The essence of the modern city lies in density and mobility.

• Since the people or use of an area often change, the character of a natural habitat at any given point in time would be a function of constant competition for space and a hierarchy of dominance. These had strong economic overtones. For example, according to one of the ecological theories, the concentration of vice and gambling just outside the Central Business District (CBD) of cities found its explanation in the land values associated with the growth of the CBD. The argument runs as follows: the CBD, the home of the highest land values in town, is in a constant state of growth, thereby encouraging speculators to purchase property on its periphery in anticipation of eventual incorporation of the property into the CBD. So as to minimize expenses and maximize short term profits the interim landlords retain older buildings without improving them. These they subdivide into a number of rooms, each of which rents for little, but which in aggregate return a handsome rental. Attracted to these accommodations are the newcomers to the city who can afford no other residence and who stay only minimal commitment, with little personal contact among neighbours or between the landlord and the tenant.