F.E. Clement postulated the concept of Vegetation/Ecological succession which is the process of change in the species structure of an ecological community over time. The time scale can be decades (after a wildfire), or even millions of years after a mass extinction (Permian, Triassic, cretaceous). The development of vegetation community in any ecosystem or habitat is affected and controlled by:
- Edaphic factors
- Biotic factors
- Physiographic factors
- Fire factors
Characteristics of Ecological Succession
1. It results from the modification of the physical environment of the community. Ex. Development of pioneer species like algae and lichens on the bare surfaces like Krakatoa Island, etc.
2. It is an orderly process of community development.
3. The time required for the development of climax vegetation in secondary succession is much less than primary succession.
4. Nutrient variation determines the settlement of the new community.
5. Deflected climax due to arresting factors like jhuming cultivation results in plagioclimax.
6. Ecological succession is disrupted in communities like Savanna, Chaparral, etc. by fire.
7. At climax vegetation:
- An ecosystem is stable and self-perpetuating
- Biomass increases to maximum
- Net community production decreases
- The food chain becomes highly complex changing to the food web.
Types of Ecological Succession:
Clements has divided succession into two types:
- Primary Succession: Primary succession refers to the developmental sequence of vegetation in those bare areas where there were no vegetation and animals earlier. Ex. Newly emerged seafloor, island of Krakatoa, etc.
- Secondary Succession: Secondary succession refers to the developmental sequences of vegetation in those areas which had vegetation earlier but now have been rendered nude due to destruction by:
- Natural processes: Lava flow, forest fires, catastrophic floods, etc.
- Anthropogenic processes: Jhuming cultivation, overgrazing, etc.
Phases of biotic succession:
- Phase of Nudation: creation of bare area devoid of vegetation. Ex. Newly emerged volcanic island.
- Phase of Migration: Arrival of seeds in to the new bare area
- Phase of Ecesis: Seeds are germinated
- Phase of Reaction: Competition between plants and physical environment.
- Phase of Stabilization: Equilibrium condition of populations of plant species
- Climax Vegetation:
The vegetation community developed at the last stage of biotic succession is called climax vegetation which is indicative of a mature ecosystem wherein the dominant vegetation is in equilibrium with the environment.
- Ecosystem is stable and self-perpetuating.
- Biomass increases to maximum.
- Net community production decreases.
- Food chain becomes highly complex changing to food web.
- Plagioclimax Vegetation:
A Plagioclimax community is an area or habitat in which the influences of humans have prevented the ecosystem from developing further.
- The ecosystem may have been stopped from reaching its full climatic climax or deflected towards a different climax by activities (arresting factors) such as: jhuming cultivation, large scale afforestation, overgrazing, etc. In each case, human activity has led to a community that is not the climax community expected in such an area.
- It is of two types:
- Arrested succession: The natural succession would continue if the arresting factors are removed. Ex. If cultivation is stopped in Gangetic plains and Great Plains of USA, normal sere of vegetation succession will start.
- Deflected succession: Even if the interferences are removed, succession to the original climax community is no longer possible. Ex. Shola forest (Nilgiris), Sahara Desert, Thar Desert, etc.