The climate and vegetation of the city differ from the climate and vegetation of its hinterland. Man, in building the city, alters the site. Plants are removed, and are reintroduced on a much smaller scale. Much of the city is paved, markedly altering the runoff pattern of rainfall, and the ground water of the city is considerably altered by the system of sewers. Rainwater that would have been intercepted by roots is likely to end up in the sewer system. Much heat is liberated in the city, and in recent decades city weather stations have reported noticeable differences in temperature between the central city and its suburbs or airport. Smoke, automobile fumes, wastes from factories and feeding yards, sewage, and other wastes are liberated into the atmosphere and water of the city, creating smog, fouling waterways, and otherwise degrading the environment.

In addition, man has deliberately altered the water regime of the city (by watering lawns, for example) and the nutrient status of the city's soil. He has introduced trees, shrubs, and other plants of his own choosing—horticultural varieties that please him. Species from other regions of the world, introduced into places in which they would otherwise never have occurred, may flourish in the new environment: an example is the Australian eucalyptus brought into southern California. Almost universally man has replaced the herb cover of the forests, the desert pavement, and the wild grasses and herbs with grass of his own choosing and selection—and remarkably few species of grass at that. (In Phoenix, lawns may be turf, desert pavement, or colored gravel.)

The factors that tend to control the behavior of living organisms in the urban environment are the same ones that influence the behavior of living organisms in the natural environment. There are some differences, of course, but for the most part they are differences in quality resulting primarily from the engineering activities of man. All biological communities—man-dominated or not—are affected by three sets of factors: climatic, biotic, and edaphic (soil and topography). Man's activities constitute the engineering factors and the management factors. The polluting activities of man tend to degrade the ecosystem to the extent of making it unfit for all life.