Tuesday, May 27, 2008

Ecosystem -Ecological Succession

Ecological Succession
The successive replacement of communities in an area over a period of time is known as ecological succession.
Types of succession
Primary succession: succession occurring on previously unoccupied sites, such as a bare rock, newly cooled lava, newly created pond or reservoir.
Secondary succession: It occurs in an area where the natural vegetation has been destroyed or removed e.g. the forests destroyed by fire. The reappearance and establishment of communities in such areas is called secondary succession.

Pioneer community- The first biotic community which develops in a bare area is called pioneer community. This stage takes the longest time to change the environment for invasion of the next community.
Pioneer species-The species that invade the bare land initially are called pioneer species.
Seral communities- The pioneer community is replaced by another community. This second community is replaced by a third community, and so on. The different communities or stage like mosses, herbs, shrubs, and trees replacing one another during succession are called seral stages or seral communities.
Climax community- is the stable, self perpetuating and final biotic community that develops at the end of biotic succession and is in perfect harmony with the physical environment. It is termed as climatic climax community. The climax community is stable and does not show changes in species composition, as long as the environmental conditions remain the same.
Sere- The entire sequence of development stages of biotic succession from pioneer to a climax community is known as sere.
Biotic Succession on Bare Rock (Xerarch)
v Lichen and Moss stage
Lichens are the first species to invade the bare rock. They secrete acids to dissolve rock. Helping in weathering and soil formation.
Lichens are normally followed by small plants like mosses (Bryophytes) which can thrive in the small amount of soil. Lichens and mosses 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 are the pioneer species forming he pioneer community
Annual Herb Stage
The mat formed by mosses on the partially fragmented rock provides suitable substratum for the germination of seeds of annual herbaceous plants. These plants have more sand binding properties. Their death and decomposition accumulate more soil so the annual grasses are replaced by perennial grasses.
Shrub Stage
Due to further weathering of rocks and death of the herbs, more soil is accumulated. So the habitat becomes suitable for the growth of shrubs. The shrubs are large in size and their roots penetrate more deeply in the rocky substratum causing more weathering and soil formation. This favours the invasion of the area by next seral stage.
Forest Stage
It is the climax community. Further weathering of rocks and increasing humus content of the soil favors the growth of more trees. Type of climax community depends upon the climate e.g. a rain forest in a moist tropical area; a coniferous forest or deciduous forest in temperate area; grassland in area with less rainfall.
Xerophytic habitat gets converted into a mesophytic one.

Biotic Succession on Water Body (Hydrarch)
In water bodies silting is done as a result of soil erosion from surrounding areas.
In a pond the phytoplankton and zooplankton makes the pioneer community.
Submerged aquatic plants, with their roots are present in the mud.
The dead remains of these organisms settle at the bottom .Floating plant species invade the pond.
Stages of Hydrarch
Phytoplankton and zooplankton stage
Submerged plant stage
Submerged free floating plant stage
Reed swamp stage
Marsh- meadow stage
Shrub stage
Forest with trees
With the continued situation, 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 live in the pond.
This is followed by the invasion of Dragonflies, crustaceans and more rooted species of plants.
With increasing settling of silt and decomposition of dead organic mater the pond becomes shallower until it gets transformed into a terrestrial habitat. Finally terrestrial species like grasses ,bushes and trees colonies the
pond area and a climax community is established.

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