Introduction:

Ecosystems encompass a tendency to change from extremely simple to complex in a process termed as succession even if the climate of a given area remains stable year after year. A vacant open space or cleared woods slowly become occupied by an increasing number of larger and larger plants. A pond becomes filled having vegetation which encroaches from the sides and slowly turns it to dry land by means of vegetation.

Ecological Succession:

Succession is a procedure of ecological change over time in a specific area of an ecosystem from simple to complex. This change takes place even when the climate remains stable year after year. Succession is the continuous and global in scope. If for illustration a wooded area is cleared and left alone, plants will slowly regain the area. Finally traces of the clearing will disappear and the area will again be woods.

Succession is a natural change. As such, changes caused through the direct influence of humans in clearing and replanting land can't be stated as succession. The succession can either be primary or secondary.

Primary Succession:

Primary succession takes on bare, lifeless substrate similar to a rock or un-colonized ground which has never had any vegetation growing on it before or in open water like newly made lakes. It might as well take place on sand dunes, lava flows of volcanoes, new volcanic islands, landslips or lakes left behind after the retreat of glaciers. On a bare rock, the process might begin by the growth of pioneer species such as lichens, making small pockets of soil. Acidic secretions from the lichens and from the plants, which grow on the rocks, later assist to break down the substrate and add to the accumulation of soil.

Mosses might then colonize these pockets of soil, finally followed by ferns, horsetails and the seedlings of flowering plants. Over numerous thousands of years or even longer, the rocks might be fully broken down and the vegetation over an area where there was once a rock outcrop might be just similar to that of the surrounding grassland or forest. This kind of primary succession on a rock is termed as lithosere whereas when on bare ground is termed as a xerosere. In contrary primary succession which builds up in aquatic environment is termed as hydrosere. A new lake, poor in nutrients might steadily accumulate organic matter and become rich in nutrients. Plants standing all along the edges of the lake similar to cattails and rushes and such growing submerged like pondweeds altogether by other organisms such as algae plants whose spores are carried by air to the lake might contribute to the formation of a rich organic soil. As this procedure carries on, and the build-up increases, the pond might become shallower and vegetation will rise. Gradually, terrestrial vegetation will encroach from the sides and fill the pond.

Finally, the area where the pond once stood might become an indistinguishable portion of the surrounding vegetation. The procedure of succession on the rock or lake referred to above might over a very long term make them feature the similar kind of vegetation feature of the area as a whole. This made the American ecologist Clements many years ago state the concept of 'Climax Vegetation'. He felt that succession presented a unidirectional series of changes which couldn't be reversed. However now, by an increasing realization that;

a) The climate keeps varying.

b) The procedure of succession is frequently very slow.

c) The nature of regions vegetation is being found out to a greater extent through human activities, ecologists don't consider the concepts of climax vegetation to be as helpful as they once did. Clements ideas are thought to be too accurate and rigidly organized to reflect the realities of the ecosystem change.

Secondary Succession:

When a wooded area is cleared and left alone, plants will gradually reclaim the area. Finally, traces of the clearing will disappear and the area will again be woods. The series of vegetation developing on these formerly vegetated and cleared regions are termed as secondary succession. The changes which take place on abandoned cultivated land or that occurs when a fire has burned off an area are as well secondary succession. Humans are frequently responsible for initiating secondary succession.

Human Influence on Succession:

A big amount of the terrestrial surface of the earth carries vegetation that is no longer in natural succession. Human interference for agricultural production has halted vegetation change or fully destroyed the natural vegetation. The most obvious consequences of farming practices are found on arable land where the natural vegetation has been cleared and monocultures of crops are grown rather. These plants are only grown for a few months before they are harvested, the soil turned over and new crop planted. The only wild plants capable to survive these practices are quick growing annual weeds that can flower and set seed before the crop is harvested. The other main managed vegetation kind is grassland.

Grazing by cattle, sheep or other domestic animals or mowing either for short turf or once a year for feed, all maintain grassland and prevent succession to scrub. The consequences of human land management on preventing and diverting succession are apparently seen around the world today. Big areas of natural vegetation have been cleared the most controversial possibly being the clearance of tropical rainforest for the creation of grassland for cattle. This large scale clearance generally modifies the ecosystem involved.

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