Urban Ecology - Ecological Systems - page 284

measurements intended to help managers in Baltimore City define tree planting
goals and identify priority areas for planting were conducted solely within the city
limits. It is the responsibility of the investigators to state what the boundaries of
their study units are and what the criteria for inclusion in the urban system are [ 40 ].
Such considerations suggest a methodological principle for urban ecology:
Principle 12 : Definition of the boundaries and content of an urban study system is
set by the researchers based on their research questions or the spatial scope of its
intended application.
The second methodological concern is how to frame comparisons within urban
systems. Because urban systems are hybrid human-natural complexes and because
they are heterogeneous, there are many possible axes of comparison within them
[ 27 ]. Comparisons may be based on criteria as divergent as the proportion of
impervious surface, the amount of woody vegetation, the density of the human
population, the surface area of buildings, the accumulated wealth of residents, the
lifestyle characteristics of neighborhoods, or the power relationships of local
institutions. Many other criteria are possible. However, different criteria are appro-
priate bases for different comparisons. For example, hydrology and flood risk may
be compared on the basis of impervious surface [ 25 ]. The amount of woody
vegetation may be used in comparisons of the exchange of heat or carbon dioxide
between the surface and the atmosphere. The density of human population may be
used as a predictor of water quality. Lifestyle contrasts may form the basis for
comparisons of environmental decisions made by households in different parts of
a metropolis, or in different metropolitan areas [ 41 ].
The fact that these many criteria are often patchily arranged suggests that
comparisons will rarely be accomplished by running linear transects across metro-
politan maps. Although raw distance is sometimes useful in making coarse-scale
urban comparisons, as it was along the urban-rural transect in metropolitan New
York [ 42 ], in many cases the spatial heterogeneity as it exists on the ground may
better be presented as a rearranged, abstract gradient [ 43 ]. An example of a gradient
abstracted from an array of plots is the gradient of wealth along which bird
biodiversity was distributed in Phoenix. Thus, while linear transects may be appro-
priate bases for initial comparison, ordinated gradients are in fact the general case
which should be used to frame comparisons over different scales. The principle is as
follows:
Principle 13 : Urban comparisons can be framed as linear transects or as abstract
gradients, and the abstract comparisons acknowledge the spatial complexity of
urban heterogeneity.
The baker's dozen of principles present above frame the current state of urban
ecology. As a biological discipline closely related to social and economic sciences
and the theory and practice of urban design and planning, it is a relatively new and
outward-looking field [ 13 ]. Hence, these principles are not the final roster that the
field may exhibit. Nor are they in what is likely to be their final form. However,
these broad ideas should continue to be useful over the next decade as this field
consolidates by accumulating more data on specific urban places through time and