Coupled? Hybrid? Or Just Systems?

Having recently returned from the first Congress of the Society of Urban Ecology, I can report that there was a lot of talk in the plenary sessions about the nature of cities-suburban-exurban areas as systems.  In particular, it was emphasized that they were “hybrid” systems, incorporating social and biophysical components and the interactions that involve both these kinds of features.  The attendees seemed to be rather excited by the terminology of hybridity.

One advantage of the idea of hybridity of urban areas is that it avoids the conceptual distinction of human or social on one hand, and natural or biophysical on the other.  The label of “coupled” human-natural systems, while attempting to point to connections, still maintains that there are these two kinds of systems that might be separated. 

The idea of hybridization may in fact be a better choice than system coupling for C-S-E areas.  A hybrid in the biological sense cannot be taken apart.  The genotype and the phenotype seamlessly combine the characteristics and features of the two parents.  There is no way, for example, to take the horse or the donkey out of the mule.

So cities may usefully be thought of as hybrids.  There is the intent and use for human wellbeing, delight, and productivity as one parent, and the sometimes subtle processes of nutrient transformation and retention, the biological activities in soils, substrates, streams, and pipes, and the behavior, distribution, and reproduction of feral and volunteer plants, animals, and microbes on the other.  While the engineering and architecture of urban systems seem to be traditionally designed and operated as though they were purely built systems, in fact, they embody both intended and unexpected biology.  The supposed purity of the built and the biological parents of our urban systems is a myth, and a reality that cannot be maintained.

An example of the hybridity of cities is found in the large, but nearly invisible transfers and cross-contamination between supposedly distinct components of the flux of water.  Biology and the natural world might reasonably be able to claim the streams that run through and adjacent to cities, while built infrastructure might claim the water supply pipes, storm drain systems, and sanitary sewers.  In reality, these seemingly different pathways of the flow of water are surprisingly interconnected.  Leaks from the pressurized water supply pipes end up in the surface streams, or in the loosely sealed ceramic tile pipes of sewers.  Similarly, the unsealed joints of ceramic tile pipes of many older sewers release fouled water, loading bacteria, nitrates, phosphates, and pharmaceuticals, among other contaminants, into streams and ground water.  Some of this contamination enters storm water pipes where it will not be treated.

Similarly, the entanglement of human decisions, people’s wellbeing, and the structure and workings of the environment, including biological and built components, is irrevocable.  Speaking in terms of hybridity is a more powerful metaphor for the kinds of successful models and understanding of urban systems than coupling distinctly human or natural models.  Urban areas – those spatially heterogeneous but highly interlinked mosaics of city, suburb, exurb, and rural – are “just” systems.  The integration required by the concept of system as an entity comprising interacting parts is already a good enough.  Still, the label of hybrid reminds us of something important about the urban realm.

Resilience: Ecology, Evolution, and Engineering

The concept of resilience is key to BES III. This powerful concept is ideal for understanding and working with complex, human ecosystems. However, it is the object of some confusion because there are two contrasting ways to frame and theorize the concept.

Originally, engineering and physical systems were the source of the concept as applied in ecology. Under the equilibrium paradigm predominant at the time, resilience was conceived of as the ability of a system to absorb a shock or deformation and to return to its original, or equilibrium, state. A rubber band is a perfect example of such a system. The loose, floppy form can be stretched repeatedly, and still return to its general band-like shape.

Engineering or equilibrium resilience is the concept used to describe this dynamic. This sort of resilience can be 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 to system failure. 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.

Equilibrium or engineering resilience may be of value when it is possible to identify a desirable state that is expected to persist over some specified time or at a particular spatial scale. But for many purposes, it is best considered a special case of the concept of resilience. A more inclusive concept suitable to systems that are not at equilibrium, or which are undergoing periodic or constant change is ecological or evolutionary resilience.

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. Resilience is about the ability to receive shocks and still stay in the game. Systems that can adapt to change are said to be resilient. This then is an evolutionary kind of concept since adaptation is a central feature. This is in contrast to engineering resilience which is concerned with stability or permanence. So, ecological and evolutionary resilience are concerned with adaptive capacity and adjustment to change, not to 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 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.

Changing concepts of resilience are relevant to the BES III main theme of Sanitary to Sustainable City. The sanitary city identifies a desired state, and seeks to keep structures or processes at that level. Given that societal and regulatory decisions identify legal or desirable targets for some features 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. Recognizing that feedbacks among social, economic, and environmental factors and processes are an unavoidable part of urban ecosystems, this suggests that we learn how to go beyond the engineering resilience concept and understand and use the contemporary concept of ecological or evolutionary resilience.

Here are some references about this contrast and the nature of ecological resilience that are relevant to socio-ecological systems.

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.