The susceptibility of an SES to suffer a loss or a damage from exposure to external stresses and shocks is defined as vulnerability or fragility. It is an inherent system measure of the propensity to change induced by the coupling of agents of pressures on the system and the system sensitivity to such exposures. The two terms are basically equivalent, though the second is generally adopted when only natural systems, like ecosystems, are considered. Vulnerability/fragility is inversely related to ecological resilience and as resilience it is linked to diversity and redundancy of functions and components in an SES at the same and across a range of spatial and temporal scales. Whenever sources of novelty are eliminated (e.g., loss of biodiversity or cultural homogenization) and as functional diversity and cross-scale functional replication are reduced, system vulnerability/fragility is increased because the ability of an SES to cope and recover from disturbances (i.e., ecological resilience) or to persist is compromised. As an example, the elimination of structuring species or processes can cause an ecosystem to reorganize, thus to change to a different state or flip to a different stability domain (Figure 1). Whenever there is a change in system vulnerability/fragility, a compensation in resilience level is triggered by adaptive measures to anticipate and reduce future harm.
As other systems' properties, vulnerability/fragility is difficult to quantify in terms of actual or future values. But when detectable system changes could be determined, as a result of natural as well as human-induced disturbances, a retrospective measure of displayed vulnerability/fragility may be advanced (i.e., the change or difference between the two states before and after the system variation). The major disadvantage of such a retrospective evaluation lies in the fact that it does not allow distinguishing between external pressure and resistance or sensitivity factors, which could have determined changes. Moreover, the possibility of recognizing driving forces causing pressure without confounding and overlapping could be compromised when time and spatial scales of change observation are mismatched compared to the ones characterizing disturbance regimes and perturbations.
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