Community is an assemblage of species living close enough together for potential interaction. Species diversity or richness is the number of species that make up a community which include relative abundance.
Some species are quite rare in a community where as other species are plentiful. Coevolution is an interspecific phenomenon which is of great importance in community ecology. Interspecific competition for limited resources determines species diversity in some communities. Closely related species can coexist if there are one or more significant differences in their niches. Succession involves changes in species composition of a community over ecological time.
Biogeography is the study of the past and the present distribution of species, deals with species diversity and composition in realms that have boundaries, ultimately associated with the patterns of continental drift.
Ecosystem is the level of ecological study that includes all the organisms in a given area along with the abiotic factors with which they interact. The environment of an organism can be divided into two: Abiotic and biotic environment. Biosphere is thin, life supporting skin consisting of seas, lakes, stream, land and atmosphere.
Biosphere is a mosaic of habitats differing in abiotic factors such as temperature, rainfall and light. These affect the distribution of biotic factors such as the type of vegetation. Environments change in time as well as space. Individual organisms can respond to changing environments by mechanisms that are behavioral, physiological or morphological. The world's major communities like deserts, tropical rain forests, grasslands and the like are known as Biomes. The aquatic communities can be divided as fresh water and marine. Rivers and streams are bodies of water continuously moving in one direction.