232 Basic principles for molding land mosaics
heterogeneity is arranged as a (gradual) gradient, rather than being
patchy, species movement is either greater or less depending on gra-
dient orientation relative to direction of movement.Natural networks
(A) Major natural-vegetation network. The primary network (emerald network)
of large natural patches and connecting corridors helps maintain dis-
tinct sections across a landscape, preventing coalescence of development
and promoting a sense of community, local culture, and care for the
land.
(B) Loops in a network. Loops or circuits in a network provide alternative
routes for movement, thus reducing the effects of gaps and less suitable
spots, and increasing the chance of successfully reaching a destination.
(C) Landscape connectivity.Most species evolved in highly connected hetero-
geneous natural landscapes, have had relatively little time to adapt to
human fragmented ones, and occur in greater numbers (species rich-
ness) in more connected areas.
(D) Species dispersal.Sincespecies disperse different distances and directions,
anatural corridor and patch network with a relatively high average
number of linkages per patch provides good dispersal opportunities
which enhance the persistence of most species.Transportation modes
This third set of principles involves highways and roads, commuter-rail
lines, and walking. Transportation is a core spatial attribute and plays a major
functional role in the urban region. It is a key factor in economic investment
and development, as well as natural systems and their use. The following refer-
ences are particularly useful for the principles here: National Research Council
(1997), Warren (1998), Cervero (1998), Forman and Alexander (1998), Forman and
Deblinger(2000), Ravetz (2000), Bullardet al.(2000), Simmonds and Hack (2000),
Calthorpe and Fulton (2001), AASHTO (2001), Benfieldet al.(2001), Williset al.
(2001), Formanet al.(2003), Forman (2004b), Dittmar and Ohland(2004), Handy
(2005),Erickson(2006), Forman (2006), and Moore (2007). Also see Chapter2.
Four groupings are addressed: (a) highways; (b) commuter-rail lines and
communities; (c) roads in communities; and (d) walking and park systems.Highways
(A) Highway as source of effects.Wider and especially busier highways, as con-
centrated linear sources of ecological effects, increasingly alter local