Bottleneck phenomena also called genetic drift (variations in gene frequency), play an important role in organic evolution, like mutations and natural selection. The population of some species may get reduced by natural calamities like floods, droughts, epidemic, etc. A few individuals survive such critical stages; they contain a smaller number of genes than the genetic pool of the original population. After tight ecological conditions, these few individuals give rise to the entire future populations. This phenomenon of periodically squeezing through (as if going through a narrow bottleneck) endangered or threatened conditions by a few individuals of a species, facing tight ecological conditions and regaining the optimum concentration of its population is called the bottleneck phenomenon.

The bottleneck phenomenon is also seen when a few individuals of the species stray out of their normal population and establish a separate successful population in a new habitat. For example, Zoroastrians like a few Parsis and Iranians who migrated to India from Persia have now established their own population in India.

All organisms living in a particular ecosystem constitute a community, while the group of organisms of a single species in an ecosystem is called population. A community is thus a localized group of several populations. For example, a pond may contain insects, frogs, fish and plants, together forming a biotic community, but it is made up of different populations (e.g. frog population, insect population, fish population, etc).

Structure of community: community contains three fundamental groups producers, consumers and decomposers-which are dependent on each other and maintain a dynamic, stable community in spite of many deaths and new arrivals.

Dominant species: The species which exerts a major controlling influence on the community because of their size or number is called a dominant species, for example in grasslands grazing animals like sheep, cows, horses, etc. are dominant, while in thick forest, carnivores like lions, tigers, wolves, etc. are dominant.

Transitional species: These are the species which are found where two communities meet, i.e. between forest and grassland, between freshwater and marine habitat, between deep waters of sea and estuarine (towards the shore). These regions contain some organisms of both the communities. Thus, it shows a greater number of species and a higher population density, and may show some new species which are adapted to live in that mixed region.

A complex of communities is known as a biome. It is characterized by climatic and soil conditions. Each one is known by the presence of distinct plants and animals.

As stated above, animals depend on plants for food, oxygen and energy, while plants depend on animals for pollination and dispersal of fruits and seeds. Thus all living organisms are interdependent and show "ecological kinship" for a number of things. In other words, they always live in harmony on the principle of "give and take" to maintain equilibrium and to establish a stable and self regulating ecosystem. The larger communities of both land and water show stratification and influence the effects of climatic and other abiotic and biotic environmental factors on the organisms.

Communities of pond : A fresh-water pond as a whole represents a complete self-maintaining and self-regulating system. It contains all four basic units, such as producers, primary and secondary consumers and decomposers, knit together forming food chains and food webs.