The process of community development is called succession. Community changes alter the ecosystem . This change favours the competitors. Thus the species replace one another. Finally a stable. self sustaining climax community is reached. Succession is a kind of community relay. in this case. plants and animals replace One another in a sequence. [hat sequence is predictable. The diverse and precise changes occur during succession. These changes take place according to changes in the environments in which succession occurs. Certain general stages of succession can be easily recognized.
Each succession begins by a few hardy invaders called pioneers. Succession ends with a diverse and relatively stable climax community. A community in which population of different species remain uniform is called climax community.
Two Major Forms of Succession
Succession of dry land .takes place in two major forms, primary succession and secondary
- Primary succession
The formation of ecosystem from bare rock, sand or clear glacial pool where previous life do not exist is called primary succession. In this case, the ecosystem is formed from the start. So it is a long process. It often requires thousands of years. Primary succession may start in two habitats
(a) Hydrosere: In this case the primary succession occurs in a pond.
(b) Derosere: In this case. succession occurs on a dry soil or rock.
(c) Xerosere: In this case. succession occurs on dry habitat. Plants growing in xeric condition are called xerophytes. Xerophytes can withstand prolonged periods of water shortage. Water is stored in large parenchyma tissue in succulent plants like cacti. Other xerophytes have leaf modifications Ibr storage of water.
(d) Psammosre: The succession which occurs on sand dunes is called psammosere.
(e) Halosere: The succession which occurs on saline soil is called halosere.
- Secondary succession
The formation of a new ecosystem after the disturbance of an existing ecosystem is called secondary succession. ‘Die disturbance may be in the form of ,forced fire or an abandoned farm field. The previous community leaves some mark in the form of improved soil and seeds. Therefore, secondary succession occurs more rapidly than primary succession.
Autogenic and’Allogenic succession
- Autogenic succession
The succession in which organisms themselves bring change in the environment during succession is called autogenic succession. The organ isms cause change in the soil. These changes include accumulation of organic matter in form of humus or litter alteration of soil nutrients and change in pH of soil. The structure of the plants themselves can also change the community. For example, larger species like trees produce shade on to the developing forest floor. It destroys the light-requiring species. Shade-tolerant species-establish in the area.
- Allogenic succession
The succession in which external environmental factors cause change in the environment during succession is called allogenic succession. Soil erosion, leaching or the deposition of silt can changes the soil. Similarly, clays can alter the nutrient content and water relationships in the ecosystems. Animals also play an important role in allogenic changes. They act as pollinators, seed dispersers and herbivores. They can also increase nutrient content of the soil in certain areas.