02 May 2013


Resilience, in the ecological sense, refers to the ability of a system to absorb external or internal shocks and still retain its fundamental form and function.  In other words, it remains in the same regimen of controlling factors.  The ability to restructure after such shocks is a characteristic of resilient systems.  A metaphorical description of resilience is an evolutionary one: resilient systems are those that can adapt to change, and “remain in the game.”  Notably, resilience does not mean that a system will not experience internal change in response to a shock, stress, or disturbance.  Rather, it means that the system can adjust itself after the shock and keep the same general identity.  

Figure 1. A resilience conceptual landscape.  Two different sets of controling factors are represented by either the solid or the dashed curve.  The black ball represents the system composition and function under the solid regime, and the threshold to a different realm of environmental control is shown at the top of the hill to the right.  If disturbances, represented by the single-headed arrows, do not move the system beyond the threshold, the system is said to be resilient.  It can absorb the shocks of the disturbance and retain its qualitative identity, though there may be internal changes as a result of the disturbance, and those would elicit adaptive responses.  If a disturbance pushes the system beyond the threshold, or if the environmental conditions change so that it becomes easier or harder to reach and exceed a threshold, the adaptive requirements for resilience, and hence the level of resilience would change, as represented by the double headed arrows and the contrast between the solid and the open “systems.”  From the arctic-council.org.

It is important to recognize that there are two contrasting definitions of resilience in circulation.  The older one, engineering resilience, is the ability of a system to absorb a shock or deformation and to return to its original, equilibrium state.  A rubber band is a perfect example of such a system.  The loose, floppy form can be stretched but, when released, it returns to its general band-like shape.  This sort of resilience is called engineering resilience because it characterizes built structures and infrastructure.  Key to this idea is that there is an acceptable, desired, or equilibrium state of the system.  Of course, extreme deformation or perhaps consistent strong deformation over time can lead the system to fail: The band breaks when stretched too far, or after years of material degradation while encircling a thick wad of forgotten papers in a hot attic. 

The concept of engineering  resilience is consistent with the old, equilibrium paradigm of ecology, which has been replaced by a more dynamic, non-equilibrium paradigm.  Equilibrium or engineering resilience may be of value when it is possible to identify a desirable or designed state that is expected to persist over some specified time or at a particular spatial scale.  But for many purposes, engineering resilience is best considered a narrow, special case of the concept of resilience.  The second definition of resilience is the one introduced at the beginning of this post, and explained further below.

Ecological resilience is the definition suitable to systems that are not at equilibrium, or which periodically or constantly change and adjust.  Such a dynamic view of resilience is appropriate to cities, as they are complex, adaptive systems that have no fixed end point of development, but which embody learning and adjustment.  Furthermore, they are affected by shocks of economic, human migrational, climate change, and biophysical origin.  For example, economic investment or disinvestment, the arrival of a mass migration, the effects of sea level rise, or the occurrence of hazards such as floods, can all shock socio-ecological urban systems.

A biological example of resilience can be found in an extensive area of broad-leaved forest in a moist climate where fire is rare.  In such a climate, severe windstorms, such as tornadoes, would be the primary external agents of disturbance.  In such a forest region, an old, continuous canopy dominated by long-lived trees, such as beech, maple, and hemlock, might be blown down by a tornado.   

Figure 2. A tornado blowdown 31 May 1985, in an old-growth forest in Western Pennsylvania.  The dark red in this false-color infrared image represents intact canopy, while the lighter tones represent dying foliage of downed trees and exposed forest floor.

The disturbance event sets in motion a reorganization phase, with some of the younger damaged trees able to resprout, while trees that have been absent from the forest at that spot for decades, but whose seeds have lain dormant in the soil, germinate in response to the altered light and temperature at the surface.  The seeds of other tree species that require high light levels, arrive on the wind, or are deposited by birds perching on the woody debris and snags left by the tornado.  Understory herbaceous species flourish for a time, reproducing and producing large numbers of propagules.  The forest regrows as the light-demanding species give way to dominance by the more shade-tolerant species that will ultimately occupy the overstory.  This process of episodic disturbance, reorganization, and regrowth are all part of the same forest system.  The system as a whole is resilient, although individual components are killed by the tornado, while others take advantage of the changing conditions produced by the regrowing forest itself.  This kind of dynamic is a source of the insights embodied in the ecological resilience concept (Holling  and Gunderson 2002).

A social model of resilience is represented by the adaptive response of the Chacoan culture of the US Southwest (Tainter 1988).  Within the arid San Juan Basin of New Mexico, the Chaco Canyon stands out as an arid, but heterogeneous setting.  Here drought is a patchy and asynchronous event.  The ancient population initially organized in dispersed settlements, each experiencing high and low agricultural production at different rhythms than its nearby neighbors.  The principal settlements included large storage capacities for maize, and were connected by an efficiently laid out road network.  Presumably, such a physical arrangement would have required administrative capacity, organization of dispersed labor, and sharing of information to assess, store, and distribute surpluses.  This strategy, a variety of energy averaging, was highly adaptive in this environment.  Below a certain density of settlements, including both the administrative and grain storage centers represented by Great Houses and the smaller dependencies, would have effectively averaged energy.  The system was resilient, since different areas had different temporal patterns of agricultural production, and therefore differentially contributed to or drew on the centralized surpluses.

Resilience is, notably, not guaranteed forever.  As the Chacoan population grew based on increased food security allowed by energy averaging, more settlements were added.  This decreased the average distance between settlements and would have increased the likelihood that larger numbers of them would experience synchronous drought and poor harvests.  

Figure 3. The topographically heterogeneous landscape of Chaco Canyon, which allowed the agricultural risk spreading in a drought prone, arid environment.  Photo by Peter Potterfield, http://www.greatoutdoors.com/published/from-chaco-canyon-to-sky-city

As a result, the adaptive benefit of resource averaging was no longer available.  After that time, the return on the investment in administration, infrastructure, grain distribution, and labor sharing became insufficient to purchase the loyalty of outlying settlements and the system then shifted to a completely different realm of control.  In other words, the complex Chacoan civilization collapsed.  This example shows both that social-ecological systems can exhibit resilience through adaptive behavior, but that it is possible for the interaction of external events and the structure of the system to cause a collapse into a different regimen of control.

Why important: 
Ecological resilience does not ask whether a complex system returns to a previous or equilibrium state. Rather, it asks about the changes that a system can experience and still persist in the same dynamic form.  This is an evolutionary kind of resilience since adaptation is a central feature.  So ecological and evolutionary resilience are concerned with adaptive capacity and adjustment to change, and not with return to a stable point.  Rather than asking about the ability of a rubber band to return to its unstressed state, evolution asks about the rubber band becoming something else that is better adapted to the new conditions.  It is of course silly to think about a simple, physical-chemical system such as a rubber band changing in such a radical way, but evolution, adaptation, learning, and adjustment are familiar capacities of both biological and social systems.  In other words, they are complex systems that can adapt.  Resilience in the more evolutionary sense is the idea that points toward the question of how--and how well--a particular system can adapt to changing conditions or sudden shocks that come at unexpected times.

The concept of ecological resilience is relevant to the BES III main theme of transition from the Sanitary to the Sustainable City (Pickett et al. 2013b). The sanitary city identifies a desired state, and seeks to keep structures or processes at a specified level.  Given that societal and regulatory decisions identify legal or desirable targets for things that people must manage, a classical or engineering definition provides guidance about how to measure success.  However, under changing environmental conditions, including social, economic, and environmental alterations, it may be more appropriate to ask about the capacity of the system to adjust to those changes.  Because feedbacks among social, economic, and environmental factors and processes are integral parts of urban ecosystems, we must learn to go beyond the engineering resilience concept and understand and use the contemporary concept of ecological or evolutionary resilience.  It is this concept that can support the desirable goals identified by socially adopted visions for urban sustainability.  Resilience, and its contributing adaptive processes, are the mechanisms that can promote or inhibit sustainability.

For more information:
·         Gunderson, L. H. 2000. Ecological resilience - in theory and application.  Annual Review of Ecology and Systematics 31:425-439.
·         Holling, C. S. 1996. Engineering resilience versus ecological resilience. Pages 31-44 in P. C. Schulze, editor. Engineering within ecological constraints. National Academies of Engineering, Washington, DC.
·         Holling, C. S. and L. H. Gunderson. 2002. Resilience and adaptive cycles. Pages 25-62 in L. H. Gunderson and C. S. Holling, editors. Panarchy: understanding transformations in human and natural systems. Island Press, Washington, DC.
·         Pickett, S. T. A., M. L. Cadenasso, and B. McGrath, editors. 2013. Resilience in ecology and urban design: linking theory and practice for sustainable cities. Springer, New York.
·         Pickett, S. T. A., C. G. Boone, B. P. McGrath, M. L. Cadenasso, D. L. Childers, L. A. Ogden, M. McHale, and J. M. Grove. 2013. Ecological science and transformation to the sustainable city. Cities.  http://dx.doi.org/10.1016/j.cities.2013.02.008
·         Redman, C. L. and A. P. Kinzig. 2003. Resilience of past landscapes: resilience theory, society, and the longue durée. Conservation Ecology 7(1): 14. [online] URL: http://www.consecol.org/vol7/iss1/art14
·         Resilience Alliance.  http://www.resalliance.org/index.php/resilience (accessed 29 April 2013)
·         Tainter, J. A. 1988. The collapse of complex societies. Cambridge University Press, New York.
·         See also BES Urban Lexicon terms: Adaptive Processes; Sustainability; Sustainable City.


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