Border fence, Tijuana Mx

In ecology, an ecosystem is a community of organisms (plant, animal and other living organisms) together with their environment, functioning as a unit. It is a dynamic and complex whole, a structured functional unit in equilibrium, characterized by energy and matter flows between the different elements that compose it. (wordnet)

Edge effect is the effect on an ecosystem of juxtaposition to an contrasting environment. When an edge is created to any natural ecosystem, and the area outside the boundary is disturbed or unnatrual system, the natural ecosystem is seriously affected for some distance in from the edge.(wikipedia)

The cities of Tijuna and San Diego each represent the furthest edge of their respected ecosystems, and it is here that the two units of equilibrium collide. The juxtaposition of these two systems creates a third ecology blurred and influenced by the two, it is the urban manifestation of the edge effect.