<P> Walking and cycling ("non-motorized transport") enjoy increasing favor (more pedestrian zones and bike lanes) in American and Asian urban transportation planning, under the influence of such trends as the Healthy Cities movement, the drive for sustainable development, and the idea of a carfree city . Techniques such as road space rationing and road use charges have been introduced to limit urban car traffic . </P> <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>

As a city grows it usually experiences an increase in the size of its