The descriptors are fundamental for defining such reference states and further to elaborate threshold values between the levels in the model based on ecological attributes. Extreme conditions may contribute to lower resilience in ecosystems of the north, thus requiring more attention. The degradation history of Iceland after human arrival illustrates how new pressures brought by people on fragile ecosystems can have catastrophic impacts (Arnalds 1987). The removal of trees and introduction of grazing animals triggered degradation processes that lead to loss of birch woodlands (Aradottir & Arnalds 2001) and vegetation cover (Thorsteinsson etai.