16 6Built systems, built areas, and whole regions
communities? How should the different ecosystems be distributed across the city,
and how can the answer be effectively meshed with recreational opportunities?
The questions are tractable and important, but answers are scarce.
Forinstance, ponds and lakes, much-valued by the public for visual quality
values, often exist in some city greenspaces. Large and small, shallow and deep,
ringed by walkways or almost inaccessible, clean or polluted, these water bodies
support very different aquatic ecosystems and species. They also provide many
societal functions from water supply and sewage overflow to boating/fishing
recreation, solid-waste disposal, and flood control. Mosquitoes and other insects
are often pests or disease vectors. Meshing these characteristics over a heteroge-
neously distributed array of large, small, squarish, and elongated city parks is a
worthygoal.
Large and small natural patches provide somewhat different benefits in rural
than urban areas. In the city large greenspaces typically serve as major hydro-
logic sponges against flooding, maintain the best facsimile of a natural commu-
nity, and cool the summer city temperature for hundreds of meters outward
(Forman and Hersperger1997). On the other hand, large patches in the urban-
region ring may help protect an aquifer or lake, connect headwater streams, and
sustain large-home-range vertebrates.
The other major role of urban greenspaces is functional, facilitating natural
flows and movements across the city (Figure7. 1). Linear spaces usually reflect
the presence, or previous presence, of a stream or river. Often streams have
become transformed into rushing water in large straight underground pipes.
Agreen strip on the ground over a former stream normally facilitates walking
and wildlife movement. Riversides and floodplains, except during flood stages,
may also be effective for the movement of people and animals. Urban flood-
plain areas, however, also commonly serve as major infrastructure conduits for
cities. Gas, oil, electricity, water supply, stormwater, sewage wastewater, com-
muter trains, intercity trains, trucks, and cars flow along different floodplains.
Furthermore, little roads commonly cross and go along floodplains for the main-
tenance and repair of the infrastructure conduits.Effects of proximity of built and natural areas
Diverse flows and movements between a built area and an adjacent nat-
ural area may be both positive and negative for each receiving side (Forman
1995,Harriset al.1996,Hersperger and Forman2003,Hersperger2005). The
strongest interactions are the negative effects of built areas on natural areas.
Veryfeweffects of built areas on natural areas are positive for natural sys-
tems and the human activities that depend on them. In the opposite direction,