Urban Ecology - Ecological Systems - page 288

of flood plains and reduction of riparian and in-stream ecological function, and with
simplification of the heterogeneity and biotic diversity in stream channels [ 47 ]. On
a more hypothetical level are generalizations about the increasing similarity of
carbon budgets of cities based on planting and maintenance of mesic vegetation in
all types of climates [ 48 ]. Although these generalizations seem robust so far, they
must be challenged, refined, or rejected based on further comparisons with addi-
tional cities. Of course, to conduct appropriate comparisons, cities may have to be
more explicitly classified according to some globally relevant schema than they are
now [ 27 ]. City-suburban-exurban systems differ based on location relative to rivers,
streams, or coastal waters; regional climate and form of original vegetation; kind of
geological substrate; and exposure to natural hazards. In the broad social realm,
cities experience different forms of governance; different histories with respect to
development, industry, shipping, and ground transportation; access to commodities
and sources of wealth; cultural context and diversity; and porosity of social groups.
Not all of these may be major axes of comparison, but they point to some of the
many dimensions that may affect differences among cities in their bioecological
structure and function.
An important aspect of the comparison of cities is their ability to represent global
change processes. Cities may well stand in as a laboratory for global change, as
temperate cities are now often drier than their mesic surroundings, and due to the
urban heat island effect, are generally hotter. Portions of some highly irrigated
desert cities may be an exception and in fact be cooler during some hours than the
surrounding arid lands due to the evaporation of massive quantities of surface
irrigation water or introduced mesic plants [ 22 ]. Another feature of global change
that some cities may mimic is a great exposure to flood risk due to the building of
floodwalls and levees upstream.
Comparisons within cities can benefit from new land cover classifications. The
usual “Anderson Level II” classification uses such categories as commercial,
industrial, transportation, and residential in low, medium, and high intensities of
occupancy. These categories may be too coarse for some desirable ecological
comparisons [ 49 ]. Likewise, the census geography of block groups based on
clusters of approximately 400 households may be rather coarse relative to some
levels of ecological function [ 19 ]. Both the land use/land cover and the census
geographies fail, for example, to adequately match the fine-scale watershed
behaviors in urban areas. Social perceptions of edges, enclaves, corridors, and so
on may also match other scales of observation than the classical tools of differenti-
ation within urban areas. One example of a classification scheme devised to expose
aspects of joint biophysical and social differentiation is the High Ecological
Resolution Classification for Urban Landscapes and Environmental Systems (HER-
CULES) [ 49 ]. Rather than assuming land use as the determining criterion, it focuses
on land cover in order to provide a structural base to test against bioecological
functioning. HERCULES classifies patches in urban systems based on (1) surface
characteristics, whether bare or paved, (2) presence and amount of either tall or
short vegetation, and (3) cover and kinds of buildings. The classes may be extracted
from continuous variation along these three axes, or may be defined to represent