping point, exist in social and economic systems as well. In the United States, for example, public intolerance to second-hand-smoke from cigarettes reached a certain critical level and smoking rapidly moved from being publicly acceptable to being unacceptable.

• Managing for resilience requires being aware of thresholds, determining what its characteristics might be, favorably altering the position of thresholds where possible, and keeping systems away from thresholds that lead to undesirable states.

Resilience Assessment

In the following assessment you will identify potentially critical thresholds in your system and explore which disturbances might be pushing your system closer to these thresholds. It is not essential to know the exact position of thresholds – which is rare, and they are dynamic in any case. You will also look at slowly-changing variables in the system, which often strongly influence the position of thresholds in the system.

Return to the alternate states diagram you developed in the previous section. True alternate (as opposed to transient, intermediate) states are separated by thresholds. Consider each of the processes you identified as having the potential to move the system from one state to another. Attempt to identify which of the transitions are smooth and gradual and which of the transitions are abrupt and jumpy. For example, the transition in a rangeland as it shifts from being grass-dominated to shrub-dominated passes a threshold level of woody biomass beyond which the feedback to fire intensity changes, and the system then goes all the way to a woody state even if all livestock are removed. The change in the system's directional dynamics is certainly sudden, at that point, but the change in amounts of observable variables (grasses and shrubs) is gradual. In contrast, when a lake ecosystem passes the threshold in phosphate content that determines the shift from the clear to eutrophic regime, the change in dynamics of the system is sudden, and so is the change in the observable variables (algae biomass, turbidity).

Looking at the transitions, identify the cases for which reversal to the previous state are difficult. Such transitions may represent critical thresholds in the system. Can you determine the approximate position of the threshold? For some processes this may be easy—it is either ‘on’ or ‘off’ (for instance, one process may be the implementation of a policy of subsidies—once the policy is enacted, the threshold has been crossed). For others it may be difficult or impossible to determine a threshold position. Assign a value, or a range of values, to thresholds wherever you can. On your state and transition model(s), indicate the threshold value and the degree of reversibility (easily reversible e.g., a step function, somewhat reversible, or highly irreversible).

Describe any additional characteristics of the threshold(s) of potential concern.

Does the focal system appear to be near or approaching any of the thresholds of potential concern? If so, which ones? Start a list of thresholds of potential concern.

Do any of the thresholds lead to undesirable system states? Do any of them lead to more desirable system states? Include on the list of thresholds of potential concern whether the alternate state is more/less desirable and how reversible the transition appears to be.

Consider the suite of disturbances you identified earlier (section 1.4). Do any of these disturbances move you closer to a threshold? Are these disturbances—either singly or combined, changing in such a way that crossing a threshold is becoming more likely? Which thresholds? Are they somewhat irreversible, and do they separate the current state of the focal system from a less desirable state? List these thresholds of potential concern in the table below and add the disturbances that might push the focal system over one of these thresholds.


Table 2.2.1 Thresholds and disturbances of potential concern

THRESHOLDS OF POTENTIAL CONCERN DISTURBANCES OF POTENTIAL CONCERN

 

 

 

 

 

 

 

 

 


Consider again the thresholds separating your current system state from less desirable states. List the factors or processes that might gradually be altering the position of those thresholds over time, if any (not all thresholds will necessarily move)? How manageable are these influencing factors? Note that these factors or processes are often related to slow variables in the system (e.g., such as phosphorus in lake sediment from the lake example). If possible, try to identify any slowly-changing variables that appear to be system drivers and list them in the following table.


Table 2.2.2 Factors or slow variables shifting thresholds

FACTORS OR SLOW VARIABLES SHIFTING THRESHOLDS
 
 
 
 


Consider the list of thresholds of potential concern. What management strategies, if any, can be employed to move you further away from those thresholds? What does this mean for strategic management in your focal system? Enter any action items on the sheets provided.

Consider the list of disturbances of potential concern. Which disturbances, if any, can be managed so that they are less likely to push the focal system over an undesirable threshold, or closer to a threshold leading to a more desirable state? (Keep in mind that trying to tightly control some disturbances can erode resilience in the system.) What does this mean for strategic management in your focal system? We will return to this section and utilize the information gathered here again in chapter 5 when considering management interventions. Enter any action items on the sheets provided.

Consider the factors, or system attributes, that influence the position of thresholds on the controlling (slow) variables. Are these attributes and their effects on thresholds adequately known and are the changes in controlling variables monitored? If not, devise a plan for monitoring them. The list of system attributes that underpin the changes in slow variables is a very important outcome of this resilience assessment, since it is these attributes that need to be managed in order to influence the changes in slow variables. Enter any action items on the sheets provided.

Are there any thresholds or disturbances of potential concern for which you have too little information? If so, devise a plan for obtaining further information—e.g., either through accessing historic data or modeling the system. Enter any action items on the sheets provided.



2.1 Alternate States

2.2 Thresholds

2.3 Scenarios

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