88 Natural systems and greenspaces
along a generalized urban-to-rural gradient (McDonnellet al.1997).Usually, how-
ever, rock, soil, and water conditions are patchy with fairly abrupt boundaries,
so the vegetation ispatchywith relatively distinct edges. Human activities nor-
mally accentuate the sharpness of edges. Indeed, in greenspaces, abrupt and
relatively straighthard edgesbetween vegetation types tend to be depauperate in
species (Forman1995,Fortin 1999).Soft edgesmay appear as a squeezed-together
gradient, as a curvy or convoluted boundary, or as a narrow strip of fine-scale
patchiness or mosaic pattern. Soft edges, typically rich in biodiversity and much
used by wildlife, offer many design opportunities in the urban region.
Thespecies structure of a communityrefers to the richness, relative abundance,
and composition of species present (Morin1999,Perlman and Milder2005).
Species richness,the number of species present, is the core of the biodiversity con-
cept.Relative abundanceranges from strong dominance or abundance of a single
species to a high degree of evenness, where several species are relatively abun-
dant and no species dominates.Species compositionrefers to the particular species
present, in contrast to their richness and relative abundance. While species rich-
ness is a key to biodiversity, a strong dominant species in a community may
be quite natural, or may be eliminating other species and warrant evaluation
for management. Species composition emphasizes that species are not created
equal, but that certain species are rare, dominant, interior, keystone, non-native,
pest, and so forth. Planning, designing, and managing greenspaces is not just
about species richness, but especially about species composition (Zipperer and
Foresman 1997). Diversifying street trees increases bird diversity, with avian rich-
ness and composition also depending on which tree species are used. Replacing
native trees with non-native trees can be expected to decrease bird diversity.
Also, communities change naturally in the process ofecological succession,
adirectional sequence of natural communities replacing one another over
time. An abandoned field that is progressively dominated by herbaceous plants,
shrubs, small trees, and large trees illustrates a successional sequence in which
the relative abundances of numerous species change over time. Plant and animal
species may differ at each stage and gradually replace one another through var-
ious mechanisms. At some stage called anold-growth community,thelargetrees
may becomeself-reproducing,soanother stage does not replace them. However,
individual trees or groups of trees die from time to time, e.g., from blowdowns
and lightning strikes, and the resulting small space or gap is often filled by
species of an earlier successional stage. This process of forming and filling holes
in the vegetation is calledgap dynamics,with large gaps, especially, mimicking
theoriginal abandoned-field sequence. Protecting old-growth in an urban region
is a high priority, because of its scarcity and the presence of old-growth-related
rare species. Maintaining successional species, those characteristic of the early