A biotic community (biocoenosis) is any assemblage of different species living in a geographical area or habitat. It is a heterogeneous collection of different groups of organisms, both plants and animals. Each unit is called Population.
The organisms of different populations have features in common due to abiotic factors in the environment, like light, water, temperature, radiation, etc. The biotic environments, i.e. plants and animals present in a particular area also influence the density of a particular group of population. All the organisms in a community are not equally important in determining the nature of that community. The species which exert a major controlling influence on the community are called ecological dominants.
The nature of relationship between species is determined by the requirements of member-community. Major communities are those which are of sufficient size and members are relatively independent. They need only sun energy from outside and are independent of output and input from adjacent communities. Minor communities are those which are more or less dependent on neighboring aggregations.
Communities have definite functional unity with characteristic structures and patterns of energy flow and also have compositional unity. Biotic community is constantly changing. Very frequently Communities blend gradually into one another so that there are no sharply defined boundaries. (The place and area where two major communities meet and blend together is called Ecotone whereas habitat of an organism or community is the place where it lives.
The position or status that a given species occupies within the community or in an ecosystem on account of its structural adaptations is called Ecological Niche. (Different species living under different geographical regions, but under similar environmental conditions are known as ecological equivalents.
Types and Characteristics of Biotic Community
Within community, animals may exhibit any of the following 6 characterized associations, namely Predation, Scavenging, Parasitism, Commensalism, Symbiosis and Competition.
Animals that live by preying on other animals are called predators. Here, there is direct food relationship between two species of animals. One animal (predator) captures and feeds on the other (prey). A species may be prey and predator at the same time. Each would ;lay a slayer and in turn would be slain for purposes of food.
In this case, an animal would feed on other animals that have been dead naturally or killed by other animals. Vultures feed on dead animals and render scavenging for the community. Here also there is direct food relationship.
Parasite is an animal or plant that lives on or in another animal or plant at the latter’s expense or to its detriment. The parasite is an organism living on in the body of a larger living organism and derives nourishment from the tissues of the host organism. There are two types of parasites, viz., partial parasite and permanent parasite. The. former spends only a part of its life-cycle on the host; while the latter spends its entire life- cycle as parasite on the host.
Endoparasites live inside the body of the host and Ectoparasites live outside drawing sustenance. At times parasites themselves are parasitised by other organism. This is called Hyperparasitism. Parasites bring about profound changes on their host which are always detrimental to the host. At the same time, parasites undergo many changes and modifications in their pursuit of successful living at the expense of host organism. This is called Parasite adaptation.
Commensels are those animals which live together as intimates but without parasitic association. There will he no physiological dependence nor any injury. In commensalism one animal might get benefit from the association while the other is neither benefited nor injured. The relationship would be just like table-mates at the dinner.
Symbiosis means living together of two dissimilar organisms to their mutual advantage. It is an association of two species of organism where both the partners derive benefits from each other. Many instances of symbiosis could be found in biotic community. Lichens are composed of algae and fungus. In this, the green algae synthesize food for both, while fungus gives protection and raw materials for the preparation of food. In the case of leguminous plants, the nodules work in the fixation of atmospheric nitrogen which is used by plants, while plants supply water, minerals and organic food to bacterium.
While symbiosis is collusion of activities for mutual benefits in the biotic community, competition is the race between species for getting a common resource. Active demand by two or more individuals of the same species or members of two or more species at the same tropical level will result in density and diversity of population causing congestion. This is a common feature in forests where trees, shrubs and herbs strive for sunlight, water, nutrients etc., and vie with each other.