Cities or for that matter all human settlements have evolved from interaction between nature and humans. But as civilizations evolved, we were at a point where we no longer perceived the role of natural processes in the sustenance of cities and humans. This apparent unawareness led us to functioning in an isolated way with no concern to nature around us oblivious to the fact that ecosystem of the entire earth is strongly interlinked. We tend to forget the fact that we are a part of an ecosystem and that affecting the ecosystem will in turn affect us.
So it is imperative that for our own well-being and that of our cities, we understand and take care of our ecosystems. From here stems the concept of urban ecology, which I feel is to study how the environment and the other living things function as part of a whole around the urban settings and design our cities to support and enhance them. Also central to concept of urban ecology is the fact that it deals with complex interdependency within the ecosystems and the actions we take as a part of our urban systems.
So to put it in other words we should study urban ecology and practice ecological urbanism (in the sense that our urban interventions promote and enhance ecosystems and their diversity).