Ecological Succession

"The occurrence of relatively definite sequence of communities over a long period of time in the same area resulting in establishment of stable or climax community, is known as ecological or biotic succession".

The first community which is inhabiting the area will be referred as 'pioneer community' and the last and stable community formed in the area will be referred as 'climax community'. The intermediate communities are called 'transitional or seral communities'. The whole series of changes in community characteristics from pioneer stage to climax stage constitute a 'sere' and the intermediate stages are the 'seral stages'.

Usually the initial stages of succession are comprised of lower forms.

Causes of Ecological Succession

Causes of ecological succession are classified under two categories.

1) Biotic factors

Interactions among the organisms in a community, as called biotic factors, influence the structure, composition and function of a community. In succession, during period of time a community makes the area less favourable for itself and more favourable for the next serial community.

2) Physiographic factors

Includes physical and chemical factors of the environment such as landslides, erosion, catastrophic factors, etc.

Basic types of succession

1) Primary succession

Also referred as 'perisere' is a type of biotic succession that occurs on a substratum devoid of life earlier like bare rock, sand dunes, new island exposed out of the sea etc. where there was no previously any sort of life.

Primary succession takes a very long time (more than thousands of years in case of climax forest on bare rock).

2) Secondary succession

It is the biotic succession that occurs in an area, which had an existing biotic community and have become bare due to destruction by fire, landslide, earthquake etc.

The sequence of successional stages is called 'subsere', and time required for the completion of sere is much shorter than primary succession.

General process of ecological succession (Mechanism of succession)

Process of primary succession occurs through a number of sequential steps, which follow one another.

a) Nudation

is the development of a bare area without any form of life.

Causes of nudation may be topographic (soil erosion, landslide, earthquake etc.) or climatic (glaciers, hailstorm, fire etc.) or biotic (human activities, epidemics etc.)

b) Invasion

- is the successful establishment of a species in a bare area, which happens in three steps.

i) Migration - reaching of seed or spores in a bare area through various agencies (wind, water etc.)

ii) Ecesis - also called 'establishment' involves the adjustment of the migrated species with the prevailing conditions of the area.

iii) Aggregation - once established, the organisms increase in number through the process of reproduction.

c) Competition and co-action

Involves the development of intraspecific and interspecific competitions among the members (as there is natural resistance like limited food and space).

d) Reaction

Involves the modification of the environment, influenced by living organisms, and existing community are replaced by next seral community and the process is repeated.

e) Stabilization

It is the stage where the final terminal community (climax community) becomes stabilized and can maintain itself in equilibrium with climate of the area.