Urban Ecology - Ecological Systems - page 277

Urban form has changed dramatically through time. Early walled cities were
physically and socially distinct from the surroundings and often were centered on
a cosmologically significant, ceremonial core. The industrial city spread beyond the
location of former city walls and established porosity for the purposes of exchange
of goods and the accumulation of immigrants needed as labor. After World War II,
the industrial city was further disaggregated into a mixture of new suburbs, dis-
persed commercial districts, and older core cities that were reduced in density. This
is the city as part of a network in a dispersed megalopolis [ 24 ].
This simple, linear typology does not necessarily reflect the temporal trends in
all parts of the world. Indeed in some cities of the global south, no industrial period
existed before they began the sprawling growth driven by migration of persons
from the provinces seeking opportunity and a better life in the modern city of
consumption. Shanty towns outpacing the urban infrastructure are one feature of
such cities. Hence, urban form is evolving, which leads to the following principle of
urban form:
Principle 3 : Urban form is a dynamic phenomenon and exhibits contrasts through
time and across regions that express different cultural and economic contexts of
urbanization.
Why is urban form important ecologically? The establishment of street grids and
major road patterns alters surface drainage [ 25 ] and presents barriers and corridors
for the movement of native and introduced plants and animals [ 26 ]. The adding of
new vegetation and the obliteration of the forest, savanna, desert, wetland, or
grassland that had previously occupied the urban site follows the dictates and
needs of the new urban form. In addition, the infrastructures for supplying clean
water and for dealing with fouled water are large alterations to watershed structure
and regional water balances. The placement of main sanitary sewers and main
storm drains in stream valleys and the shortcutting of ground and surface water
flows are components of the universal urban alteration of hydrology. Furthermore,
the shapes of heterogeneity introduced by urban form are often rectilinear and of
different scale than the former regional and local patterns of environmental
gradients and patches. In addition to spatial heterogeneity of the biophysical
components of urban systems, the social features of cities, towns, and suburbs are
notably patchy [ 27 ]. Urbanists have long acknowledged the fine-scale change -
often from block to block - in economic activity, wealth, social group, architecture,
and land use that characterize cities. Although traditional postwar suburban devel-
opment typically occupies rather large tracts which they homogenize, the abrupt
shifts in urban structure persist. This patchiness is reinforced by the near universal
employment of zoning in large urban areas of wealthy countries. All of these
heterogeneous structural alterations can have significant effects on urban and
adjacent natural ecosystem function, as will be detailed in a later section “ Principles
Concerning Urban Function ”. These insights about the nature of urban form at
scales as small as residential parcels or a stroll to the corner of the block can be
summarized in a principle: