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Resilience

The term resilience is sometimes used differently, but its meaning focuses around the potential of a system to react to perturbations by maintaining its essential characteristics. Introduced into ecology by Crawford S. Holling in the 1970ies, it is often understood in one of the two following ways:

  • as Engineering resilience, referring to the time an ecosystem needs to return to an equilibrium or a steady state after perturbation (aka recovery rate), or
  • as Ecological resilience, referring to the magnitude of disturbance a system can tolerate before it shifts into a different state and indicating the capacity of this system to absorb perturbation and to reorganize while undergoing change so as to still retain essentially the same function, structure, identity, and feedbacks.
Resilience

The first definition is often used in the context of physical systems and draws on the theoretical conception of stability as derived from (mathematical) system models. Holling (1973) however stresses the difference between resilience and stability. While stability refers to the theoretical concept of equilibria (fixed points, limit cycles ...) which can be found in mathematical analysis of idealized systems, Holling emphasizes that natural systems can show high resilience just because of their instability. He illustrates this on the example of spruce budworm outbreaks that repeatedly have afflicted Canadian forests.

Usually, in these forests, the budworm is a rather rare species kept at low population size by various natural enemies. In this state, it does not impair young fir from growing, together with spruce and birch, to form dense stands in which the spruce and the birch, in particular, suffer from crowding. Their reproduction is hampered by the density of trees. Still, fir, spruce and birch together are able to produce stands of mature and overmature trees with fir being predominant. Spruce budworm outbreaks have been found to be correlated with a sequence of unusually dry years. After such years, when there are mature stands of fir, the budworm starts rapidly reproducing and thus escapes the control by predators and parasites. These outbreaks cause major destruction of fir trees in mature forests, leaving only the less susceptible spruce, the non-susceptible white birch, and a dense regeneration of fir and spruce. At some point however, the increase of the budworm population eventually causes enough tree mortality to force its own collapse and the reinstatement of control around the lower equilibrium.

"In brief, between outbreaks the fir tends to be favored in its competition with spruce and birch, whereas during an outbreak spruce and birch are favored because they are less susceptible to budworm attacks. This interplay with the budworm thus maintains the spruce and birch which otherwise would be excluded through competition. The fir persists because of its regenerative powers and the interplay of forest growth rates and climatic conditions that determine the timing of budworm outbreaks." (Holling 1973)

Told in this way, the forest dynamics could be seen as following a limit cycle with large amplitude. But regarding the budworm by itself, it shows a widely fluctuating and hence rather unstable dynamic. Depending on climatic conditions, the outbreaks occur highly irregular. Nevertheless, they enable the overall system to persist, to absorb change and disturbance, and to maintain the same relationships between populations.

Holling, C.S. (1973). Resilience and stability of ecological systems. Annual Review of Ecology and Systematics 4: 1–23.

Walker, B., Holling, C. S., Carpenter, S. R., Kinzig, A. (2004). Resilience, adaptability and transformability in social–ecological systems. Ecology and Society 9 (2): 5.

A similar example can be seen in irregularly occurring smaller bush fires which clean out loose underwood and branches from forests and thus prevent the outbreak of large devastating fires.