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Population and Biotic Community


     Biotic community is an association of a number of interrelated and independent populations belonging to different species, in a common environment which can survive in nature.

Organism and Its Characteristics

     An organism is a distinct unit of life in nature. It is said to be the smallest level and basic unit of ecological hierarchy.

Population and Its Characteristics

     Population is the next level in ecological hierarchy the word population has its origin in the Latin word populus meaning 'people'.

Factors Affecting Population

     Natality rate is the rate at which new individuals are added to a particular population by reproduction (birth of young ones or hatching of eggs or germination of seeds/spores).

Population Growth

     The growth of population is measured as increase in its size over a period of time and populations show characteristic patterns of growth with time. These patterns are known as population growth forms.


     The sum of all the populations of same kind of organisms all over the world is called species. Generally, a species is often defined as a group of organisms similar in structure, function and behaviours.

Reproductive Isolation

     Reproductive isolation is the inability of members of different species to interbreed in nature.


     Is also referred as 'Origin of species'. All new species develop from pre existing ones. The formation of one or more new species from the pre existing species is called 'Speciation'.

Interaction Between Members of a Species

     In nature there is always a kind of interdependence between different kinds of living organisms among themselves and also between biotic and abiotic components of ecosystem.

Cooperative Interactions - Mating and Parental Care

     In Cooperative interactions, the members of the same species interact and increase the chances of survival of one another. Such interactions are essential for reproduction and continuation of species.

Cooperative Interactions - Group Formation

     Purpose of reproduction leads to the formation of groups of individuals, called a family, which is an instinct of individuals of different sexes of a species.

Cooperative Interactions - Dominance and Subordination

     It is a social organisation with hierarchial order. The ritualised behaviour of hierarchial dominance avoids infighting between the members.

Cooperative Interactions - Altruism, Leadership and Territoriality

     It is a kind of cooperative intraspecific, self destructive behavioural interaction in which an individual, called alturist, increases the chances of survival of other individuals of same species due to its sacrificial behaviour.

Cooperative Interactions - Society Formation, Communication and Cannibalism

     It is a highly organised, integrated and complex society, which involves the division of labour at the individual level. This high level degree of division of labour is due to well-developed instinctive behaviour.

Competitive Interactions

     Competitive interactions are seen more as a result of rivalry between two or more organisms, which depend on the same resource.

Characteristics of Biotic Community

     Biotic community occupies particular area with specific physical environment, which acts as a limiting factor, regulating the population size of various species within the community.

Interspecific Interactions

     Interaction between different populations occur for their needs like, 1) Nature and mode of food gathering, 2) Space to rest or shelter, 3) For specific habits like aggregation, breeding etc.

Positive Interactions

     It is a positive inter specific interaction in which both the partners are mutually benefitted and increase the chance of their survival. However, the interaction is not obligatory for their survival as both can live without this interaction.

Negative Interactions

     It is a negative, direct food related interspecific interaction between two species of animals in which larger species called predator attacks, kills and feeds on the smaller species called prey.

Neutral Interaction

     Lack of interaction between individuals of different species is called 'neutralism'. Examples: Rabbits, deer, frogs, live together in a grassland with no interaction between them.

Biotic Stability

     Since a biotic community is formed of a large number of diverse type of populations of plants and animals and microbes, all these populations are essential for biotic stability.

Ecological Succession

     The occurrence of relatively definite sequence of communities over a long period of time in the same area resulting in establishment of stable or climax community, is known as ecological or biotic succession.

Lithosphere (Xerarch)

     It is a type of xerosere and involves the ecological succession on bare rock surfaces.


     Lichen species like Graphis Rhizocarpon, Rinodina and Lacanora forms the pioneer community in a lithosere, as they can tolerate desiccation. Organic acids produced by lichens corrode the rock surface and release minerals for proper growth of lichens. Wind borne soil particles and organic matter collect in the depressions, which initiates soil formation, leading to the growth of foliose lichens like Parmelia, Physcia etc.


     Hydrosere, also called hydrarch involves the ecological succession in the newly formed pond or lake.


     A group of individuals, having similar structural and functional characters, capable of potential interbreeding is termed as species. In nature all apecies exhibit cooperative and competitive interactions among the members of the species (intraspecific) and also with members of other species (interspecific).

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