Ecology is the study of the relationships of living organisms with each other and their non-living or physical surroundings. Living organisms can be studied at different level of organization, like population, community, ecosystem. Population is a group of organisms of one species occupying a defined area and usually isolated to some degree from other similar groups. Community in ecology represents any group of organisms belonging to a number of different species that coexist in the same habitat or area and interact through trophic and spatial relationships. Ecosystem is a community of organisms and their physical envirronment interacting as ecological unit. The different ecosystems form the biosphere, which includes all living organisms and the physical environment in which they interact. Thus, the oceans, land surface and lower parts of the atmosphere all form part of the biosphere