Ecological Succession

Ecological succession involves an orderly process of community changes, which are directional and hence predictable. It involves the modification of the physical environment, culminating in the establishment of a stable community.

A sequence of temporary communities replace one another in a given site, thus bringing about changes in the physical environment, which in its turn determines the pattern of succession. The transitional series of communities which develop in a given area are termed as sere or seral stages, while the final stable and mature community is termed as the climax.

Ecological succession is therefore community controlled although the environment determines the successional pattern.

Characteristics of succession

Following are the characteristic features of ecological succession features:

The interacting organisms change the physical environment. Besides this, the interaction between biotic and physical factors also changes the environment under which a new set of organisms grow and develop.
Changes in this local environment are more suitable for the survival of other organisms.
The replacement of one community is followed by an orderly series of communities gradually one after the other. This community change is related to set of conditions and is termed sere. It is made of number of biotic successions (communities). The last succession or a community in a sere is termed as climax or climatic climax.

Causes of succession

Following are the three basic causes of succession:

Initial or initiating causes: Climatic as well as biotic factors are responsible for starting a succession. Climatic causes include soil erosion, soil deposits, wind, fire, etc., while the biotic causes are interaction amongst organisms. These factors either produce bare areas or eliminate the existing vegetation.
Ecesis or continuing causes: The processes like migration, aggregation, competition, reaction,etc., allow the effect of changes to continue.
Stabilizing causes: Climate of an area is the chief agent that permits the new community to settle down.

Types of succession

Following are two basic types of succession; primary and secondary succession.

Primary succession:

This is a type of succession that occurs on a previously sterile area such as bare rocks, sand dunes etc. Hardy species inhabit these areas. Such first group of organisms to establish in these areas where no living matter was present is termed pioneer, primary community or primary colonizers. Lichens generally form pioneer community in barren rocky areas.

Water seeps in cracks in the rocks; it may freeze expand and break the rock into small pieces. The surface of rock weathers by the beating of wind and rain. Carbonic acid forms when atmospheric carbon dioxide dissolves in water, helps to dissolve minerals in the rocks providing nutrients. Dead lichens also contribute organic matter to the forming soil. This community is followed by mosses which may gain a hold even in very thin layer of soil. Mosses are followed by annual grasses, perennial herbs, shrubs and finally the trees. The forest and grasslands generally represent climax communities.

Fig: Ecological Successsion (Primary Succession)

Secondary succession:

This type of succession occurs in an area where a community was existing earlier but has been destroyed mostly due to forest fire, floods, land, etc. Only a few organisms and a considerable amount of organic matter remain in the area. The remaining species and those able to grow on organic matter form new pioneer community.

A primary succession may take anywhere up to 2000 years to reach the climax community. Secondary succession, however, takes much shorter time. For example, a destroyed grassland may take anywhere between 50 to 100 years, while destroyed forest requires more than 200 years to develop into a climax community.

Fig: Ecological Succession (Secondary Succession)

General Process of succession:

Every primary succession, irrespective of the bare area from which it starts, shows following five stages which follow one another:

Nudation: This is the development of bare areas which may be due to erosion, deposition,etc.
Invasion: This is the successful establishment of a species in a bare area. The species reaches this area from some other region.
Competition and co-action: The species having occupied new area develops competition (both intra and inter specific) for food and space. The competition between already existing species and new species, results in elimination of those of them which are unsuitable for the habitat.
Reaction: The community or the species that establishesd itself in a new area, affects the environment by modifying light, water, soil etc. This results in elimination of a community which now makes way for another community for which the modified environment is most suitable. This sequence of communities is often termed as seral communities, seral stages or developmental stages.
Stabilization: This is the final stage when a community reaches an equilibrium with the climate of an area and becomes comparatively stable.

General trends in succession

Certain trends often can be made out during the course of succession. In the first place, community productivity goes up. Secondly, biomass tends to increase because with succession the number of individuals in a community increases. Finally, species diversity of all kinds usually increases for most of the course of the succession. However, towards the end of the succession, diversity may decrease due to competitive dominance of a few or even one species.

Species composition: A change in Species composition occurs. The change occurs fast in the beginning and then more gradually.
Species diversity: Some plant species which were present in the initial stages may not be found in an advanced stage of succession. However, in the climax stage there may be more kinds of autotrophs and heterotrophs than in the earlier seral stages.
Density and biomass of organism: There is usually an increase in the number (i.e., density) in older communities. The biomass structure in climax remains very high. The total biomass gradually increases and reaches a maximum in climax stage. In very older ecosystem density may decline.
Heterotrophic population: The number of species usually goes on increasing as the food chain relationships become more complex in the climax stage.
Chlorophyll: Green pigments go on increasing during the early phase of primary succession. The ratio of yellow / green pigments remain around 2 in the early stages and increases to 3-5 in the climax stage. Pigment diversity also increases from seral to climax stage.

Functional changes in succession

There is a progressive increase in living biomass and dead organism. There is an increase in gross primary production and net primary production in initial and seral stages.
The community respiration increases but the P/R ratio remains more than 1 in the seral stages. The huge living biomass respires a lot in the climax stage and P/R ratio equals one. Thus in the early stages P is greater than R and in the climate stages P=R.
The food chain relationship becomes more complex as succession proceeds due to high species diversity large accumulation of living biomass and complex food chain relationship. The complexity of climax community increases the number of ecological niche and routes of energy flow system. All the attributes makes the climax community more stable.