· Ecological Succession
Ø The gradual change in species composition and processes of communities over time is known as ecological succession or community development.
Ø From the stand point of time, ecosystem and community changes can be divided into two types:
i. Changes occurring over geological time scale (million years), and
ii. Those occurring over medium time scale, say in 1-1000 years.
Ø The causes of succession are as follows:
i. Initial/Initiating causes: These are climatic as well as biotic. The factors
include erosion and deposition, wind, fire, activities of organisms, etc. These
causes produce the bare areas or destroy the existing population in the area.
ii. Ecesis/Continuing causes: These are the processes such as migration, ecesis,
aggregation, competition, reaction, etc., which cause successive waves of
populations as a result of changes, chiefly in the edaphic features of the
area.
iii. Stabilizing causes: These cause the stabilization of the community.
According to Clements, climate of the area is the chief cause of stabilization,
other factors are of secondary value.
DIFFERENT KINDS OF SUCCESSIONS
Ø Primary succession: If succession proceeds from a primary bare area or primitive substratum which has not been changed physically by organisms, it is called primary succession. The first group of plants establishing there are known as pioneers.
Ø Secondary succession: If succession starts in a secondary area previously colonized, but has been cleared off, it is called secondary succession. Thes series of developmental stages are called subseres. In subseres, the substratum contains already formed soil, humus and may contain seeds.
Ø Autotrophic succession: Succession characterized by early and continued dominance of autotrophic organisms like green plants.
Ø Heterotrophic succession: Succession characterized by an early dominance of heterotrophs, i.e., fungi, bacteria and animals. It begins in a predominantly organic environment, and there is a progressive decline in the energy content.
Ø Induced succession: The climax community has low productivity as compared to initial communities. In a climax community respiration almost balances the production of organic matter. Therefore, very little is left for man to harvest.
Ø Allogenic succession: Allogenic succession is due to major environmental changes beyond the control of the indigenous organisms. Dust bowls, winds, dry periods change the pattern of vegetation.
Ø Autogenic succession: Succession resulting from the resident population altering its own environment. For example, plants of a developmental state produce changes in the habitat initially to favor their growth but the changes go on beyond the optimum so that the habitat becomes unsuitable for them.
Ø Retrogressive succession: At times a climax vegetation may deteriorate and get replaced by a community of an earlier stage of succession due to destructive effects of organisms. Sometimes the development of the disturbed communities does not occur and the process of successive instead of progressive becomes retrogressive, e.g., forest may change to shrubby or grassland.
Ø Deflected succession: A succession in which the vegetation does not pass through the normal stages of development but either adds or replaces a successional type.
Ø Serule (Microsere): It refers to the miniature succession of microorganisms like fungi, bacteria, actinomycetes, etc., that occurs within a microhabitat like fallen logs of decaying wood, tree, bark, etc. Serule is heterotrophic in nature and begins on substratum rich in organic matter.
GENERAL PROCESS OF SUCCESSION
2) Invasion: Successful establishment of a species in a bare area. The species actually reaches this new site from any other area. It involves following successive stages
(i) Migration (dispersal): The seeds, spores, or other propagules of the species reach the bare area. The agencies which bring about dispersal are wind, water, animal including man.
(ii) Ecesis (establishment): After reaching the area, the process of successful establishment of the species, as a result of adjustment with the conditions prevailing there, is known as ecesis. This process is dependent upon the climatic, edaphic and biotic factors. The success of plant depends upon the climatic, edaphic and biotic factors. 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. As a result of ecesis, the individuals of species become established in the area.
(iii) Aggregation: As a result of reproduction, the individuals of the species increase in number, and they come close to each other.
3) Competition and Coaction: This phenomenon involve struggle for existence between two or more individuals growing in an area, that makes successive demands, that are similar in nature, on the soil.
4) Reaction: This is the most important stage 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. For example, plants change the structure and texture of soil in course of time by addition of humus into the soil. Due to all these, environment is modified, becoming unsuitable for the existing community which sooner or later is replaced by another community.
5) Climax community: The reactions of individuals collectively have a wider effect on the environment. The local climate is actually changed if the climax vegetation is forest. The reactions keep the vegetation in active state till the climax community is formed. Although strictly speaking vegetation can never be called as stable. Yet at maturity the community being mesic, further mesic changes being less possible the climax community can be called as relatively stable. Thus, we find the development of community living in harmony within the environment which has been the result of its reactions.
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