Figure 1: The ancient Aztec city of
Tenochtitlan, a cosmological,
political city. Heterogeneity appears
as water, land, made land, ceremonial
and residential structures, and
|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.
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):
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).