Ecological Succession
A group of plants of different species living together in a common habitat is called as an plant community.
Communities arise, grow, mature, attain old age and die from natural causes or by accident. Communities are never stable, but dynamic, changing more or less regularly over time and space. They are never found permanently in complete balance with their component species or with the physical environment.
Thus as vegetation develops, the same area successively is occupied by different plant communities.
Clements – “the natural process by which the same locality become successively colonized by different communities of plants”.
Succession results from the modification of the physical environment by the community and from competition-coexistence interactions at the population level. The whole sequence of communities that replace one another in a given area is termed as sere. The intermediate stages in the process are termed as seral communities or stages.
Then final community which stabilizes the system and which persists is called as climax community
Kinds of Succession:
Succession initiated on an area previously unoccupied or on a bare land, where there was no previously any living organisms is called as primary succession. The first group of organisms establishing there are known as pioneers.
Succession initiated on an area previously occupied by another community is called as secondary succession. In this area the existing community disappears due to action of external force like a sudden change in climatic factors, disruption by fires, biotic intervention etc.
After succession has begun, the community itself, as a result of its reactions with the environment, modifies the environment and thus cause its own replacement by new communities, such a succession is called as autogenic succession. Replacement of the existing community by external condition and not by the existing communities is called as allogenic succession.
On the basis of kind of organisms, the succession is of two types autotrophic and heterotrophic. In autotrophic succession there is early and continued dominance of autotrophic organisms like green plants. This type of succession begins in a predominantly inorganic environment. In heterotrophic succession there is early dominance of heterotrophs like bacteria, actinomycetes, fungi. It begins in a predominantly organic environment.
Depending upon the nature of the environment the following types of succession are found-
Succession which begins in water habitat like lakes, ponds, streams, etc., is known as hydrach and the different stages of development constitute the hydrosere.
Successsion which begins in the dry or xerophytic conditions like deserts, etc., is known as xerarch and the different stages constitute the xerosere.
Succession which begins in the area where adequate moisture conditions are present is called as mesarch and the different stages constitute the mesosere.
Sometimes, there are further types like –
Succession initiated on the rocks is called as lithosere, on sand is called as psammosere and on in saline water or soil is called as halosere
Process of Succession
According to Clements succession has five following stages-
I. Nudation
Development of bare area without any form of life is called as nudation. The bare area may develop due to several causes such as landside, erosion, deposition, etc.
II. Invasion
Successful establishment of a species in a bare area is called as invasion. The species reaches this new area from any other area. The whole process of invasion is completed in following three stages:
A. Migration: the seeds, spores, propagules of the species reach the bare area. This process is
generally brought about by air, water, etc.
B. Ecesis (Establishment): after reaching the new area, the process of successful establishment of
the species is known as ecesis. After migration, seeds or propagules germinate, seedlings grow, and adults start to reproduce. Thus, as a result of ecesis, the individuals of species become established in the area.
C. Aggregation: as a result of reproduction, the individuals of the species increase in number.
III. Competition :
After aggregation of a large number of individuals the species at limited space, there develops competition mainly for space and nutrition. The competition may be inter or intra specific. The species, if unable to compete with other, would be discarded.
IV. Reaction:
Modification of environment through the influence of living organisms on it is called as reaction.
As a result of reactions, changes takes place in soil, water, temperature, etc., of the environment.
Due to all these the environment is modified, becoming unsuitable for the existing community
which sooner or later is replaced by another community.
V. Stabilisation or Climax
Final community more or less becomes stabilized for a longer period and it can maintain itself in equilibrium with the climate of the area. This final community is not replaced, and is known as climax community and the stage is called as climax stage.
The various stages in the hydrosere are:-
Phytoplankton Stage:
They constitue the poineer community. Blue gree algae, green algae, diatoms, bacteria are the first organisms to colonise the pond or lake. They multiply and grow for some time.
Rooted Submerged Stage
Death and Decomposition of phytoplanktons, and their mixing with silt, brought from the surrounding areas by rain waters, there develops soft mud at the bottom of the lake. This new habitat becomes suitable for the growth of rooted submerged hydrophytes like Hydrilla, Potomogeton, Vallisneria, Utricularia.
These plants bring about buildup of the substratum as a result of their death and decay.Water level decreases making lake shallower.
Rooted Floating Stage
By now water depth is around 2-5 feet. Various rooted hydrophytes with large leaves floating on water surface. E.g – Nelumbo, Nymphaea, Trapa invade the area. Some free floating species e.g.,Azolla, Pistia, Lemna, Salvinia also become associated with rooted plants.
Water level decreases making pond more shallower. The decomposing organic matter formed due to death and decay of these plants brings about further buildup of the substratum.
Reed Swamp Stage
This stage is also known as amphibious stage as the plants are rooted but their shoot are exposed to air. These are species of Typha, Sagittaria, Phragmities. They have well-developed rhizomes and form a very dense vegetation.
Sedge Meadow Stage
Decreasing water level and increasing amount of light become unsuitable for the reed swamp communities. Further changes in the substratum becomes suitable for Cyperaceae and Gramineae species such as Carex, Cyperus, Juncus to colonise the area
The form mat like vegetative towards the centre of the pond with the help of their branched rhizomatous system. As a result of hight rate of transpiration there is a rapid loss of water due to which mud is exposed to air. Thus mesic conditions approach and marshy vegetation disappears gradually.
Woodland Stage
Due to disappearance of the marshy vegetation, oil becomes drier for most time of the year. This area is now invaded by terrestrial species, which are shrubs (Salix, Cornus) tress (Almus, Populus).
Due to accumulation of humus with rich flora of microorganisms, it becomes suitable for arrival of the tree species.
Forest Stage
This is the climax community. The area is now invaded by several trees. In tropical climate with heavy rainfall there develops tropical rain forests, whereas temperate climate there develops mixed forest of Almus, Acer Quercus. In regions with moderate rainfall, there develop tropical deciduous or monsoon forests.
The poineers to colonise this primitive substratum are crutose lichens, and through a series of successive seral stages the succession terminates into a forest which constitutes the climax community.
The various stages in the xerosere are:-
Crustose Lichen Stage
Substratum is poor in moisture and organic matter with extreme temperature it is unsuitable for many plants. Crutose lichens alone are the pioneers which are able to grow. Species of Rhizocarpon, Rinodina, Lecanora estasblish in this substratum as these lichens are slow growers and can withstand extreme desiccation.
When there is rain they absorb water like sponge and flourish rapidly. They produce some acids which bring about weathering of rocks. Dead organic matter of lichens becomes mixed with the rock particles that make conditions for growth of other vegetation
Foliose Lichen Stage
As soon as little soil is accumulated, some foliose lichen like species of Dermatocarpon, Parmelia, Umbilicaria appear. They have large leaf-like thalli that can absorb and retain more water. Humus and dust particles are accumulated which help in the further build up of the substratum. Weathering of rocks and mixing of humus result into the development of fine thin soil layer.
Moss Stage
Development of thin soil layer on rock surface favours development of Xerophytic mass – Polytrichum, Tortula, Grimmia.
Dead and decay produce mat on the rock surfaces. This mat like surface has more water holding capacity. Due to their death and decay there is further addition of organic matter and this leads to increase in the thickness of the soil layer.
Herbs Stage
Due to extensive growth of mosses there is more accumulation of soil. This changed habitat favours the growth of herbaceous plant – first annuals and later binennials and perennials make there appearance. Eg.,Aristida, Festuca, Poa, Solidago.
The process of rock disintegration and accumulation of humus is accelerated as the tangled network of root increases. Evaporation and temperature decreases.
Shrub Stage
Due to much accumulation of soil, the habitat becomes suitable for shrubs. Species of Rhus, Phytocarpus with dense shrubby growth makes soil shaded and evaporation is reduced. With the enrichment of soil and greater water holding capacity favours the growth of trees.
Forest Stage
Some xerophytic tree species invade the area. Further weathering of rocks and increase humus which favours arrival of trees and vegetation finally becomes mesophytic. Finally a forest community develops
A group of plants of different species living together in a common habitat is called as an plant community.
Communities arise, grow, mature, attain old age and die from natural causes or by accident. Communities are never stable, but dynamic, changing more or less regularly over time and space. They are never found permanently in complete balance with their component species or with the physical environment.
Thus as vegetation develops, the same area successively is occupied by different plant communities.
Clements – “the natural process by which the same locality become successively colonized by different communities of plants”.
Succession results from the modification of the physical environment by the community and from competition-coexistence interactions at the population level. The whole sequence of communities that replace one another in a given area is termed as sere. The intermediate stages in the process are termed as seral communities or stages.
Then final community which stabilizes the system and which persists is called as climax community
Kinds of Succession:
Succession initiated on an area previously unoccupied or on a bare land, where there was no previously any living organisms is called as primary succession. The first group of organisms establishing there are known as pioneers.
Succession initiated on an area previously occupied by another community is called as secondary succession. In this area the existing community disappears due to action of external force like a sudden change in climatic factors, disruption by fires, biotic intervention etc.
After succession has begun, the community itself, as a result of its reactions with the environment, modifies the environment and thus cause its own replacement by new communities, such a succession is called as autogenic succession. Replacement of the existing community by external condition and not by the existing communities is called as allogenic succession.
On the basis of kind of organisms, the succession is of two types autotrophic and heterotrophic. In autotrophic succession there is early and continued dominance of autotrophic organisms like green plants. This type of succession begins in a predominantly inorganic environment. In heterotrophic succession there is early dominance of heterotrophs like bacteria, actinomycetes, fungi. It begins in a predominantly organic environment.
Depending upon the nature of the environment the following types of succession are found-
Succession which begins in water habitat like lakes, ponds, streams, etc., is known as hydrach and the different stages of development constitute the hydrosere.
Successsion which begins in the dry or xerophytic conditions like deserts, etc., is known as xerarch and the different stages constitute the xerosere.
Succession which begins in the area where adequate moisture conditions are present is called as mesarch and the different stages constitute the mesosere.
Sometimes, there are further types like –
Succession initiated on the rocks is called as lithosere, on sand is called as psammosere and on in saline water or soil is called as halosere
Process of Succession
According to Clements succession has five following stages-
I. Nudation
Development of bare area without any form of life is called as nudation. The bare area may develop due to several causes such as landside, erosion, deposition, etc.
II. Invasion
Successful establishment of a species in a bare area is called as invasion. The species reaches this new area from any other area. The whole process of invasion is completed in following three stages:
A. Migration: the seeds, spores, propagules of the species reach the bare area. This process is
generally brought about by air, water, etc.
B. Ecesis (Establishment): after reaching the new area, the process of successful establishment of
the species is known as ecesis. After migration, seeds or propagules germinate, seedlings grow, and adults start to reproduce. Thus, as a result of ecesis, the individuals of species become established in the area.
C. Aggregation: as a result of reproduction, the individuals of the species increase in number.
III. Competition :
After aggregation of a large number of individuals the species at limited space, there develops competition mainly for space and nutrition. The competition may be inter or intra specific. The species, if unable to compete with other, would be discarded.
IV. Reaction:
Modification of environment through the influence of living organisms on it is called as reaction.
As a result of reactions, changes takes place in soil, water, temperature, etc., of the environment.
Due to all these the environment is modified, becoming unsuitable for the existing community
which sooner or later is replaced by another community.
V. Stabilisation or Climax
Final community more or less becomes stabilized for a longer period and it can maintain itself in equilibrium with the climate of the area. This final community is not replaced, and is known as climax community and the stage is called as climax stage.
Hydrosere
Ecological succession that takes place in water bodies like ponds, lakes is called as hydrosere. It starts with colonization of phytoplanktons which form the pioneer plant and finally terminates into a forest, which is a climax community.The various stages in the hydrosere are:-
Phytoplankton Stage:
They constitue the poineer community. Blue gree algae, green algae, diatoms, bacteria are the first organisms to colonise the pond or lake. They multiply and grow for some time.
Rooted Submerged Stage
Death and Decomposition of phytoplanktons, and their mixing with silt, brought from the surrounding areas by rain waters, there develops soft mud at the bottom of the lake. This new habitat becomes suitable for the growth of rooted submerged hydrophytes like Hydrilla, Potomogeton, Vallisneria, Utricularia.
These plants bring about buildup of the substratum as a result of their death and decay.Water level decreases making lake shallower.
Rooted Floating Stage
By now water depth is around 2-5 feet. Various rooted hydrophytes with large leaves floating on water surface. E.g – Nelumbo, Nymphaea, Trapa invade the area. Some free floating species e.g.,Azolla, Pistia, Lemna, Salvinia also become associated with rooted plants.
Water level decreases making pond more shallower. The decomposing organic matter formed due to death and decay of these plants brings about further buildup of the substratum.
Reed Swamp Stage
This stage is also known as amphibious stage as the plants are rooted but their shoot are exposed to air. These are species of Typha, Sagittaria, Phragmities. They have well-developed rhizomes and form a very dense vegetation.
Sedge Meadow Stage
Decreasing water level and increasing amount of light become unsuitable for the reed swamp communities. Further changes in the substratum becomes suitable for Cyperaceae and Gramineae species such as Carex, Cyperus, Juncus to colonise the area
The form mat like vegetative towards the centre of the pond with the help of their branched rhizomatous system. As a result of hight rate of transpiration there is a rapid loss of water due to which mud is exposed to air. Thus mesic conditions approach and marshy vegetation disappears gradually.
Woodland Stage
Due to disappearance of the marshy vegetation, oil becomes drier for most time of the year. This area is now invaded by terrestrial species, which are shrubs (Salix, Cornus) tress (Almus, Populus).
Due to accumulation of humus with rich flora of microorganisms, it becomes suitable for arrival of the tree species.
Forest Stage
This is the climax community. The area is now invaded by several trees. In tropical climate with heavy rainfall there develops tropical rain forests, whereas temperate climate there develops mixed forest of Almus, Acer Quercus. In regions with moderate rainfall, there develop tropical deciduous or monsoon forests.
Xerosere
Succession initiated in the substratum which is deficient in water and lacks any organic matter is called as xerosere.The poineers to colonise this primitive substratum are crutose lichens, and through a series of successive seral stages the succession terminates into a forest which constitutes the climax community.
The various stages in the xerosere are:-
Crustose Lichen Stage
Substratum is poor in moisture and organic matter with extreme temperature it is unsuitable for many plants. Crutose lichens alone are the pioneers which are able to grow. Species of Rhizocarpon, Rinodina, Lecanora estasblish in this substratum as these lichens are slow growers and can withstand extreme desiccation.
When there is rain they absorb water like sponge and flourish rapidly. They produce some acids which bring about weathering of rocks. Dead organic matter of lichens becomes mixed with the rock particles that make conditions for growth of other vegetation
Foliose Lichen Stage
As soon as little soil is accumulated, some foliose lichen like species of Dermatocarpon, Parmelia, Umbilicaria appear. They have large leaf-like thalli that can absorb and retain more water. Humus and dust particles are accumulated which help in the further build up of the substratum. Weathering of rocks and mixing of humus result into the development of fine thin soil layer.
Moss Stage
Development of thin soil layer on rock surface favours development of Xerophytic mass – Polytrichum, Tortula, Grimmia.
Dead and decay produce mat on the rock surfaces. This mat like surface has more water holding capacity. Due to their death and decay there is further addition of organic matter and this leads to increase in the thickness of the soil layer.
Herbs Stage
Due to extensive growth of mosses there is more accumulation of soil. This changed habitat favours the growth of herbaceous plant – first annuals and later binennials and perennials make there appearance. Eg.,Aristida, Festuca, Poa, Solidago.
The process of rock disintegration and accumulation of humus is accelerated as the tangled network of root increases. Evaporation and temperature decreases.
Shrub Stage
Due to much accumulation of soil, the habitat becomes suitable for shrubs. Species of Rhus, Phytocarpus with dense shrubby growth makes soil shaded and evaporation is reduced. With the enrichment of soil and greater water holding capacity favours the growth of trees.
Forest Stage
Some xerophytic tree species invade the area. Further weathering of rocks and increase humus which favours arrival of trees and vegetation finally becomes mesophytic. Finally a forest community develops
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