In ecology, a community is an assemblage of populations of two or more different species occupying the same geographical area. The term community has a variety of uses. In its simplest form it refers to groups of organisms in a specific place or time, for example, "the fish community of Lake Ontario before industrialization". Community ecologists study the interactions between species in communities on many spatial and temporal scales, including the distribution, structure, abundance, demography, and interactions between coexisting populations.[1] The primary focus of community ecology is on the interactions between populations as determined by specific genotypic and phenotypic characteristics. Community ecology has its origin in European plant sociology. Modern community ecology examines patterns such as variation in species richness, equitability, productivity and food web structure (see community structure); it also examines processes such as predator-prey population dynamics, succession, and community assembly. On a deeper level the meaning and value of the community concept in ecology is up for debate. Communities have traditionally been understood on a fi

e scale in terms of local processes constructing (or destructing) an assemblage of species, such as the way climate change is likely to affect the make-up of grass communities.[2] Recently this local community focus has been criticised. Robert Ricklefs has argued that it is more useful to think of communities on a regional scale, drawing on evolutionary taxonomy and biogeography,[1] where some species or clades evolve and others go extinct. A population is all the organisms of the same group or species who live in the same geographical area and are capable of interbreeding.[1][2] In ecology the population of a certain species in a certain area is estimated using the Lincoln Index. The area that is used to define a sexual population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals from other areas. Normally breeding is substantially more common within the area than across the border.[3] In sociology, population refers to a collection of human beings. Demography is a social science which entails the statistical study of human populations. This article refers mainly to human population.