(a) The concept of ecosystem considers human beings, animals, plants and environment as parts of a single system called ecosystem.

(b) The human beings, animals and plants are living components, while the environment is inorganic or non-living component of this system. The ecosystem functions and exists due to interrelation of these components.

These three models of city are related with three theories. These are :

(i) The concentric zone theory,

(ii) The sector theory, and

(iii) The multiple nuclei theory.

(b) Environment : It means geographic environment of human beings, i.e., temperature, water, land, air and everything surrounding them.

(c) Ecology : Study of relationship between living things (plant, animal and human beings and environment).

The environment has been destroyed by human beings for comfortable living. Destruction of plants, trees and forests, industrial pollution, automobile pollution, water pollution and noise pollution have degraded the balance in the ecosystem. In the modern industrial societies, exploitation of environment has become alarming. Any economic development seems directly related to destruction of nature.

(b) Social Ecology : The inter-relationship of physical, biological and cultural features of a religion is the subject matter of social ecology. Social ecology is a study of relationships between communities and environment.

(c) Invasion and Succession : The incoming process of the rural people to the cities is called invasion and the process of occupation of urban region by replacing the original settlers in known as “succession”.