Definition:
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.
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with the old, equilibrium paradigm of ecology, which has been replaced by a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.
Examples:
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.
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).
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.
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.
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.