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centers and their surrounding landscapes is well documented and inarguably immense. In urban areas of the eastern United States, only species with the most general habitat and resource requirements have remained in urban corridors. Moreover, the prospect of further erosion of biological diversity looms. In Great Britain, where the sustained assault on the environment is measured in millennia rather than in centuries, and where most vertebrate species are distant memories, a cascade of invertebrate extinctions is now being observed. For example, 80% of the resident butterfly species have declined in number in at least a major part of their British ranges during the past decade (Thomas, 1984). A number of those survive only on reserves and under rigorous management regimes. An estimated 18% of all European butterfly species are considered to be vulnerable to or imminently faced with extinction (Heath, 1981).
Unfortunately, losses of animal and plant species are restricted neither to temperate zone urban areas nor to the developed world. Urban impacts on biological diversity reach their most devastating in the Third World. Less than 2% of the Atlantic forests of coastal Brazil within the urban reach of Sao Paulo remain, and it has been estimated that thousands of species from this region of high endemism have been driven to extinction, most never having been described by taxonomists.
Although the full extent of this urban environmental degradation is virtually impossible to convey, its underlying causes are comparatively simple to identify. With few exceptions, losses of naturally occurring biological diversity are incidental to human activities. Thus, urban areas are effectively synonymous with ecosystem disruption and the erosion of biological diversity. Natural habitats are replaced directly by houses, condominiums, hotels, and malls, as well as by streets, highways, and utilities that support them. Historically, urban areas were the first regions subjected to local overkill of wildlife for food, fur, and feathers, and through misdirected predator control programs. They were also the first to experience logging and weed eradication programs. The biological diversity of urban areas has also been among the most severely affected by the introduction of animal species, which prey on native animal populations, compete for limited resources, and act as vectors for novel diseases and parasites to which native organisms can be particularly susceptible.
Great effects on biological diversity in urban areas also can result from less direct sources, including many of the air- and water-borne pollutants that imperil human health. Toxic by-products of industrial production, such as polychlorinated biphenyls (PCBs), sulfur dioxide, and oxidants as well as pesticides directed at noxious species, have been found to disrupt natural ecosystems (Ehrlich and Ehrlich, 1981). Airborne pollutants are especially insidious, since they expand the reach of urban blight far beyond city limits. More subtle impacts on biological diversity result from overdrafting local aquifers, dropping water tables, and ground subsidence. These processes are often compounded by changes in natural patterns of groundwater percolation caused by the destruction of wetlands and diversion of runoff.
This wide array of obvious and subtle factors contribute to the disruption of ecosystem function, the decoupling of interactions among species, and the disappearance of populations of organisms from urban locales. Why should that concern us? Because losses of just a few populations can result in a great destabilization