From a systems perspective, the urban environment could conceptualized as a polysystem formed by a dynamic set of open systems that exchange matter, energy (receive products , emit waste) and information with the external environment. The urban environment is characterized by devour all and, consequently, also excrete, after digestion, vast amounts of waste. It is understood the city as a heterotrophic system because all foreign matter and exports entropy, energy and matter in degraded form, which accelerates the degradation of the outer areas of the city that feeds.Thus, the city maintains an open cycle that generates and ejects entropy. You need huge amounts of matter and energy for sustenance over-explotation which favors rural and implementation of farming practices unsustainable. It also absorbs large amount of population leaving rural communities. In exchange, the city exports huge amounts of degraded material in multiple forms: solid waste , contaminated water , polluted air , etc. To this must be added what the report termed peripheralization Blue Plan, i.e. the extension of the city on its district or hinterland with expansive urban models that use a lot of space. The surrounding land that once nourished the city have become speculation zones where it develops a new concept of city, following the American model that represents a huge waste of space.The strong pressure, in environmental terms, exercises rural town, leads to outline a new conflict with serious consequences. The bigger the city, the more dependent is the supply of external resources and reinforces the development of unsustainable production practices that promote the development of the desertification process. Thus, urbanization produces a double effect might be called centrifugal and centripetal in their participation in the process of desertification.We understand it as centripetal to become cities poles of attraction for farmers leaving the land and to the flows of matter and energy that consumes the urban system , allowing the advance of desertification. The centrifugal effect is used to describe both the process of physical expansion of the city into the rural areas as the process of cultural diffusion Urban lifestyle and urban ways of thinking and understanding the relationship between society - nature that are at the base of the ecologically unsustainable processes belonging to desertification.
Cited: ALEDO TUR, Alonso (1999): desertificación y urbanización: el fracaso de la utopía. Biblioteca CF+S CIUDADES PARA UN FUTURO MÁS SOSTENIBLE. Instituto Juan de Herrera. Av. Juan de Herrera 4. 28040 MADRID. ESPAÑA.