Description - How Can We Visualize Resilience in the Real World?
The concept of resilience is often illustrated through the “ball-in-cup” model as illustrated in Figure 1. The balls in each diagram represent the current condition of the system (Gunderson 2000). Each cup or “basin of attraction” represents potential states or “regimes” in which the system can exist (for example, a desert, grassland, or forest). The ecological resilience concept recognizes that there is more than one basin of attraction (known as “alternative states”) to which the system may transition. The peak that divides two alternative states is known as the “threshold”. Ecological resilience is focused on quantifying how much “perturbation” the system can undergo before moving to an alternative state.
Figure 2 shows two examples of the ball-in-cup model as a grassland transitions to two possible alternative states.
Over half of the world's grassy ecosystems are considered “disturbance-dependent” ecosystems, in that they require disturbance to persist long term and will transition to a woody alternative state in the absence of disturbances (Bond et al. 2005). In the Great Plains, fire suppression and introduction of Juniperus propagules in grasslands has led to regime shifts from grassland to a Juniperus woodland alternative state (Twidwell et al. 2013).
Grassland resilience is supported by stabilizing feedbacks between herbaceous fuels, fuel continuity, and ignitions (Ratajczak et al. 2014). Continuous herbaceous fuels facilitate the spread of fire across grasslands that limit the distribution and abundance of Juniperus trees. Interruption of stabilizing feedbacks in grasslands reduces the resilience (i.e., shrinks the grassland cup in the ball-in-cup heuristic) of grasslands to Juniperus woodland regime shifts while supporting feedbacks that promote the Juniperus woodland state. For example, removing/suppressing ignitions, fragmenting herbaceous fuels, or reducing fuel loads can suppress stabilizing feedbacks in grasslands, increasing the likelihood of Juniperus establishment and further fragmentation and reduction of herbaceous fuels. Thus, high fire occurrence and intensity supports grassland resilience while low fire occurrence and intensity supports Juniperus woodland resilience (Fuhlendorf et al. 2008).