Dynamics of biological communities

Communities are assemblages of large number of species  that  occur together in space and time and all interact directly or indirectly  with each other. A biotic community comprises plants and animals present in a particular area. E.g. microbial community, plant community, animal community. Community concept is one of the most important principles in Ecology. This concept defines community as diverse organisms usually live together in an orderly manner.  Victor E. Shelford has defined “the community as an assemblage of populations living in a prescribed area or physical habitat with a definite trophic organization and metabolic pattern”. Communities are stable, integrated, and orderly entities.

 Community diversity-plants and animals belonging to different taxonomic groups are present in a community. The abundance of populations and number of species varies greatly in a community. Species diversity is the product of richness and evenness.

Community stratification-All communities have a physical structure and biological pattern. Stratification is a layering or series of separations that differently distributes the organisms occurring in an ecosystem. Stratifications can be described in terms of separations between organisms in space (vertical or horizontal) or in terms of separations between organisms in time (periodicity). An ecosystem can be stratified in space either vertically (layers) or horizontally (concentric circles).Stratification of terrestrial communities reflects the life forms of plants. Stratification results in economy of space and per unit area of the land surface.  Organisms in a community display daily, lunar, seasonal or irregular patterns that separate them in time.

Community dominance -All the organisms in a community are not equally important in determining the nature of that community. Only a few species generally exert the major controlling influence on the community due to their size, number or activities. These groups largely control the energy flow, which are known as ecological dominants. In pasture grass and cattle are ecological dominants.

Community structure - A community is a composite biotic unit. The individual populations of different species depend on each other and on the abiotic environment for their food, shelter and reproduction. A community consists of mainly 3 groups: Producers are autotrophic organisms; Consumers can be herbivores and carnivores; Decomposers are microbes.

Community periodicity - The rhythmic changes occur in the activities of organisms. E.g. Seasonal periodicity – temperature, rainfall and photoperiod determine the reproductive cycle of its inhabitants; Generally, animals of terrestrial community are active during the day and inactive during night which is known as dial periodicity.

Community fluctuations - Even within permanent and stable community, fluctuations in species abundance and numbers occur due to the interplay of biotic potential and environmental resistance. It may be regular or cyclic. E.g. host-parasite or predator- prey fluctuations in an ecosystem.

Community stability - Community stability is the degree of fluctuations in the size of the populations comprising the community. Environmental stability is the fluctuations in the abiotic factors of the ecosystem. Both the types of stability are closely related. The structure of community has a major role in determining the degree of environmental variability. For e.g. the organisms of a forest floor is protected from winds, evaporation and temperature variations by the thick forest canopy.

Community diversity - Species diversity is a key feature of biological communities. Species diversity affects the stability and productivity of communities. A latitudinal gradient of species diversity exists for many taxa. Species diversity declines as latitude increases.  High diversity in the tropics leads to high productivity. Diversity is positively correlated with stability.

Communal interdependence - A community is a self-sustained unit. The interdependence among different species of a community is found for food, reproduction and protection. Most communities are interdependent, or share a mutual responsibility for group survival.

Community trophic structure - Trophic structures are the feeding relationships within communities  i.e. who's eating whom. The first trophic level is made up of the primary producers, the organisms that obtain the energy from inorganic sources. Primary producers  are photosynthetic organisms; more generally, primary producers are autotrophs (i.e., they fix CO₂). Consumers are the heterotrophs, i.e., organisms that obtain their carbon from other organisms.

Community succession - It is the development of a community. It is a directional change in the community structure as a function of time.

There two fundamental types of succession: primary and secondary. Primary succession is the series of community changes which occur on an entirely new habitat. Secondary succession is the series of community changes which take place on a previously colonized, but disturbed or  damaged habitat.

Biotic community – salient features - A community is an aggregate of organisms, which form a distinct ecological unit. The size of community unit may be large (forest community) or small (community of invertebrates). Different community occurs in different habitats. The composition and character of a community is an indicator of the type of environment that is present. The structure of a community is dependent on: 1) the number of species.2) the relative number individuals in each species. Species diversity indices give quantitative expression of community structure. Communities are constantly changing. The replacement of one community by another is called community succession. The final stage in community succession is a climax community. On the whole a community is considered as a highly integrated self-contained organic unit.

Concept of biocoenosis - A biocoenosis is a biotic community, coined by Karl Mobius in 1877. This concept defines community as the interacting organisms living together in a habitat (biotope). Zoocoenosis refers to faunal community; Phytocoenosis refers to floral community; Microbiocoenosis refers to microbial community.

Community concept is one of the most important principles in Ecology. This concept defines community as diverse organisms usually live together in an orderly manner. Victor E. Shelford has defined “the community as an assemblage of populations living in a prescribed area or physical habitat with a definite trophic organization and metabolic pattern”.

The individualistic hypothesis was proposed by H.A. Gleason( American Ecologist),1917.  This hypothesis depicted a community as a chance assemblage of species found in an area because they have similar abiotic requirements. In other words “vegetation is continuously variable in response to a continuously varying environment”.

The interactive hypothesis was proposed by F.E. Clements in 1916. According to him, each community is an assemblage of closely linked species having mandatory biotic interactions that cause the community to function as an integrated unit.

Community types – terrestrial community e.g.( 1.Tropical  rain forests; 2.Temperate Forests; 3. Grasslands; 4.Deserts), aquatic community(Pond community )

Community succession - All ecosystems change over time. There are  two types of ecosystem changes. Non-directional directional changes-Replacement-Fluctuations; Directional changes-Biogeographic change- Succession. All these types of changes may occur simultaneously within a single ecosystem.

Community succession is defined as a gradual, continuous and unidirectional change  in the species composition  of a natural community over a long time.

Stages of community succession - Early stages are characterized by low biomass, low nutrient levels, low species diversity, short food chain and high productivity; Middle stages are characterized by rich  biomass, rich  organic nutrients and more  species diversity; Final stages are characterized by stable high biomass, high organic nutrients & low productivity, complex food chain and high species diversity &competition.