Succession

Succession, types of succession, mechanism of succession and mechanism of succession

1. Succession Dr. Emasushan Minj Assistant Professor Department of Botany

2. Introduction Succession is a universal process of directional change in the community. Its composition and structure constantly change in response to environmental condition . It is orderly and sequential parallel changes with the physical environment. These changes lead finally to the equilibrium of the community with the environment called a climax community. Climate community marks the endpoint of succession . Ecological succession is a gradual and fairly predictable change in the species composition of a community in a given area. An intermediate stage in the successional process towards the climax community is known as seral community and the temporary stage is called the seral stage .

3. Types of succession Primary succession S econdary succession Section occurring in bare areas such as new volcanic Island, where no community existed before is called primary succession . In primary succession the unoccupied terrestrial site is first colonized by a few pioneer species which are often microbes, lichens and mosses . Soil develops gradually as rocks weather and organic matter accumulates from the decomposed remains of the early colonizers. Secondary succession occurs after the destruction of existing community by some natural disturbance such as fire or flooding; or by human activity. Hence , secondary succession is the development of biotic community after the complete or partial destruction of the existing community. For example , the forest destroyed by fire reoccupied by herbs in the initial stages. The reappearance and establishment of communities in such area is secondary succession.

4. Autogenic succession is self driven, resulting from the interaction between organisms and their environment. In this case successional changes are brought about by the organism themselves . Allogenic succession occurs as a result of the changes brought about in the habitat by external agencies and not by the existing community itself . The succession where initially the green plants are much greater in quantity than the animals is known as autotrophic succession . Such succession takes place in a rich inorganic substances medium . In heterotrophic succession , the population of heterotrophic organisms like animals and fungi are present in greater quantity in the initial stages. It begins, prominently in an organic environment and there is a progressive decline in the energy content.

5. The kind of succession takes place from single and few forms of complex and several kinds of forms are known as progressive succession . A change from grassland community to forest community is an example of progressive succession. In retrogressive succession the community become simplistic and contents fewer species and less biomass over time. A forest changing into a grassland community is an example of retrogressive succession.

6. Mechanism of succession According to Clements succession is a process involving several steps. These steps or processes are : Nudation - Succession begins in a bare area without any form of life . Invasion or migration- Many species invade or reach a new site from any other area. The seeds, spores or other propagules of the plant species reach the bare area . Ecesis - After reaching the new area the process of successful establishment of the species as a result of adjustment with the condition prevailing there is known as ecesis and able to reproduce sexually in the particular area . Aggregation - After successful establishment of a species as a result of reproduction, the individuals of the species increase in number.

7. Competition - The aggregation of individual in an area leads to interspecific and intraspecific competition. The competition is usually for water, particularly when there is a shortage of water nutrients or if the plant grows the shade of another, shortage of carbon dioxide, oxygen and space takes place . Reaction- The mechanism of modification of the environment, through the influence of living organism on it is known as a reaction. As a result of the reaction, changes takes place in soil, water, light condition, temperature and many other factors of the environment. Stabilization (Climax)- The whole process of succession results in stabilization of the vegetation which is now in complete harmony with the environment of the place, where the condition remains unchanged. The soil is fully occupied by plants and the community is closed. This final community is not replaced and is known as climax community and the stage as climate stage.

8. Three major theories have been proposed to define, identify and interpreter the climax communities. These theories are Monoclimax theory, Polyclimax theory Climax pattern theory . Monoclimax theory Monoclimax theory is also known as climatic climax theory. It was proposed by Frederic Clements . This theory recognizes only one climax, no matter how great the variety of environmental condition is at the start . Preclimax - Climax communities developed under similar climatic conditions. If the community has a life forms lower than those in the expected climatic climax is known as preclimax . Postclimax - Community that has life forms higher than those in the expected climatic condition of climatic climax is postclimax . Subclimax - The stage in succession just preceding the climatic climax is subclimax . Disclimax - When a stable community which is not the climate climax for the given site is maintained by man for his domestic animals, it is designated as disclimax .

9. Polyclimax theory - It was proposed by Tansley . The theory considers that the climate vegetation of a reason consists of not just one type, but a mosaic of vegetational climax controlled by several factors such as soil moisture soil nutrients, topography slope exposure, fire and animal activities. Tansley recognized following climax types : Climatic climax- Climax community, which is determined by the general climate of the region . Edaphic climax- Climate communities, which are determined by the soil properties such as soil moisture, soil nutrients . Topographic climax- Climax community, which is determined by the topography of the region . Fire climax- Climax resulted by repeated fire is called fire climax.

10. Climax pattern theory A variation of the polyclimax was proposed by R.H. Whittaker (1953). The natural community is adapted to the whole pattern of environmental factors in which it exists . In addition to the climatic factors, e.g. wind, there are a whole range of edaphic factors, fire and biotic factors including grazing, disease and mutualistic and competitive interactions. Whereas, monoclimax theory allows one climatic climax in a region and the polyclimax theory allows several climaxes in the same area, the climax pattern hypothesis allows a community of climax types, varying gradually along environment gradients and not neatly package into discrete climax types.

11. Model of succession There are three models to explain the ecological processes of community change during succession. These models are- F acilitation model T olerance model I nhibition model.

12. Facilitation model The classical model that explains the mechanism of succession is the facilitation model according to this model certain Pioneer species with quality ideal for early succession can colonize the newly exposed landforms after and ecological disturbance. This initial species modify the site, making it more suitable for invasion by other species, for example, by carrying out the earliest stages of soil development. Once established, the latter sectional species eliminate the pioneers through competition . This ecological dramatic proceed through a progression of stages in which early species are eliminated by later species, until the climate stage is reached. This model seems to be most appropriate in explaining changes in many primary successes, but less so for secondary succession.

13. Tolerance model According to tolerance model, new Pioneer species neither inhibit nor facilitate the growth and success of other species. All species in the succession are capable to establish on the newly distributed site, although with exhausting successors in turn of the rapid attainment of a large population size and biomass. In contrast with the facilitation model the early occupations of the site do not change environmental condition in ways that favour the subsequent invasion of later successional species rather with increasing time the various species short them self out through their difference is tolerance of the successional increasing intensity of biological stresses associated with competition. In the tolerance model competition tolerant intolerant species are relatively successful in early successional stages when site conditions are characterized by a free availability of resources. However these species are eliminated later on because they are not as competitive as later species which eventually develop a climate climax community.

14. Inhibition model T olerance model, both early and the late successional species can establish populations soon after distribution disturbance . However some early species make the site less suitable for the development of other species. For example some plants are known to secret toxic biochemical into the soil these are called aloe chemicals which inhibits the establishment and growth of other species. Eventually however the inhibitory species die and this creates opportunities that later successional species can exploit. All three of these models facilitation tolerance and inhibition can be supported by selected evidence from many ecological studies. All these models differ significantly in their predictions about the organizing principles of successional dynamics it appears that none of them is correct all of the time .

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