Introduction - What Is Resilience?
Have you ever wondered how some systems, such as ecosystems or societies, can undergo massive changes, like forest fire or foreign invasion, and somehow recover to a similar state as before? Perhaps one forest burns and recovers in a few years, while a similar forest instead regrows as a different kind of forest, or as a mix of forest and grass. Why do some cities fall permanently to the first invading army, and others are rebuilt many times? Why do destructive forces affect forests and cities so differently? The concepts of ecological resilience help us understand why some complex systems like forests and cities stay the same and why others transform completely.
Ecological resilience is defined as the amount of disturbance that a system can withstand without altering self-organized processes and structures (Gunderson 2000). This concept can be applied to help explain why some communities continue to thrive following a natural disturbance and why other communities are devastated.
Ecological resilience was introduced as a concept in 1973 by C.S. Holling, who attempted to provide insight into three “reactions” ecosystems may have to a disturbance such as fire or climatic change:
1. Persistence of the relationships among components of a system in the face of change; for example, the ability of an ecosystem’s members (like animals and plants) to continue their daily interactions despite a disturbance
2. The capacity of a system to absorb disturbances and continue functioning; for example, the ability of an ecosystem to continue providing the same ecosystem services (such as water purification, carbon sequestration, etc.) despite having been disturbed
3. The sudden and discontinuous change ecosystems undergo following a disturbance that cannot be absorbed; for example, a quick and significant change in ecosystem services or plant/animal interactions as the ecosystem becomes fundamentally different than it was before the disturbance
One example of resilience can be found in grasslands. A problem many grasslands around the world face is the encroachment of woody plants (Walker and Salt 2012). Woody encroachment occurs when the processes that limit woody plants in grasslands are altered. As woody plants expand they block sunlight from reaching the herbaceous plants below the canopy, like grasses and forbs, leading to the loss of herbaceous biomass. As herbaceous plants die off due to lack of sunlight, the woody plants fill in the gaps until the only thing left is a dense, woody thicket with little to no understory (Walker and Salt 2012).
Now let’s look at this example through a resilience lens. In the early stages of encroachment, there were woody species within the grassland but the system still functioned as a grassland. As time progressed, the woody species grew denser and the grasses died off. Its resilience was overcome as the ability of the system to resist the invasion of woody species was reduced. Once the original grassland structures were gone, the system no longer functioned as a grassland. The structure, functions, and relationships of a grassland disappeared and were replaced by the structure, functions, and relationships of a forest. This is an example of the third reaction above.