Concept of Community and Succession

Community

A community is an association of a number of living organisms living in a certain prescribed area or physical habitat with usually of different species. The assemblage of plants in a certain area or habitat is considered as plant community and assemblage of animals in a certain area or habitat is considered as the animal community. Communities are of two types, major communities include dependent and self-reliant living organisms in their habitat while minor communities are greatly determined by inputs from adjacent communities.

Characteristics of community

1. Biotic community is a natural assemblage of a large number of plant and animal of various species in an area. The range of environment that a species can tolerate is called its ecological amplitude.
2. The different population of a community has various growth forms and structure that determine the structural pattern of the community.
3. Species living in their particular habitat do not live in a complete isolation as pure cultures but co-exist in mutual adjustment obligatory in one direction or in both.
4. Species with number and biomass are considered to be dominants. By virtue of their activity, size or number they have controlling influence upon other populations.
5. In a community, a different population exists in a specific proportion that is relatively distributed in a given habitat.
6. There are different species of different trophic levels between which there exists a food chain like herbivorous, primary and secondary carnivorous.
7. The different types of plant form more or less distinct strata or layers on vertical as well as in horizontal planes. This is characteristically known as stratification like subterranean in grassland, limnetic in pond ecosystem, etc.

Composition of community

The community has a certain size which varies from different inches like bigger in the ocean, forest, but smaller in microhabitats. The number of the population varies depending upon natural resources. There are always dominants populations. Type of community in a particular habitat is determined by climate and other abiotic conditions as well as the biotic environment.

Quantitative and qualitative characters of community

Quantitative characters:

Frequency: It is the percentage of a particular species in each bisect, quadrate, point center or point quarters.

Frequency =\(\frac{No.of sampling units in each habitat}{ Total No. of sampling units studied}\) × 100%

Density: It is the number of individual of the species in a unit area.

Density =\(\frac{ total no of individuals of the species in all the sample units}{Total No. of sampling units studied}\)

Abundance: It is the number of individuals of any species per sampling unit of occurrence.

Abundance = \(\frac{Total no. of individuals of the species in all the sample units.}{No. of sampling units in which the species occurs}\)

Cover and basal area: An area in a medium, occupied by the particular individual or its units such as stems, leaves, and flowers, as seen from above is called cover area. It is estimated by chart-quadrate. Basal area refers to the area of ground actually penetrated by stems. It is measured by calipers, line interception or point centered quadrate method.

Dominance: It is measured in terms of cover. In a grassland area, a single tree may occupy quite a large cover.

Qualitative characters:

1. Physiognomy: The growth form of dominant species influence the general appearance of vegetation like grassland, forest, etc. This is known as physiognomy.
2. Phenology: Plants of different species have the different time period of germination and development in relation to climate.
3. Stratification: It is the arrangement of different types of the population on vertical and horizontal planes.
4. Sociability: It refers to the closeness of the species to one another in the habitats.
5. Vitality: Survival of the species is determined by the capacity of organisms for its normal growth and reproduction.
6. Growth form: It is the sum of adaptation of individual to its environment and determines the structural pattern of the community.

Concept of succession

The gradual replacement of one type of ecological community by another in the same area, involving a series of orderly changes, especially in the dominant vegetation. Hult (1885) was the first person to use the term 'succession.'

Causes of succession

1. Initial or initiating causes: It involves both climatic causes and biotic causes like heavy snowfall, lightening, etc.resulting from the total destruction of community. Biotic includes grazing, cutting etc. due to living agencies. These are directly responsible for a vegetational change.
2. Ecesis or continuing causes: Here processes like migrations, ecesis, aggregation, etc. causes succession waves of the population as the result of edaphic features of the area.
3. Stabilizing causes: For succession, climate also plays the important role.

Basic type of succession

1. Primary succession:It starts when a new area that has never previously supported an ecological community is colonized by new plants and animals. Eg: Succession in a pond (hydrosere) and succession in a desert (xerosere).
2. Secondary succession:It starts when an area with an ecological community is disturbed or changed that the original community is destroyed and is replaced by another community.

General process of succession

The general process of primary autotrophic succession is carried out through following steps:

1. Nudation:It is the process of development of bare or naked land without any ecological community. It is formed due to natural calamities.
   
   • Topographic: Here, the existing community may disappear due to soil erosion by silting, volcanic activity, etc.
   • Climatic: Community when destroyed by glaciers, drought, storms, etc.
   • Biotic: Human, insects, pests, fungi, etc. also play an important role in destroying forests and grasslands destruction.
     
     
2. Invasion: It is the establishment of new species or new ecological community in a bare area is called the invasion. It includes following stages.
   
   • Dispersal or migration: Here, seeds, pores, etc. germinates whereas insects, birds or animals migrate to nude area from another area. (Animal behavior)
   • Ecesis: It is the successful establishment of a species after reaching in the new area adjusting with conditions prevailing there.
   • Aggregation: After their successful establishment, they reproduce in a widespread way so as to increase their population.
3. Coaction and competition: Due to aggregation of the different population, there develop coactions and competitions for food and space among the species. The weak species may be replaced by another the community.
4. Reaction: It is the process of modification of environment due to the influence or force of living organisms on it. The environment change as a result of reaction which becomes difficult for the existence of communities and sooner or later will be replaced by another community.
5. Stabilization or climax: The final and comparatively stable stage are known as climax stage. When the final terminal community becomes stabilized for a longer period of time and it can maintain itself in equilibrium with the climate of the area, it is known as the climax community.

Process of succession in hydrosere

Hydrosere is primary succession sequences which develop in aquatic environments such as lakes and ponds. It results in the conversion of the waterbody and its community into a land community. Various stages in succession in hydrosere are as follows:

1. Phytoplankton stage: It starts with the formation of the pioneer community due to germination of encysted spores reached through wind or animals. It includes minute autotrophic diatoms, Phyto flagellates, etc. After decaying they mix with slits and forms soft mud.
2. Rooted submerged stage: Due to death and decay of rooted submerged hydrophytes like Hydrilla, Vallisneria, etc. and deposition of sand and slit, there is a slow rise in the bottom level (soil layer) of the pond. Buried older plants form good humus.
3. Rooted floating stage: Floating plants like Nymphea, Trapa, Monochoria, etc. appear helping water to become rich in mineral and organic matter. Later free floating plants like Azolla, etc. also grows. This increase the bottom layers of the pond and makes water shallower.
4. Reed swamp stage: Here, floating plants are replaced by the other plants like Typha, Scirpus, etc. which produce organic wastes and loose huge amount of water by transpiration. It increases substratum of the pond and becomes unsuitable for growth of amphibious plants. It is also called amphibious stage.
5. Sedge meadow stage:Also called marsh meadow stage where plant species like Carex, Juncus, and herbs develops forming mat-like vegetation with the branched rhizomatous system. It may disappear due to the development of mesic conditions.
6. Woodland stage: The soil remains drier for most of the time and becomes suitable for the development of wet woodland i.e. shrubs and trees such as willow, alders, etc. It lowers the water table by transpiration, building up the soil and accumulation of humus with associated microorganisms.
7. Forest stage: It is the development of climax community which is greatly influenced by climatic conditions like a tropic rainforest in areas with heavy rainfall, mixed forest in temperate regions, etc.

Climax concept of succession

1. Monoclimax Theory: This theory was developed by F.E. elements in 1916. According to this theory, one climate is the sole cause of the stabilization of only one climax. All other communities besides climax community are related to climax by successful development and are recognized as sub-climax, disclimax, pre-climax, and so on.
2. Polyclimax Theory:This theory was developed by Tansley in 1935 and tells that the climax vegetation of region consists of more than one vegetation climax controlled by environment factors like soil, moisture, etc.
3. Climax pattern Hypothesis: This theory was purposed by R.H. Whittaker in 1953. According to this theory, succession involves a directional change which climax presents fluctuation around an average.