Ecological succession

 Ecological succession is the gradual and orderly process of ecosystem development brought about by changes in community composition and the production of a climax characteristic of a particular geographic region. Succession is a community-controlled phenomenon, which results due to the action and co-action on living organisms. 

Physical environment often determines the nature, direction, rate and optimal limit of change. When succession begins in a sterile area such as a bare rock or in an area not previously occupied by a similar community or when a lake community is eventually replaced by a forest community, it is called primary succession. Secondary succession results when there are severe changes in climate or other factors such as fire, cultivation and grazing which cause the ecosystem to revert to an earlier stage.

 Farm areas, which have been cleared and then abandoned, are examples of secondary succession. Secondary succession progresses more rapidly than primary succession because soils and physical conditions have been altered to a certain extent by previous communities, which have not been completely eradicated. The plants that invade the bare land firstly, are called pioneer species. 

The assemblage of pioneer species forms the pioneer community. Generally, the pioneer species show high rate of growth but short life span. Once established, a community brings about changes in the environment such as addition of humus to soil, changes in pH and increased water retention of the soil. Eventually, the total environment is altered to the point that another community can replace the pioneer community from the area. Community after community establishes itself and in turn is replaced until a climax community is established. The different communities or stages represented by combinations of mosses, herbs, shrubs and trees replacing one another during succession are referred to as seral stages or seral communities. The plant species, which get established later, during the course of succession, are known as late successional species. These species are slow growing and long lived. The terminal stage of succession is represented by the climax community. The climax community is a community which will not be replaced by another, unless there is a basic change in climate or landform. The sequence of communities succeeding each other during the course of succession represents the sere. 

Succession on a Bare Rock (Xerarch) Xerosere is the characteristic sequence of communities reflecting the developmental stages of a plant succession that begins in terrestrial areas with low moisture (for example, rock, sand). The process of succession proceeds on a bare rock in the following steps. The first colonisers are lichens and certain mosses. Acids secreted by the lichens attack the rock and provide bits of soil. Additional soil particles may be formed by weathering or be blown in from elsewhere. Damage and decay of the lichens supplies some humus. 

Lichens are normally followed by mosses, which speed up the process of soil accumulation by trapping wind-blown particles. Mosses grow in bunch and together with lichens, make a mat over the substratum. Lichens and mosses, which get established on barren rock, are the pioneer species forming the pioneer community. The accumulation of soil particles in the lichenmoss carpet provides suitable substratum for the germination of seeds of herbaceous plants that are dispersed in it. Now the seeds of higher plants germinate and grow successfully in pockets of newly formed soil on the rock. Their roots penetrate deeper, causing more weathering of rocks. Progressively, more soil is accumulated and herbaceous species make way for the invasion of shrubs followed by trees. Their dead, decaying leaves add organic matter that makes soil more fertile and moist. Passing through the seral stages in course of time, climax community gets established. The climax community is determined by the climate and amount of soil formation. 

Trees normally dominate the climax community. The changes in biotic community from the pioneer to the climax stage may take hundreds of years.

 Succession in Aquatic Environment (Hydrarch) Hydrosere is a sequence of communities that reflects the developmental stages in a plant succession, which commences on a soil, submerged by fresh water. The process of succession proceeds in aquatic environment in the following steps: Water bodies are prone to silting as a result of soil erosion from surrounding areas. In a pond, the phytoplankton and zooplankton comprise the pioneer community. Dead plankton mix with the bottom mud that becomes soft and fertile and consequently suitable for the growth of the next serial stages. Submerged aquatic plants, with their roots attached in the mud, are next to colonise the pond. Silt and decayed organic matter goes on gathering under these plants, raising the bottom and also increasing its fertility. Besides, floating plant species invade the pond. 

With the continued siltation, the pond bottom is gradually raised and water layer becomes shallow and rich in nutrients. As a result, rooted, emergent plants with aerial leaves, such as reeds, are able to colonise the pond. The invasion of dragonflies, crustaceans and more rooted species of plants accompany this. Consequently, the species composition of the pond keeps changing with time. 

With increased settling of silt and deposition of dead organic matter derived from floating and rooted species, the pond becomes shallower until it gets transformed into a terrestrial habitat. Finally, terrestrial species, like grasses, bushes and trees, colonise the pond area and a climax community is established. 

The colonisation by land plants generally progresses from margins toward the centre of the pond area. In a similar example involving hydrarch succession, an oligotrophic lake may gradually, by the accumulation of organic matter, become eutrophic. Communities at early successional stages have a lower total biomass, higher net productivity, fewer species, many fewer heterotrophic species and less capacity to regulate the cycling of nutrients than do communities at later successional stages.