All organisms such as microorganisms, plants, animals and human beings, along with the physical surroundings, interact with each other and maintain a balance in nature. All these interacting organisms, along with non-living constituents of the environment, form an ecosystem. Food-chain is explained in detail with examples according to nature, such as in a forest, grassland and pond. Human activities have an impact on the environment. The use of chemicals like CFCs has endangered the ozone layer.