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 heterogeneous
manner.
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 heterogeneous
in 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.
|
References
Lefebvre, H.
2003. The urban revolution. University of Minnesota Press, Minneapolis.
Leibold, M. A.
2011. The metacommunity concept and its theoretical underpinnings. Pages
163-183 in S. M. Scheiner and M. R.
Willig, editors. The theory of ecology. University of Chicago Press, Chicago.
Lovett, G. M.,
C. G. Jones, M. G. Turner, and K. C. Weathers, editors. 2005. Ecosystem
function in heterogeneous landscapes. Springer, New York.
Lynch, K. 1960.
The image of the city. MIT Press, Cambridge, MA.
McGrath, B. and
S. T. A. Pickett. 2011. The metacity: a conceptual framework for integrating
ecology and urban design. Challenges 2011:55-72.
Pickett, S. T.
A., M. L. Cadenasso, and C. G. Jones. 2001. Generation of heterogeneity by
organisms: creation, maintenance, and transformation.in M. L. Hutchings, E. A. John, and A. J. A. Stewart, editors.
Ecological consequences of habitat heterogeneity, the annual symposium of the
British Ecological Society. Blackwell, London.
Scheiner, S. M.
and M. R. Willig. 2011. A general theory of ecology. Pages 3-18 in S. M. Scheiner and M. R. Willig,
editors. The theory of ecology. University of Chicago Press, Chicago.
Tilman, D. and
P. Kareiva, editors. 1997. Spatial ecology: the role of space in population
dynamics and interspecific interactions. Princeton University Press,
Princeton
Tischendorf, L.
and L. Fahrig. 2000. On the usage and measurement of landscape heterogeneity.
Oikos 90:7-19.
Wiens, J. A.
1995. Landscape mosaics and ecological theory. Pages 1-26 in L. Hansson, L. Fahrig, and G. Merriam, editors. Mosaic
landscapes and ecological processes. Chapman and Hall, New York.
Wirth, L. 1945.
Human ecology. American Journal of Sociology 50:483-488.
Wu, J. G. and J.
L. David. 2002. A spatially explicit hierarchical approach to modeling complex
ecological systems: theory and applications. Ecological Modelling 153:7-26.
