Conceptual Threshold Framework
We assembled a conceptual framework to help focus thinking about the dynamics of ecological sites and ecological thresholds (Fig. 1). We integrated published frameworks to take advantage of different perspectives, but largely relied on the resilience-based state-and transition framework of Briske et al. (2008). Associated with our conceptual framework is a narrative template to aid in capturing current understanding of the key features of reference and alternative states, to identify plausible transition triggers, and to document possible resilience properties of states (Table 1).
The conceptual framework starts the reference or native state of an ecological site, and community phases of the state (Fig. 1 - 1). Native or reference conditions typically are the desired state for conservation management. Negative feedbacks confer system resilience and maintain the community phases within the state. For instance, a negative feedback that inhibits shrub dominance in grasslands is the interaction between amount of grass cover and fire return interval. Given sufficient grass cover, wildfire events are often and large enough to inhibit the establishment of shrubs across a grasslands. Resilience is defined as the amount of change required to overcome stabilizing, negative feedbacks. Phases comprising the natural range of reference conditions differ in their vulnerability to disturbances and stressors. Phases with diminished structure or partially impaired functioning are more vulnerable (e.g., 1-III in Fig.1). In the absence of severe disturbances and stressors, however, a vulnerable phase may develop to a more resistant phase (e.g., 1-I in Fig. 1). Alternatively, the intensity of a driver or stressor and the temporal coherence of these change agents can exceed the resilience of a phase, and initiate a threshold crossing with equal development of negative and positive feedbacks mechanisms. Narrative information about the reference state includes a description of the indicators of non-vulnerable phases (key compositional, structural, and functional properties), the feedbacks that confer resilience, and the indicators of vulnerable or at-risk phase(s).
A threshold crossing occurs due to biotic or abiotic triggers which can span spatial scales from site level (e.g., trampling) to regional (e.g., climate). A key feature emphasized in the framework is the recognition that a single driver or stressor (trigger) may initiate a threshold crossing, or temporal order or spatial convergence of multiple triggers may be critical for a threshold crossing. For example, drought or excessive livestock grazing may not significantly alter a site, but the combination of these may precipitate soil erosion and the eventual development of alternative states. For each trigger or combination of triggers, subsequent alternative states will differ. For all triggers, the following generalized threshold progression is representative, but the sequence and types of alternative states will differ. The narrative requires a description of the triggers known or hypothesized to initiate threshold crossings.
The threshold progression is characterized by increasing dominance of positive feedbacks, and changes in compositional and functional properties. Over time there may be a substantive change in the structural properties of a site followed by a loss of reference-condition species and the establishment of other native or exotic species (3 in Fig. 1). Inherent in this progression is the continual loss of properties of the reference condition. The early portion of a threshold progression is referred to as preventative thresholds (Bestelmeyer, 2006), indicating that residual components of reference conditions and resilience still exist. Properties of resilience include those, for example, which promote soil nutrient and water retention, and the regeneration of native plants. Many alternative states can occur in this threshold progression phase, with some becoming stable alternative states. Occurrence of alternative stable states as well as the direction and rate of threshold progression is mediated by feedback dominance, and the influence of drivers and stressors. With intervention (facilitating practices (Brandon B. cit)– 5 in Fig. 1) these states may be reverted to reference conditions. This may require the removal of the dominant species of the post-threshold state or simply the removal of the change agent (e.g., social trailing, livestock grazing). The degree of residual reference conditions and resilience influences the probability of reversibility. The narrative for this includes procedures likely to result in the reversal to pre-threshold conditions. The emphasis on preventive thresholds is to highlight the fact that a threshold crossing doesn’t equate to degraded conditions that can never be restored to pre-threshold conditions.
Continued increase in the dominance of negative feedbacks that stabilize alternative states can proceed to a point where features of the reference condition are effectively no longer present, and negative feedback mechanisms maintain the stability of a degraded state (4 in Fig. 1). The pattern and process thresholds leading to this phase determine the degraded state. Thus, for an ecological site, multiple stable, degraded states are possible. Active restoration methods (accelerating practices – 6 Fig. 1) are required to revert to reference conditions (Brandon.B. cit). The energy and costs to revert a degraded state may be prohibitive from a management perspective. Also, reversibility may not be biologically possible due to the extinction of native biota (i.e., species, genomes), and the loss of inherent properties necessary to support reference conditions. The narrative for degraded states includes a description of structural and compositional properties of plausible states, properties conferring resilience of these states, and potential restoration actions, including the probability of restoration success. In some cases, complete restoration to native conditions may never be possible due to the extinction of inherent system properties. Thus, a description of the most likely conditions that can be restored is included.
Post-threshold states have consequences for ecosystem services and resource management (7 – Fig. 1). These consequences increase along the gradient from preventative to restoration thresholds. Degraded states especially may no longer afford provision of services such as water (amount and quality), livestock forage production, or desirable recreational opportunities, and may no longer support the biodiversity of native systems. Consequences to resource management include the cost of restoration actions, where possible. The narrative for consequences of threshold crossings includes a description of the impacted ecosystem services, and the practicality and costs of restoration actions.
Table 1. Narrative guide to capture the essential properties and dynamics of reference states and of alternative states resulting from a threshold crossing (see Fig. 1). Section 1 is repeated for each community phase of reference conditions. Sections 2-4 are repeated for each possible threshold crossing and threshold progression.
Ecological Site name
1. Reference State; Community Phase_________.
Structural properties
Functional properties
Negative feedback mechanisms
Inherent properties conferring resilience (properties that tend not to change; e.g., soil properties)
Dynamic properties
conferring resilience (properties with the potential for large change; e.g., vegetation structure and composition)
Vulnerability to threshold crossing
Possible management-assessment point (process, pattern characteristics for early-warning of an impending threshold crossing)
2. Threshold Crossing
Transition triggers
(list biotic, abiotic trigger(s), and temporal order & spatial convergence properties)
3. Preventive Threshold
Transition triggers leading to this state from other alternative states (if applicable)
Likelihood of occurrence
Structural properties
Functional properties
Negative feedback mechanisms
Positive feedback mechanisms
Condition of the Inherent properties conferring resilience in the reference state
Condition of the Dynamic properties conferring resilience in the reference state
Mechanism(s) of reversibility (facilitating practices)
Probability of reversibility (probability of complete restoration or probability of restoring near-similar reference conditions and attributes of these conditions)
Consequences of this state to ecosystem services, resource management
Consequences of this state to resource management
4. Restoration Threshold
Transition triggers leading to this state from other alternative states (if applicable)
Likelihood of occurrence
Structural properties
Functional properties
Negative feedback mechanisms
Positive feedback mechanisms
Inherent properties conferring resilience
Dynamic properties conferring resilience
Mechanism(s) of reversibility (accelerating practices)
Probability of reversibility (probability of complete restoration or probability of restoring near-similar reference conditions and attributes of these conditions)
Consequences of this state to ecosystem services
Consequences of this state to resource management
No comments:
Post a Comment