Ecological Succession

• An important characteristic of all communities is that their composition and structure constantly change in response to the changing environmental conditions. This change is orderly and sequential, parallel with the changes in the physical environment. These changes lead finally to a community that is in near equilibrium with the environment and that is called a climax community. The gradual and fairly predictable change in the species composition of a given area is called ecological succession. During succession some species colonise an area and their population become more numerous whereas populations of other species decline and even disappear.
• The entire sequence of communities that successively change in a given area are called sere(s). The individual transitional communities are termed seral stages or seral communities. In the successive seral stages there is a change in the diversity of species of organisms, increase in the number of species and organisms as well as an increase in the total biomass. The present day communities in the world have come to be because of succession that has occurred over millions of years since life started on earth.
• Actually succession and evolution would have been parallel processes at that time. Succession is hence a process that starts in an area where no living organisms are there – these could be areas where no living organisms ever existed, say bare rock; or in areas that somehow, lost all the living organisms that existed there. The former is called primary succession, while the latter is termed secondary succession.
• Examples of areas where primary succession occurs are newly cooled lava, bare rock, newly created pond or reservoir. The establishment of a new biotic community is generally slow. Before a biotic community of diverse organisms can become established, there must be soil. Depending mostly on the climate, it takes natural processes several hundred to several thousand years to produce fertile soil on bare rock.
• Secondary succession begins in areas where natural biotic communities have been destroyed such as in abandoned farm lands, burned or cut forests, lands that have been flooded. Since some soil or sediment is present, succession is faster than primary succession.
• Description of ecological succession usually focuses on changes in vegetation. However, these vegetational changes in turn affect food and shelter for various types of animals. Thus, as succession proceeds, the numbers and types of animals and decomposers also change. At any time during primary or secondary succession, natural or human induced disturbances (fire, deforestation, etc.), can convert a particular seral stage of succession to an earlier stage. Also such disturbances create new conditions that encourage some species and discourage or eliminate other species.

Causes of Succession

• 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.
• 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.
• 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 Succession

1. 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. The series of development stages are called prisere. For example, succession on a bare rock (lithosere).
2. Secondary succession: If succession starts in a secondary area previously colonized, but has been cleared off, it is called secondary succession. The series of developmental stages are called subseres. In subseres, the substratum contains already formed soil, humus and may contain seeds. The rate of change is more rapid and the time required for the completion of sere is much shorter than in primary succession. For example, succession in a forest area where vegetation has been devastated by natural catastrophes.
3. Autotrophic succession: Succession characterized by early and continued dominance of autotrophic organisms like green plants. It begins in a predominantly inorganic environment and the energy flow is maintained indefinitely. There is gradual increase in the organic matter content supported by energy flow.
4. 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.
5. 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.
6. Allogenic succession: It is due to major environmental changes beyond the control of the indigenous organisms. Dust bowls, winds, dry periods change the pattern of vegetation. The habitat is changed by the action of outside factors like change in climate, leaching of soil nutrients, increase in salt concentration of the soil and deposition of salt or sand.
7. 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. It paves the way for the growth of another type of plant community.
8. 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.
9. 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.
10. 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

1. Nudation: Development of a bare area without any form of life. The area may develop due to several causes such as landslide, erosion, deposition or other catastrophic agency. The causes of nudation may be:

• Topographic: Soil erosion, landslide, volcanic activity, earthquake, etc.
• Climatic: Glaciers, dry period, hails and storm, frost, fire, etc.
• Biotic: Destruction of forest due to industrialization, agricultural expansion, urbanization, diseases due to fungi, bacteria, etc.

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:

• 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.
• 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.
• 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. The struggle is usually between two individuals of same kind that have similar demands such as space, nutrients, water, light, etc. Competition can either be interspecific (between two different species) or intraspecific (within the individuals of same species). As a result of competition, the weak individuals are eliminated and the stronger ones are retained. The dead bodies of the eliminated plants and animals’ decay and form humus enriching 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.