Urban Ecology - Ecological Systems - page 282

Native biological diversity, or biodiversity, is usually reduced in urban areas,
with native species declining in richness compared to species introduced from
elsewhere [ 35 ]. An example of altered species distribution that may have an impact
on the future composition of urban forests is the greater number of vine species,
especially introduced species, in the treefall gaps found in urban compared to rural
forests. Such vines can impede the regeneration of trees in canopy gaps. Still, the
total number of species, including both native and exotic species, can be larger in
urban areas than in nearby rural areas in the temperate zones. However, in some
central German cities, the richness of even native species is greater in urban areas
than in the adjacent countryside. Urban wildlife is subsidized by purposeful feed-
ing, by the incidental increase in resources found in garbage, or for fruit-feeding
birds, by the large abundance of fleshy-fruited horticultural species planted in
settlements [ 36 ].
Urban fauna and flora can respond to the novel conditions of urban ecosystems.
For example, bird populations in San Diego exhibit genetically different plumage
from rural counterparts [ 37 ]. Although that difference is not necessarily adaptive,
other changes are functional. Urban bird populations have been demonstrated to
raise pitch and volume of their songs, presumably selected by the higher noise levels
in cities than in the countryside [ 38 ]. Plants are well known to genetically adapt to
the heavy metals found in brownfields or associated with urban construction.
An important question is the contribution of exotic and native species to ecosys-
tem services in urban areas. Biophilia, the affinity that humans express for natural
settings and living things or the salubrious effects of plants and wild animals in the
surroundings, are benefits of wildlife and plants in cities. Other services are
performed by the biota of cities, including moderation of climate through shading
and transpiration, stabilization of soil surfaces and stream banks, and absorption of
pollution. These services can be performed by both introduced and native species.
Indeed, some introduced species, such as the exotic Norway maple, were imported
due to their tolerance of urban stresses. Ironically, this species is one that has
escaped into the wild, and due to the deep and seasonally long-lasting shade it
casts, poses a threat to the herbaceous species of the forest understory and to
regeneration of other canopy tree species [ 39 ].
Because of the damage that exotic species can do to native biotic communities
and the functional changes they can cause in ecosystems, they are often targeted for
removal. Although the introduction, either purposeful or accidental, of new exotic
species should be avoided, in urban areas the removal of exotics as a blanket policy
may be problematic. The contribution of the species to ecosystem services in urban
areas, the ability of native species capable of performing that function to tolerate
urban conditions, or the potential to breed native species for these capacities are all
issues that must be considered in planning the management of exotic species. Some
exotics can contribute to ecological functions in city-suburban-exurban habitats.
A principle that emerges from knowledge about urban biota is this:
Principle 10 : Urban biodiversity has multiple components, and the contributions of
each kind of species to ecosystem services in cities must be evaluated.