Urban Ecology - Ecological Systems - page 275

A second example is the disconnection between riparian zones of urban streams
and the water table [ 18 ]. This disconnection limits the capacity of urban riparian
zones to convert nitrate, a pollutant in groundwater derived from fertilizer and
sewage, to gaseous forms that do not pollute streams. Research in agricultural
landscapes has suggested that riparian restoration, inserting woody and grass
vegetation between crops and stream banks, is an effective strategy to mitigate
nitrate pollution in streams. When the capacity of urban riparian zones to accom-
plish such mitigation was examined in Baltimore, MD, USA, it was discovered that
riparian zones had become disconnected from the groundwater sources that control
their ability to convert nitrate to nitrogen gas. With reduced infiltration of
stormwater into the ground due to impervious surfaces, and with high incision
leaving stranded droughty floodplains in cities, urban riparian zones no longer
support the anaerobic conditions and high organic matter required to fuel
denitrifying bacteria. Hence, the expected denitrification in urban riparian zones
may not always occur [ 19 ]. This example demonstrates that knowledge obtained in
nonurban environments may not apply to urban situations.
Guide to the Article
It is now clear that urban areas express different combinations of ecological
processes than do nonurban areas and that urban areas are an increasingly important
component of the global biosphere. It is now possible to examine key principles that
are emerging from the new ecological focus on urban research. Future directions for
research and management are addressed at the conclusion of the article.
Urban Ecology Principles
Thirteen principles characterize the contemporary science of urban ecology. Of
course, such principles are likely to be improved or replaced with advances in
this rapidly growing scientific field. The principles can be divided into four groups:
(1) the human ecosystem, (2) urban form, (3) urban function, and (4) methodology.
Principles Concerning Human Ecosystems
Much of the history of urban research has proceeded as though city and ecology
were different and mutually exclusive. At the same time that the social scientists of
the Chicago School were applying bioecological concepts of the day to their work
in the cities, another member of the University of Chicago faculty was establishing
the concept of plant succession. A member of the Botany Department, Henry