Category : 12th Class
Community are never stable but keep on changing. This relatively definite sequence of communities over a period of time in the same area is called ecological succession.
Types : Succession is of two types :
(1) Primary succession : It includes changes which occur when living things become established on a previously uninhabited area such as a newly exposed sea floor, lake sediments or sand dunes.
(2) Secondary succession : It occurs where early communities have been damaged, leaving a few organisms and considerable organic matter. These remnant species, along with some new ones, regenerate a new community.
(1) Autogenic succession : When the succession has begun, the vegetation itself is responsible for replacing itself by changing existing environmental conditions.
(2) Allogenic succession : When in succession other conditions (not vegetation itself) are responsible for replacing communities, then it is called allogenic succession.
(1) Hydrosere or Hydrarch : Succession beginning in fresh water, e.g., ponds, lakes, streams, etc.
(2) Xerosere or Xerarch : Succession beginning in dry conditions. It is further classified into three types :
(i) Lithosere : When succession starts on bare rocks.
(ii) Psammosere : When succession begins on sand.
(iii) Halosere : When succession starts in saline conditions.
(3) Mesarch : When succession begins in mesic conditions.
General processes of succession
Succession is completed in following steps :
(1) Nudation : Formation of bare area without any form of life is called nudation. The cause of nudation may be climatic, topographic or biotic.
(2) Invasion : Successful establishment of a species in this bare area is called invasion. It has three steps :
(i) Migration (ii) Ecesis or establishment (iii) Aggregation
(3) Competetion and coaction : Competetion may be interspecific or intraspecific.
Intraspecific competetion is called coaction.
(4) Reaction : Modification of environment due to organisms in it is called reaction.
(5) Stabilization : The terminal community becomes stabilized in the prevailing conditions.
Biotic succession on bare rock (Lithosere, Xerosere).
The sequence of successional stages that occur on bare rocks is called lithosere. Because the bare rock is deficient in water, the lithosere is also called xerosere.
The various seral stages are as follows :
(1) Lichen stage : Bare rock is invaded first by crustose lichens (e.g., Graphis). They corrode the rock at places causing foliose lichens to invade, eliminate crustose lichens and creating conditions for invasion by mosses. In tropics, blue green algae are pioneers instead of lichens.
(2) Moss stage : Mosses are of larger size, have gregareous habit and their rhizoids penetrate deeper in the rocks. They shade the lichens and hence replace the same. Mosses accumulate more soil and organic matter.
(3) Annual grass stage : Annual hardy grasses and herbs invade the humus rich moss dominated rock surface, e.g., Aristida, Poa. Their roots cause fragmentation of the rock, creating more soil, humus and moisture.
This increases moisture and soil. The soil becomes favourable for growth of longer lived annual grasses. The process of soil accumulation continues.
(4) Perennial grass stage : Annual grasses are replaced by perennial grasses due to increased moisture and soil in the rock crevices. The perennial grasses have runners and rhizomes which rapidly spread the grasses, e.g., Cymbopogon, Heteropogon, Shade, moisture, soil, perennial vegetation and seeds invite several small animals.
(5) Shrub stage : Shrubs begin to grow in area occupied by perennial grasses, e.g., Zizyphus, Caparis, Rhus, Rubus. Shrubs are larger and their roots reach greater depth causing further cracks in the rock substratum and hence helping in more soil formation. The shrubs shade the area, make it more moist and invite hardy trees and several types of animals.
(6) Climax community : Initially hardy light demanding small trees invade the area. They make the habitat shadier and more moist. Ultimately, trees shrubs and herbs representing the climax community begin to grow in the area.
Biotic succession in newly formed pond/lake (Hydrosere) : Seres of biotic communities that develop in a newly formed pond or lake is called hydrosere. It starts as soon as the muddy water becomes clear. The various successional or seral stages of hydrosere are :
(1) Plankton stage : Phytoplanktons (diatoms, flagellates, blue green and green algae) are the pioneers in a freshly formed water body. They are almost immediately followed by zooplanktons that feed on phytoplanktons.
(2) Submerged stage : The bottom lined by soft mud having organic matter is favourable for growth of submerged plants like Hydrilla and Najas. They are rooted in the mud and form dense growth. As a result sand and silt get deposited around the plants. The bottom level, therefore, rises slowly. The older plants and buried parts of other plants form humus on their death and decay.
(3) Floating stage : In the shallower regions appear plants with tuberous rhizomatous and creeping stems and leaves floating on the surface of water, e.g., Nymphaea, Nelumbo. At places, free floating plants also appear (e.g., Azolla, Wolffia, Lemna) to cover the water surface. Humus rich bottom begins to rise making water shallower.
(4) Reed swamp stage : Amphibious plants grow where the water body becomes shallow (0.3-1.0m), e.g., Phragmites, Typha, Scirpus, Sagittaria. The plants of swamp stage transpire huge quantities of water. They also produce abundant organic matter. Their tangled growth accumulates silt.
(5) Sedge/Marsh meadow stage : On newly built up shores, Carex (Sedge), Juncus, Cyperus, some grasses and herbs (Themeda, Caltha, Polygonum) grow rapidly and lower the water table. The plants transpire rapidly and add abundant humus. Therefore, soil is build up to invite next stage.
(6) Woodland stage : Rhizome bearing shrubs and small trees capable of tolerating excessive light and water logged conditions appear on the edges of sedge/marsh meadow, e.g., Cornus, Cephalanthus, Populus, Alnus.
The shrubs shade away the plants of sedge meadow stage. They invite invasion by trees capable of bearing bright sunlight and water logging, e.g., Populus (Cottonwood), Alnus (Alder). The plants of woodland stage lower the water lable by their transpiration. They also built up more soil. Shade loving plants come to grow below them.
(7) Climax forest : New trees, shrubs and herbs appear which are in perfect harmony with the climate of the area.
Importance of biotic succession
(1) Sequence of biotic succession is usually fixed. Ecologists can immediately recognise the seral stage of a biotic community found in an area.
(2) It tells us how a biotic seral stage like grasses and herbs of a pasture can be maintained by not allowing the biotic succession to proceed further through interference like grazing and fire.
(3) Information gained through biotic succession is used in having controlled growth of one or more species by preventing their superiors to invade the area, e.g., maintenance of teak forest.
(4) Dams are protected by preventing siltation and biotic succession to occur.
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