Climate change, species extinction, water scarcity 331where one can harvest resources, even colonize, if hard times arrive. Emeralds
can be stolen, right in an urban region.
Thecorridorsof an emerald network have a low interior-to-border ratio and,
as noted in the preceding section, are difficult to protect in an urban region
with so many people and ongoing urbanization (Briffettet al.2000,Jongman
and Pungetti2004,Hiltyet al.2006). Perhaps the ‘‘string of pearls” corridor type
(Chapters4 and 10 ;Color Figure 41 )(Forman2004a), where a tree-lined path is
well used and widely known by people, is most likely to persevere over time in
an urban region. A sequence of small patches (pearls) of semi-natural vegetation
along the path may persist long term as treasured neighborhood parks.
Urban regions are the great centers fornon-native and invasive species(the non-
native species that colonize and spread in natural ecosystems) (Kareivaet al.
2007). Ships and planes and trains and vehicles bring most non-native (exotic)
species to a city. Rail yards are hotspots of non-natives (Muehlenbach1979,
Kowarik and Langer2005), which also are typically abundant along railroads
and roads (Formanet al.2003). Of particular importance are the extensive res-
idential areas and house lots surrounding a city, where non-native species are
purposely and widely planted for diversity and human delight. This massive area
teeming with exotics, which are continuously being transported by people and
vehicles, is probably the prime source of non-native and invasive species into
natural areas across the urban region, and beyond.
Some ecologists consider invasive species to be a major current threat to
biodiversity, and others consider this to be a minor tangential issue compared,
e.g., with habitat loss and habitat degradation due to human activities. Two
guidelines seem useful in reducing the chance of biodiversity problems associ-
ated with invasive species. First, eliminate known and suspected invasive species
from the market, especially the nursery business that supplies wholesale and
retail plantings. Second, since emeralds are about the only areas in an urban
region where sustained management against invasives is worthwhile for long-
termbiodiversity protection, surrounding these large natural areas with mod-
erately large adjacent properties containing a low density of non-native species
generally makes sense. Also some ‘‘invasives” are really naturalized species.
Connectivityforspecies movement across a whole urban region should reduce
the chance of biodiversity loss there. This generally requires greenspace corri-
dors, normally with walking paths, which interconnect with large semi-natural
areas around the region’s unbuilt donut (or coastal half-donut) geometry. The
corridors should also appropriately connect to adjoining regions, e.g., in the
cardinal directions. Transportation corridors can be crossed with major wildlife
underpasses and overpasses (Chapter4)(Trocmeet al.2003,Iuellet al.2003,
Formanet al.2003). However, strip development along a highway requires a wide