<P> Housing of residents presents one of the major challenges every city must face . Adequate housing entails not only physical shelters but also the physical systems necessary to sustain life and economic activity . Home ownership represents status and a modicum of economic security, compared to renting which may consume much of the income of low - wage urban workers . Homelessness, or lack of housing, is a challenged currently faced by millions of people in countries rich and poor . </P> <P> Urban ecosystems, influenced as they are by the density of human buildings and activities differ considerably from those of their rural surroundings . Anthropogenic buildings and waste, as well as cultivation in gardens, create physical and chemical environments which have no equivalents in wilderness, in some cases enabling exceptional biodiversity . They provide homes not only for immigrant humans but also for immigrant plants, bringing about interactions between species which never previously encountered each other . They introduce frequent disturbances (construction, walking) to plant and animal habitats, creating opportunities for recolonization and thus favoring young ecosystems with r - selected species dominant . On the whole, urban ecosystems are less complex and productive than others, due to the diminished absolute amount of biological interactions . </P> <P> Typical urban fauna include insects (especially ants), rodents (mice, rats), and birds, as well as cats and dogs (domesticated and feral). Large predators are scarce . </P> <P> Cities generate considerable ecological footprints, locally and at longer distances, due to concentrated populations and technological activities . From one perspective, cities are not ecologically sustainable due to their resource needs . From another, proper management may be able to ameliorate a city's ill effects . Air pollution arises from various forms of combustion, including fireplaces, wood or coal - burning stoves, other heating systems, and internal combustion engines . Industrialized cities, and today third - world megacities, are notorious for veils of smog (industrial haze) which envelop them, posing a chronic threat to the health of their millions of inhabitants . Urban soil contains higher concentrations of heavy metals (especially lead, copper, and nickel) and has lower pH than soil in comparable wilderness . </P>

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