The Evolution of Urban Heterogeneity Thinking

The heterogeneity of cities has been acknowledged as one of their most striking features for a very long time.  Spatial heterogeneity characterized the ancient, cosmologically oriented cities of the Middle East, Asia, and the Americas (Lynch 1960; e.g. Fig. 1).  Social heterogeneity of cities, compared to rural village life, was recognized by the founders of modern sociology (Wirth 1945).  Urbanists, including urban designers, planners, community organizers, architects, among others, continue to be impressed with, engaged by, and responsive to the heterogeneity of cities and urban regions (Lefebvre 2003). 

Figure 1: The ancient Aztec city of
Tenochtitlan, a cosmological, 
political city.  Heterogeneity appears
as water, land, made land, ceremonial
and residential structures, and
agricultural areas. 
Although ecologists are admittedly relatively new comers to the city and the urban region as a subject of study, they come with a significant toolkit to deal with heterogeneity (Tischendorf and Fahrig 2000).  There is also a rich conceptual foundation for understanding heterogeneity within ecology (Wiens 1995, Pickett et al. 2001, Wu and David 2002).  In particular, a recent treatise on the theory of ecology illustrates the conceptual drawer of this toolkit (Scheiner and Willig 2011).  They summarize the most inclusive foundations of ecology in eight principles (Box 1).  Fully five of these principles mention heterogeneity by name or embed the core concept of heterogeneity within their scope.  These fundamentals are operationalized with such more specific disciplines within ecology as landscape ecology, metapopulation and metacommunitiy theories, succession, biogeography, evolutionary ecology, and ecosystem ecology.  
Box 1.  Principles of General Ecological Theory, from Scheiner and Willig (2011: 13).  Quoting [With some explanations inserted in brackets, and the term heterogeneity or conceptual equivalents italicized]:
1. Organisms are distributed in space and time in a heterogeneousmanner.
2. Organisms interact with their abiotic and biotic environments.
3. Variation in the characteristics of organisms results in heterogeneity of ecological patterns and processes.
4. The distribution of organisms and their interactions depend on contingencies. [Contingencies may be defined as heterogeneities in events, processes, resources, and stresses.]
5. Environmental conditions as perceived by organisms are heterogeneous in space and time.
6. Resources as perceived by organisms are finite and heterogeneousin space and time.
7. Birth rates and death rates are a consequence of interactions with the abiotic and biotic environment.
8. The ecological properties of species are the result of evolution. [N.B. Evolution by natural selection rests on the heritable heterogeneity or variation in organisms, and the degree to which it matches the prevailing, heterogeneous environment.]
The practical tools of ecology also address heterogeneity, and do so increasingly.  Most conspicuous among these tools are those supporting landscape ecology.  In this genus of ecology, the concern is with the reciprocal relationship of pattern and process.  Consequently, ways to measure spatial differentiation are central to the discipline.  Gradients, patches, and patch mosaics are measured via field study, remote sensing, and statistical modeling.  Parameters such as patch size, patch shape, boundary thickness and porosity, nearest neighbor features, and so on, suggest and are applied to spatially-oriented questions.  Heterogeneity can be assessed in genetic, behavioral, and communication activities in populations, or in the distribution of competitive and facilitative interactions among species.  Changes in spatial heterogeneity over time is measured when concern is with such phenomena as succession, disturbance, migration, and ecosystem process rates, for example.  There is, simply, no facet of contemporary ecology that does not address and profit from understanding spatial and temporal heterogeneity (Tilman and Kareiva 1997, Lovett et al. 2005, Leibold 2011).
Fig. 2. Heterogeneity as cause and
consequence, or driver and outcome.

The urban realm – cities, suburbs, exurbs (CSE), and the urbanized regions they constitute – presents both the need and the opportunity to meld the heterogeneities recognized by the social sciences with that recognized by biophysical sciences (McGrath and Pickett 2011).  Thus sociology, economics, political ecology (a social science), diffusion of innovation, social network theory, and governance theory among others, and the various flavors of biophysical sciences, such as soil science, hydrology, biogeochemistry, plant and animal community ecology, biotic population ecology, microbial ecology, and others, must be in dialog.  And that dialog must address a variety of heterogeneities.  Not all heterogeneities must appear in all interdisciplinary models, but hypotheses about particular couplings will guide which heterogeneities are relevant over the long term.

The joint concern with heterogeneity by the social and the biophysical sciences in urban areas suggests a large hypothesis:  Spatial heterogeneity acts as a driver and an outcome that affect ecological processes in cities, suburbs, and exurbs (Figure 2).

Spiral Causality in Heterogeneity

This feedback model (Figure 2) may seem at first glance to be hopelessly circular.  But pull the circle apart, like a mental slinky, and a spiral form of hypothetical argumentation appears.  The spiral plays out over time.  The abstract spiral model of heterogeneity as driver-outcome-driver-outcome, etc., would need to be filled in by particular features and moved forward by particular ecological or social events.  This is how that might look (Figure 3):

Fig. 3. Converting apparently circular causality to spiral causality in which the action of different kinds of heterogeneity
can be understood and studied as linked outcomes and drivers.  The spiral begins with a set of boundary conditions or
initial heterogeneity.  Human or natural events convert that heterogeneity into a driver for further interaction.

Heterogeneity as Driver and Outcome: A Baltimore Scenario

A hypothetical example, likely to soon to be a testable reality in Baltimore and many other American cities located in the Eastern Deciduous Forest Biome, is the interaction of the invading emerald ash borer with the distribution of planted and volunteer ash trees (Fraxinus spp.).  Ash trees are not uniformly distributed across CSE space.  Nor are the invading beetles.  This suggests the first link in a spiral of causation involving spatial heterogeneity (Figure 4).  It is based on the interaction between the initial heterogeneous distribution of ash trees, the presumably patchy invasion of the emerald ash borer, AND the patchy management by people of both the ash population and the insect.  These interventions and events result in a second kind of heterogeneity, the spatially distributed mortality (including preemptive removal) of ash trees. The initial condition is labeled an outcome, the events of invasion or management act on that outcome to produce a new spatial pattern – ash mortality, that then becomes a driver for further spatially explicit outcomes and the interventions or events they stimulate in nature or in society.  This same logic is played out in the remainder of the cascade involving patchy altered thermal environments, human risk of heat stress, and social and individual responses to heat stress in the altered environment (Figure 4). 

Fig. 4. A hypothetical model of the relationship of different kinds of heterogeneity that might exist and be causally linked following the invasion of the emerald ash borer in Baltimore or other cities.  The boxes attached to the event arrows can be
both biophysical and human generated.

Heterogeneity and the Urban Ecosystem

This is the kind of logic we wish to explore to generate specific testable hypotheses about 1) heterogeneity as both a driver and an outcome affecting ecological processes in the urban system of Baltimore, 2) the integration of human and natural processes in the urban ecosystem, and 3) the intersection of two of ecology’s fundamental concepts: heterogeneity and the ecosystem as it is manifested in urban areas.  BES IV will investigate the role of spatial heterogeneity as a driver and outcome as it underlies and affects the basic structures and interactions in the urban ecosystem (Figure 5).
Fig. 5. The human ecosystem, consisting of biotic, physical,
social, and built components, all interacting within the
context of spatial various kinds of heterogeneity that
affect the interactions among components, and therefore
the structure and function of the components.


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