Environmental Engineering Reference
In-Depth Information
because they are in a sense pre-adapted as “opportunists” to be colonizers.
These organisms are adaptable, and those that are animals are resource gen-
eralists.They have effective distance dispersal mechanisms and broad etho-
logical and/or physiological tolerances. Rats, mice, opossums, squirrels,
pigeons, sparrows, and starlings are common city commensals in North
America. Cockroaches have been house guests around the world since we
created semi-tropical conditions in our kitchens. In other parts of the world
urban animals include similar types but different species. In India, for
instance, crows and kites are prominent urban scavengers, and monkeys
often exploit the favorable scrounging situation. In Alaska polar bears
invade towns, but are not established there, while in other parts of the world
the common mynah takes over from the starling, or sparrow, as a common
passerine.
5
Just as the accidents of zoogeography determined the regional
range of early domesticates, so it is with urban colonizers. On the plant side,
hardy undemanding species with efficient seed dispersal (often wind-
borne) are typical urban wasteland weeds. The second group of organisms
comprises those that humans have chosen to plant or to otherwise intro-
duce into the cityscape.Trees that line streets or are planted in squares and
other “civic” areas belong to this category.These and other plants were, and
still are in many cities around the world, chosen for their resistance to pol-
lution. Our atavism for being surrounded by plants is reflected in the
extraordinary range of garden cultivars that are grown by those household-
ers who possess gardens (in addition, of course, to house plants).These con-
stitute an often exotic element in the urban environment, including plants
originally from distant continents or even from obscure oceanic islands.
Introduced, unrestrained animals are comparatively rare. (Cattle in Indian
cities are an exception.) Most urban introductions are house pets, and their
feral by-products constitute a nuisance.
In relatively unrestricted large parks, bio- or otherwise, extensive areas of
trees, shrubs, and herbaceous plants and ornamental or natural ponds, lakes,
and streams attract fauna from nearby wild and semi-wild habitats. Birds can
invade these areas more easily than flightless animals. A large “rookery” of
black-crowned night herons nest at the National Zoo every year, flying to
the Potomac to catch fish for their young. In Greater Washington, bird
counts carried out by more than 100 volunteers recorded 91 breeding
species, with an estimated 115 migrants passing through. Within Washing-
ton, the highest species diversity was found in parkland and the lowest in
commercial and high-density residential areas.
6
The number of migrants is
