Ecological Succession means when one plant community is replaced by the other and the process is continued till a final stage is replaced or as vegetation develops, the same area becomes successively occupied by different plant communities. This process of development of vegetation (climax) is known as Ecological succession.
A specific sequence of development of a community is also related to particular set of physical and chemical conditions. This is known as a Sere and is composed of a number of biotic successions replacing each other in the course of time.
Characteristics of Successions
Odum (1969) has given the following three main characteristics of succession.
- Succession is an orderly process of community development that is reasonable directional and therefore predictable.
- Succession is community induced process and organisms bring for modification in the physical environment determine the pattern and rate of succession with in habit.
- Succession leads to the formation of a stable community with maximum biomass. Species diversity and of all possible niches by the organisms.
Causes of Succession
Succession is not caused by any single factor, it is the product of interactions of several abiotic and biotic factors causing changes in community structure during interaction with environment. Generally these factors are grouped into three types, which are as follows:
- Initiation factors - These factors are responsible for making an area barren to initiate succession. Climatic and biotic, both factors are responsible for this succession. Erosion, volcanic eruption, frequent flood or inundation, alluvial deposition, forest fire, indiscriminate deforestation, etc, are responsible for making an area barren to initiate succession (subsere). Succession begins soon after the action of initiating factors is over.
- Continuing factors - Processes like migration, invasion, ecesis (establishment), aggregation, competition, etc., are responsible for successive waves of communities in a succession until the climax is reached.
- Stabilizing factors - They are responsible for establishment of climax when a community gets stabilized. Neither the community modifies environment nor the environment forces the community to leave the place.
General Process of Succession
The whole process of a primary autotrophic succession is actually completed through a number of sequential steps, which follow one another. These steps in sequence are as follows:
1. Nudation - This is the development of a bare are without any form of life. The area many develop due to several causes such as landslide, erosion, disposition, or other catastrophic agency. The cause of nudation may be any of the following:
- Topographic - Due to soil erosion by gravity, water or wind, the existing community may disappear. Other causes may be deposition of sands etc., landslide, volcanic activity and other factors.
- Climatic - Glaciers, dry period, hails and storm, frost, fire etc. may also destroy the community.
- Biotic - Man is most important responsible for destruction of forests, grasslands for industry, agriculture, housing etc. Other factors are disease epidemics due to fungi, viruses etc. which destroy the whole population.
2. Invasion - This is the successful establishment of a species in a bare area. The species actually reaches this new site from any other area. This whole process is completed in the following three successive stages:
Migration (Dispersal) - The seeds, spores, or other propagules of the species reach the bare area. This process, known as migration, is generally brought about by air, water etc.
Ecesis (Establishment) - After reaching to new area, the process of successful establishment of the species, as a result of adjustment with the conditions prevailing there, is known as ecesis. In plants, after migration, seeds or propagules germinate, seedlings grow, and adults start to reproduce. Only a few of them are capable of doing this under primitive harsh conditions, and thus most of them disappear. Thus as a result of ecesis, the individuals of species become established in the area.
Aggregation - After ecesis, as a result of reproduction, the individuals of the species increase in number, and they come close to each other. This process is known as aggregation.
3. Competition - It occurs when the availability of a space and food becomes inadequate to meet the optimum requirement of all the individuals growing in that area. Competition may be for space, light, water and mineral requirement among the different individuals. It may be intraspecific (among individuals of the same species) or interspecific (among individuals of different species).
4. Reaction - This is the most important in succession. The mechanism of the modification of the environment through the influence of living organisms on it, is known as reaction. As a result of reactions, changes take place in soil water, light conditions, 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 (serai community). The whole sequence of communities that replaces one another in the given area is called a sere, and various communities constituting the sere, as seral communities, seral stages or developmental stages. The pioneers are likely to have low nutrient requirements, more dynamic and able to take minerals in comparatively more complex forms. They are small-sized and make less demand from environment.
5. Stabilization - It is a stage when the final community becomes more or less stabilized for a longer period of time and it can maintain itself in equilibrium or steady state with the climate of that area. The final community is not replaced and is known as climax community and the stage is known as climax stage.
Types of Succession
The development stages from pioneers to the climax communities is called sere and each developmental stage of succession is called seral stage. A sere can be prisere or primary succession, and subsere or secondary succession.
- Primary succession - The succession which occurs in originally barren site or area such as a pond or rocks or on sand dunes, etc. The sere in primary succession is referred to as presere.
- Secondary succession - Another general types of succession is secondary succession which starts from previously built up substrata with already existing living matter. The action of any external force, as sudden change in climatic factors, biotic intervention, fire etc., causes the existing community to disappear. Thus, area becomes devoid of living matter but its substratum, instead of primitive, is built up, Such successions are comparatively more rapid.
- Autogenic succession - After the succession has begun, in most of the cases, it is the community itself which, as 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, however, the replacement of the existing community is caused largely by any other external condition and not by the existing organisms, Such a course is referred to as allogenic succession.
- Autotrophic succession - It is characterized by early and continued dominance of autotrophic organisms like green plants. It begins in a predominantly inorganic environment 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 heterotrophs, such as bacteria, actinomyceters, fungi and animals. It begins in a predominantly organic environment and progressive decline in the energy content.
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