Natural systems within and next to built areas 167Figure 7.1Linear greenspaces serving as separating barriers or filters and as
channels for flows and movements. The river corridor separates neighborhoods and
land uses, while facilitating floodwater flows and wildlife movement. Vegetation
strips, and to a lesser extent, street tree lines channel birds and mammals along
them. Note that: street trees shade and cool impermeable surfaces; wildlife can
readily cross under the bridge on both sides; the dam outlet fixes the channel in
place so that normal river migration across a floodplain cannot take place; and
stormwater runoff from pipes enters downriver. Milwaukee, USA. Photo courtesy of
US Federal Highway Administration.i.e., effects of natural areas on built areas, both positive and negative, appear to
be intermediate in impact.
Thenegative effects of built areas on natural areasare highlighted in more detail
because these have the greatest importance for planning and the urban region.
Heat, air pollutants, chemical pollutants in stormwater, human wastewater,
vehicular traffic, non-native and invasive species, domestic animals, and espe-
cially people dispersing outward from a built area are all familiar. The last