A biotic community is defined as a group of several populations of different species. The biological community in an area or ecosystem is a complex network of interactions. The interaction that occurs among different individuals of the same species is called intra–specific interaction while the interaction among individuals of different species in a community is termed as inter-specific interaction.
Interactions between organisms belonging to the same trophic level often involve competition. Individuals of a population may compete for food, space and mates.
For example if a mouse has been eaten by a cat, other cats competing for this resource would have one less mouse to prey on. The snake another predator of the mice would also have fewer mice to eat during the night if the cat has succeeded.
Direct competition, though, between the cat and snake is not much as they prey at different times. They also eat a variety of different foods. So competition may be intra-specific as well as inter-specific. Inter-specific relationship may be direct and close as between a lion and deer or indirect and remote as between an elephant and a beetle.
This is because interactions between two species need not be through direct contact. Due to the connected nature of ecosystems, species may affect each other through intermediaries such as shared resources or common enemies.
Specific terms are applied to inter-specific interactions depending upon whether the interaction is beneficial, harmful or neutral to individuals of the species.
Community: In an environment or habitat, different types of plants and animals exist in close association and show interdependence. An actively interacting group of a number of different populations of several species in a common environment is known as biotic community.
Community Ecology: This is the study of interactions among all populations in a common environment. Examples of biotic communities: Pond community, Forest community and Lake Community.
Characteristics of Biotic Community
The characteristics of biotic community include the following:
Interdependence of animals and plants
Communities are not a random mixture of species. The animal, plant and microbial community of a biotic community show fundamental dependence on each other.
Carbon dioxide-oxygen interdependence.
Interdependence in pollination.
Interdependence in dispersal of seeds and fruit.
A community is characterized by a definite trophic organization, i.e., trophic levels. All species of a population living in a community can be divided into three trophic levels, producers or green plants, consumers or animals and decomposers or microbes.
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In large terrestrial and aquatic communities, population of each species occupies a particular strata. This is called stratification. E.g. In a forest community, the tree tops, the branches, the leaf litter and the soil bottom are occupied by different species.
A pond community has surface dwellers, bottom dwellers and those living in the intermediate depths.
In any community, one or few species dominate either in numbers or in physical characteristics or in both over the species. For example, in Grassland: Grasses; Pine forest: Pine.
Variety of Species
A community is formed of many species. These vary from community-to-community. A great variety of species are found in the tropical rainforest whereas only a few species are found in a polar community.
In a community, there is interaction among the organisms of the same species, with other organisms of their own community or among the organisms of different communities. The organisms are related to one another through cooperative or competitive actions so that they maintain a balance.
This relationship between organisms is classified into following two types: Intra-specific relationship and Inter-specific relationship
It is the relationship among the individuals of the same species. The cooperative interactions in the members of the same species or population include mating behaviour, parental care, family formation, group formation, altruism, dominance subordinate behaviour, animal societies and communication.
Communication: It maintains continuous interaction with another among members. E.g. Bees.
Altruism: It is a form of behavior among social animals in which one or several organisms sacrifice with their own interest for the welfare of the group. For example: Group of cheetahs and a wild dog.
Animal societies: It is among many species, intimate exchange between members and adaptive, cooperative, action are facilitated through formation of permanent social structures called societies. For example: Bees/ants.
The competitive interactions are dominance subordinate behaviour, leadership and home range or territoriality. The cooperative relationships are beneficial for the species.
Mutualism is an interaction between two organisms of different species where both the partners are benefited with none of the two capable of living separately. Examples:
1. Positive Interaction
These represent an intimate mutualistic relationship between a fungus and green alga or cyanobacteria. Their body is made up of an alga and a fungus, both living together in intimate symbiotic relationship.
The fungus provides substratum, water, minerals and shelter to algal component, while the alga provides fungi with food (carbohydrates).
Protocooperation is an association between organisms of different species in which both are mutually benefited but they can live without each other. It means protocooperation is equally beneficial for both but is not obligatory.
Example is commensalism;
Commensalism is an association between two different organisms or species in which one is always benefited but the other is neither benefited nor harmed. The species that derives benefit is called commensal and the other is called host.
Benefits: Commensals derive benefit from the host in the form of food, protection, shelter, living space or transportation.
Examples: Sucker fish and Shark: sucker fish gets attached to the under surface of sharks by its sucker. This provides easy transport for new feeding grounds and also food pieces falling from the sharks prey.
Scavenging is the feeding by an animal on the remains or carcasses of dead animals and on the refuse of living animals. Scavenging is a food relationship between a dead animal and its eater.
The eater is called scavenger. E.g: Animals such as foxes, hyenas, vultures etc. are the animals, which are natural scavengers. Dogs, crows, ants are occasionally seen to do the work of scavengers.
2. Negative Interaction
Predation is the direct food relation between two organisms of different species in which one animal (predator) captures kills and feeds upon other animal (the prey). Examples: Familiar examples of predator-prey relationship are between Tiger and deer, Hawk and small Bird, Snake and Rat, Frog and Insects.
Species like Frog may be both a prey and predator. It is predator for insects but a prey for other animals.
Parasitism is a food relationship between organisms of two different species in which the smaller one lives on or within the larger one and obtains its food from it. The former which obtains food is known as parasite and the latter which provides food and shelter to the parasite is called host. Example: Temporary parasitism and permanent parasitism.
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Temporary parasites visit host for a short period for feeding as Bedbugs, Leeches and Mosquitoes. These are called intermittent parasites. However, Mosquito is not a parasite because it transfers parasite to the human beings. So it is a vector.
Permanent parasites live in contact with host throughout their life. E.g. Ascaris, Taenia, Entmoeba.
One of the principles of nature is stability amidst diversity. The larger the number of diverse forms present in a community, the more stable that community will be. It means that the stability of a community depends not on the large populations of a few species but upon the number of populations of different species.
Examples: A large population of a single species of Eucalyptus is likely to be totally wiped out by a fungal disease or insect attack. When there are many species of trees, only one species may be affected by a disease or a pest whereas the rest would survive.
The stability of a community is displayed by the large population of wild animals in Africa. In Serengeti plains of Africa, about 20 species of antelopes live together in the area. Each species of antelope eats a different kind of grass or shrub. Some may even feed on the same kind of shrub but at different stages of its growth.
This division of food preference enables all animals to get adequate nutrition and also keeps the habitat productive. But it has been observed that destruction of wildlife and introduction of domestic animals for grazing in an area destroys the productivity of land.
In summary, all the populations are interdependent. It may be small as a pond community or as a grassland community. Exhibits both direct and indirect relationships.
All the species are not equally represented. Some may be abundant than others. Abiotic community is defined as a group of several populations of different species. The biological community in an area or ecosystem is a complex network of interactions.
The interaction that occurs among different individuals of the same species is called intra- specific interaction while the interaction among individuals of different species in a community is termed as inter-specific interaction.