The physical and chemical characteristics of soils largely determine the control of vegetation composition and abundance by the climatic factor of rainfall, interaction (8). Vegetation, however, is not completely passive. Plants create a physical and chemical microenvironment. Interaction (7) includes the processes by which perennial vegetation determines the extent and spatial pattern of abundance and flow of water and nutrients in the landscape (Noble and Tongway,1986). This concentration and patterning facilitates the perpetuation of the species in space and therefore leads to a persistence of the community in space and time. 

   Interaction (8) serves as a positive feedback to (7). An enhanced concentration of water and nutrients in turn enhances the establishment, growth and persistence of vegetation. The control of this positive feedback loop is the absolute levels of, and variability in, the resources of water and nutrients. It is one of the most influential and stabilizing of feedback loops in arid ecosystems.