Environmental Biology Prof.(Dr.) Punam Jeswal Head M.Sc semester IV Botany Department MECHANISM OF ECOLOGICAL SUCCESSION ECOLOGICAL SUCCESSION Ecological succession is the gradual and continuous change in the species composition and community structure over time in the same area. Environment is always kept on changing over a period of time due to - i) Variations in climatic and physiographic factors and ii) The activities of the species of the communities themselves. These influences bring changes in the existing community which is sooner or later replaced by another community developed one after another. This process continues and successive communities develop one after another over the same area, until the terminal final community again becomes more or less stable for a period of time. The occurrence of definite sequence of communities over a period of time in the same area is known as Ecological Succession. Succession is a unidirectional progressive series of changes which leads to the established of a relatively stable community. Hult (1885), while studying communities of Southern Sweden used for the first time the term succession for the orderly changes in the communities. Clements (1907, 1916) put forth various principles that governed the process of succession, and while studying plant communities, defined succession as ' the natural process by which the same locality becomes successively colonized by different groups or communities of plants. According to E.P. Odum (1971), the ecological succession is an orderly process of community change in a unity area. It is the process of changes in species composition in an ecosystem over time. In simple terms, it is the process of ecosystem development in nature. There are two main types of succession , primary and secondary succession. 1) Primary Succession - Succession occurring in bare area or newly exposed site that was not previously occupied by any sort of living organism. * In primary succession, the unoccupied terrestrial site is first colonised by a few pioneer species which are often microbes, lichens and mosses. 2) Secondary Succession - Secondary succession starts at a site that has already previously built up substrate with already existing living matter. * The action of any external forces, biotic intervention, fire etc., cause the existing community to disappear. Thus the area becomes devoid of living matter but its substratum, instead of primitive is built up. * Secondary succession starts on a well developed soil. Secondary succession faster as compared to primary succession which may often require hundreds of year. CAUSES OF SUCCESSION Since succession is a process, more appropriately a series of complex process, it is natural that there may not be a single cause for this. Generally, there are three types of cause i) INITIAL OR INITIATING CAUSE These are climatic as well as biotic. The former includes factors, such as erosion and deposits, wind, fire etc., caused by lighting or volcanic activity and the latter includes the various activities of organisms. These causes produces the bare areas or destroy the existing populations in an area. ii) ECESIS OR CONTINUING CAUSES These are the process as migration, ecesis, aggregation, competition, reaction etc, which cause successive waves of population as an results of changes, chiefly in the edaphic features of the area. iii) STABLISING CAUSES These cause The stabilisation of the community. According to Clements, climate of the area is the chief of stabilisation, other factors are of secondary values. MECHANISM OF ECOLOGICAL SUCCESSION The whole process of ecological succession is a complex process completed through a number of sequential steps, which fellow one another. Frederic Clements in 1916 for the first time proposed the sequential phase of an ecological succession. The process of succession is completed through a series of sequential steps that are as fellow : - 1) NUDATION - Succession begins with the development of a bare area without any form of life. The area may developed due to several causes such as landslide, erosion, deposition, or other catastrophic agency. The cause of nudation may be :- a) Topographic - Due to soil erosion by gravity, water or wind, the existing community may disappear. Other causes may be deposition of sand etc., landslide, volcanic activity and other factors. b) Climatic - Glaciers, dry period, hails and storm, frost, fire etc., may also destroy the community. c) Biotic - Man is most important agent responsible for destruction of forests, grassland for industry, agriculture, housing etc. Other factors are disease epidemics due to fungi, viruses etc which destroy the whole population. 2) INVASION OR MIGRATION :-This is the successful establishment of a species in a bare area. The species reaches new site from any other area. The seeds, spores, or other propagules of the species reach the bare. This process is knows as migration, is generally brought by air, water and various agents. 3) ECESIS ( ESTABLISHMENT) :- After reaching to new area, the process of successful establishment of the species as a results of adjustment with the condition prevailing there is known as ecesis. In plants, after migration seeds or propagules germinates, seedlings grow and adults start to reproduces. Only a few of them are capable of doing this under primitive harsh condition and thus most of them disappear. * Ecesis is considered to be complete, if the plants is able to sexually reproduce in the given area. As a result of ecesis the individual of the species become established in the area. 4) AGGREGATION :- After successful establishment of a species as a result of reproduction, the individuals of the species increases in number. As compared to earlier stages, there are a larger number of individual of a species that have aggregated in the given area. 5) COMPETITION AND CO-ACTION : - After aggregation of a large number of individuals of the species at the limited place, there develops competition (inter as well as intraspecific). * Competition is mainly for space and nutrition. Individuals of a species affect each other's life in various ways and this is called co-action. Competition and co-action results the survival of fit individual and the elimination of unfit individuals from the ecosystem. * A species with wide reproduction capacity and ecological amplitude only will survive. 6) REACTION :- The mechanism of modification of the environment, through the influence of living organisms present on it is knows as a reaction. As a results of the reaction, changes take place in soil, water, light condition, temperature and many other factors of the environment. * Due to all these the environment is modified, becoming unsuitable for the existing community which sooner or later is replaced by another community (Seral community) 7) STABILISATION (CLIMAX) :- Finally, there occurs a stage in the process, when the final terminal community becomes more or less stabilised for a longer period of time and it can maintain itself in equilibrium with the climate of the area. This final community is not replaced, and is known as climax community and the stage as climax stage. * It was clement who used the term climax to described the idealized endpoint of succession. The climax community is in equilibrium with its environment and represents a steady state of species composition, community structure and energy flow. NUDATION INVASION ECESIS AGGREGATION OR MIGRATION Process of Ecological Succession STABILISATION REACTION COMPETITION (CLIMAX) AND CO-ACTION Major Steps Of Ecological Succession Possible Cyclic Possible Cyclic Succession if Succession if disturbances continue no disturbances Primary Secondary Secondary Succession Succession Succession Bare Pioneer Seral Serial Climax Ground Stage Stages Fig :- Diagrammatic Representation Of The Different Stages In Succession The whole sequence of communities that replaces one another from beginning till the climax stage in the given area is called sere (sera). The succession process involves several stages, each stages having characteristics organisms together with their environment. Each such developmental is called seral stage. A seral community is an intermediate stage ecological succession advancing towards the climax community. In many case more than one seral stage evolves until climax condition are attained. Pioneers - The first species that colonise the bare area in the beginning of succession are called pioneers, primary community or primary colonisers. Pioneers species which are microbes, lichens and mosses. Fig :- Gradual Changes From Pioneer Stages To Climax Community Fig :- Primary Ecological Succession Over Several Hundred Years Of Plant Communities On Bare Rock . .