Introduction:

Ecological succession is the process by which the mix of species and habitat in an area changes over time. Gradually, these communities replace one another until a “climax community”—like a mature forest—is reached, or until a disturbance, like a fire, occurs.

Ecological succession is a fundamental concept in ecology.
The study of succession was pioneered at the University of Chicago by Henry Chandler Cowles, who was also one of the founders of ecology as a discipline.

Types of Ecological Succession:

Type of Succession	Definition	Starting Conditions	Stages of Development	Example
Primary Succession	The process that occurs when a new patch of land is created or exposed for the first time, devoid of any initial vegetation or soil.	Begins on bare surfaces with no existing soil, such as newly formed volcanic rocks, lava flows, or areas left by retreating glaciers.	1. Pioneer Species: Lichens and small plants that can survive without soil begin to colonize the area. 2. Soil Formation: Decomposition of pioneer species contributes to soil formation. 3. Successive Communities: Gradual colonization by larger plants. 4. Climax Community: A stable ecosystem, like a forest, is established.	The formation of a new ecosystem on bare rock after a volcanic eruption or in areas exposed by glacial retreat.
Secondary Succession	The process that occurs when an existing ecosystem is disturbed, leading to the reestablishment of the climax community, but not starting from scratch.	Occurs in areas where the previous community was disturbed but where soil and nutrients remain intact, such as after a forest fire.	1. Initial Growth: Grasses and small plants grow first. 2. Shrub and Tree Colonization: Shrubs and a variety of tree species start to grow. 3. Climax Community: The ecosystem gradually returns to its previous state, potentially with minor changes.	The regrowth of a forest after a fire or logging, where the soil remains, and vegetation gradually re-establishes the original community.

Autogenic succession.

After the succession has begun, in most of the cases, it is the community itself which, as a result of its reactions with the environment, modifies its own environment and, thus, causing its own replacement by new communities.
This course of succession is known as autogenic succession.

Allogenic succession:

In some cases replacement of one community by another is largely due to forces other than the effects of communities on the environment.
This is called allogenic succession and it may occur in a highly disturbed or eroded area or in ponds where nutrients and pollutants enter from outside and modify the environment and in turn the communities.

Autotrophic succession:

It is characterized by early and continued dominance of autotrophic organisms such as green plants.
It begins in a predominantly inorganic environments and the energy flow is maintained indefinitely.
There is gradual increase in the organic matter content supported by energy flow.

Heterotrophic succession.

It is characterized by early dominance of heterotrophic organisms such as bacteria, actinomycetes, fungi and animals.
It begins in a medium which is rich in organic matter such as small areas of rivers, streams; these are polluted heavily with sewage or in small pools receiving leaf litter in large quantities.

Induced succession

Activities such as overgrazing, frequent scraping, shifting cultivation or industrial pollution may cause deterioration of an ecosystem.
Agricultural practices are retrogression of a stable state to a young state by man’s deliberate action.

Retrogressive succession.

It means a return to simpler and less dense or even impoverished form of community from an advanced or climax community.
In most cases, the causes are allogenic, i.e., forces from outside the ecosystem become severe and demanding.
For example, most of our natural forest stands are degrading into shrubs, savanna or impoverished desert-like stands by the severity of grazing animals brought from surrounding villages.
Excessive removal of wood, leaf and twig litter also leads to retrogressive succession.

Cyclic succession.

It is of local occurrence within a large community.
Here cyclic refers to repeated occurrence of certain stages of succession whenever there is an open condition created within a large community

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SUCCESSION AND ITS TYPES