Examples of patches classified using a system in which the proportional cover of coarse and fine vegetation, bare soil, pavement, and buildings is scored into 5 categories (0 = none, 1 = present - 10%, 2 = 11 - 35%, 3 = 36 - 75% and 4 = > 75%). Building types are identified as N = none, S = single, C = connected, or M = mixed.   Cadenasso et al. 2007

Patch Town/Network Town: Studying Heterogeneity and Connections
 
Cities are Patchy. People who study cities, including historians, sociologists, and architects, among others, frequently remark on how patchy cities are. In the space of a city block, one can step from one world to another, as a crowded commercial array gives way to a quiet residential area. Social contrasts exist at boundaries between working class and managerial enclaves. Or architectural contrasts are revealed with a turn into an alley. In old Baltimore such a turn can transport a walker from a polished antebellum neighborhood, with its stately, richly ornamented row houses built for mill owners and wealthy merchants, to the small, unadorned row houses originally occupied by slaves or first generation immigrants in the 19th century. Right around the corner from a busy shopping mall there may be the entrance to a condominium. A new suburban development may abut an old farm woodlot, or a white-fenced pasture dotted with a few grazing horses. The number and kinds of contrasts are many, and extend from the central city to the edge of suburbia.
 
The ecologists and social scientists of the Baltimore Ecosystem Study (BES) look at these contrasts as more than just engaging diversions. Instead, our researchers and urban design scholars see such diversity as the raw material for the theories and hypotheses they will test. In fact, the big idea being examined by the research in BES is that the spatial patterns that exist in the biological, surface, and social-economic characteristics of the urban system determine how the system acts, and how it changes.
 
Links to Ecological Theory. This theoretical motivation is shared with ecology in general. It has been said in recent years, that space is the final frontier for ecology. That clever little take off on a tag line from Star Trek means that the spatial structure of ecosystems, biological populations, communities of plants and animals, and ecological landscapes is considered to be a major force in the functioning of those ecological systems. Such spatial variation and patterning had not been a major research motivation in the earlier days of ecology, so focused attention on this topic throughout the discipline has become an important modern goal for ecology. Ecologists had, through much of the earlier history of their science, sought systems that were internally uniform, or assumed that the internal heterogeneity was not important, or could be ignored. That view is a far cry from the modern hypothesis that spatial heterogeneity and ecological system function may be linked.
 
So it is in urban ecological studies. BES asks how the various kinds of patches that can be recognized in the urban area – the city, suburbs, and fringe – are organized, how they come to be, how they persist or change, and how different patches and kinds of patches relate to one another. This question is made especially complex by the fact that physical sciences, ecological sciences, and socio-economic sciences highlight different kinds of patchiness. Furthermore, the urban design disciplines, and the administrative structures of the metropolis recognize still different kinds of patches. Once the variety of patches and types of patches, ranging from the physical through the social, are understood, new questions emerge. Some of the general guiding questions are these: How do the different “layers” of patches interact? What kinds of changes in one layer of patchiness result from changes in other layers? Of course, there are more specific questions about patchiness as well, for example: How does bird diversity relate to the social status and financial resources in a patch? How is that relationship altered by the presence of other nearby social or biologically defined patches? Or, how do social processes, such as group cohesion, affect management of potentially polluting fertilizers or the presence and condition of vegetation that might store carbon in a patch? Or, as a final example, how does the combined structure of vegetation, buildings, and paved or bare surfaces relate to the ability of different patches to neutralize key pollutants or environmental stresses? Do the altered stresses feed back on social processes? The number of questions that exemplify a patch perspective is huge.
 

Urban heterogeneity: False color infrared air photo of Baltimore City taken in 1999 at submeter resolution.   Cadenasso et al. 2007

The Networked City. The power of the patch approach should not obscure another important aspect of urban ecosystem structure. Urban areas are certainly patchy, as discussed above; however, they are also knit together by various kinds of physical networks. The infrastructure of pipes, wires, and roads are examples of networks. In some cases, networks bypass certain patches, and link distant patches more closely than nearby ones. Patches do not have to be neighbors to interact. This complication needs to be taken into account in assessing the function of the urban ecosystem. Indeed, figuring out how patchworks and networks are meshed together, and how each affects the other, is an important task for urban ecology. An ultimate goal of BES is to discover the nature of each of these kinds of spatial heterogeneity, and how they jointly determine the way the urban ecosystem functions and changes in space and time.
 
The walks and careful observations of urban researchers mentioned at the beginning of this essay have led, under the guidance of patch theory, to a new view of our ecosystem – an urban patch/network. That’s the complex creation we are trying to understand.