Succession is the process by which new community gets established at an area which had been uninhabited. The community which gets established at the site is called climax community. It is the aggregation of changes of species structure in an ecological community over time.
Even though ecologists have tried to define the processes and events of succession, they are more or less a continuum of activities which are overlapped in the ecological timescale.
Stages of succession
The process begins with establishment of a few pioneer species which are replaced or reduced in abundance by species of increasing complexity. The diversity increases until the ecosystem as a whole gets stabilized with the establishment of climax community. The establishment of pioneer species at a bare site causes changes in soil structure and nutrient content. Changes in physical environment soon follow the first step.
New species of plants replace the existing plants due to the change of physical factors and soil structure. These in turn alter the existing conditions paving way for newer species to get established. These changes are often accompanied by the introduction of animal species into the area.
The cyclic process ends after reaching a stabilized community called as climax community. The ecosystem is fully balanced at this stage until they get disturbed by any other external factors.
Disturbances eventually destroy the existing climax community and the process of succession starts anew.
The processes of ecological succession are
1. Nudation. It is the development of a bare site uninhabited by any organisms. The process is usually caused by disturbances. These factors can be either topographic (soil erosion, wind action etc); climatic (hails, storm, glaciations, fire etc.); or biotic (human activities). The area thus formed can sustain only autotrophic organisms which can utilize inorganic substrates. The environmental conditions are set up for the inhabitation of new species. For example, secretion of acidic substances by lichen species helps in break down of rocks into soil.
2. Invasion or migration. The process of invasion or migration helps the arrival of seeds, spores or other reproductive propagules for establishment of species. Invasive species are non native organisms which can spread widely in a community. These are usually threatening the normal ecosystem and causative agents for community disturbance. However, in succession process, they help to alter the soil texture and function. R-selected species are often the first colonizers due to their high reproductive rates and better dispersal mechanisms.
3. Ecesis. This is the initial establishment of plant community. This is dependent on the soil structure. The stage is also called as 'colonization'. In this stage, the early colonizing species proliferate abundantly through germination, growth, and reproduction. Ecesis is due to allogenic mechanisms alone. This is the stage at which the pioneer species survive the dispersal mechanisms. The different pioneer species can have different maturation rates which allow this process to be longer and gradually allowing replacement of some species by others. The process also makes the soil structure suitable for those species whose seeds were present in the existing ecosystem but were unable to germinate for lack of suitable community support.
4. Aggregation is the increase in population of the species which has become established in the area. The shrubs replace the small herbs in most successions. This also proves as a source of food for future inhabitants.
5. Competition. Once the few initial species have become established the intra as well as interspecific competition among the species starts. This stage is called competition. The competition is usually for resources such as food, water etc. Competition is found in both plants as well as animal species. The process leads to sharing of resources (resource partitioning) or competitive exclusion.
6. Reaction. The environmental conditions get modified by the action of species occupying the habitat. These changes subsequently trigger the displacement and replacement of one species by another. The existing community will be unable to support itself due to the harsh conditions. The major underlying mechanism is autogenic succession in which the plants themselves alter the environmental conditions. Early colonizers usually facilitate the way for establishment of mature species.
7. Stabilization is the process by which the climax community gets established. A climax community is mature, self sustaining, stable and is the final stage of succession. The physical and chemical conditions are altered and stabilized to such levels that it supports the entire community. The climax communities are best adapted to the regions of succession and the community structure is likely to continue until another disturbance steps in. This represents a steady state of ecological equilibrium with specific composition, structure and energy flow.
The process of secondary succession is much faster due to the availability of enriched soil and other factors. The only limiting factor in such succession is the disturbance. The process is essentially the same as primary except for the absence of some of the typical stages. Eventually both will result in the establishment of stable communities which are made up mostly of K-selected species.
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