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Transcript
Fact sheet.........73 Ecological Succession What is ecological succession? The ecosystem is made up of living and non-living components which interact with each other to form a stable, self-sustaining system in nature. In certain habitats, however, communities tend to succeed one another in a relatively definite sequence even when there are no changes in climatic or topographic conditions. This sequence of ecological changes is known as ecological succession, and is usually the result of activities of existing organisms. How does ecological succession occur? Due to the activities of existing species, the environment is gradually changed such that it becomes more favourable to other species. These new species will compete with and eventually replace the existing species which have created the modified conditions. The new species will dominate the habitat for a period and form a closed community. In the course of time, conditions become further modified by the actions of the existing species so that they can no longer compete successfully with new invaders. In ecological succession, one community continues to follow another until, in many situations, a type of community is reached that cannot be replaced under the prevailing conditions. The community that can maintain itself indefinitely in a habitat is known as the climax community. When a climax community is established, it tends to remain in possession of the area because it does not change the environment so as to favour the growth of other species. The climax community will not be replaced unless the environment is dramatically changed. Types of ecological succession There are two types of ecological succession: primary succession and secondary succession. Primary succession is the colonisation of an 74.........Fact sheet area which has not supported life before, e.g. volcanoes, sand dunes, and glaciated surfaces. It usually occurs from the starting point of bare rock. Secondary succession is ecological succession in an area which previously supported an ecosystem has been denuded of vegetation by factors such as fire or grazing. It may occur after a forest fire or clearance of agricultural land. Examples illustrating the change underlying ecological succession 1. Establishment of a tropical rain forest In August 1883, two volcanoes exploded in Krakatoa, an island between Java and Sumatra. The lava and volcanic ash covered the whole island and all life forms disappeared. After nine months, some blue-green algae appeared on the rock surface of the shore. After three years, ferns and grasses were found over most part of the island. Seven years later, there were young coconut trees, lizards, snakes and a large number of insects and spiders. Finally, a dense tropical rain forest becomes established on the island again and the community is in a state of balance. Krakatoa has reached a climax community with eight hundred different species being recorded in 1983 one hundred years after the clearance of the island. The recovery of the forest on Krakatoa illustrates how progressive changes in a community can occur over a long period of time. At first, the bare rocks could hardly support any organisms except the primitive algae and lichens. These first organisms to move in are called pioneer species which can penetrate the rock surface and trap organic materials to begin the formation of humus. Weathering of the rock and action of algae and lichens on the rock formed a thin layer of soil which favoured the survival of mosses. The mosses outgrew the algae and lichens. They trapped more dust and formed more soil when they died. The accumulating soil in turn favoured the establishment of grasses and ferns, and they in turn broke the rocks further. Finally the soil was deep enough for the establishment of the trees which became the dominant species. Fact sheet.........75 2. Changes in mangrove community In coastal areas of tropical regions, the mangroves grow into the sea by producing special seedlings called droppers that will root in shallow water. The spreading prop roots of the mangroves trap soil particles and dead organic matter between them thus raise up the sea bottom. As the soil becomes drier, this change leads to the gradual elimination of the mangroves and allows the establishment of the more terrestrial shrubs. 3. Changes in a deserted land A deserted field or construction site is usually first invaded by weeds. As more soil is formed by the activities of the weeds, the weeds are succeeded by shrubs which in turn add more soil to the land. Finally, trees can establish on the deeper soil and replace the shrubs. 4. Changes of a forest after a fire When a forest is cleared by fire, if conditions become suitable once again, the bare land will begin to change back into a forest. happens very slowly and gradually. This Spores, seeds and organs of vegetation may remain viable in the soil and thus influence the succession. There may also be an influx of plants and animals through dispersal and migration from the surrounding area. In these circumstances the succession will not begin with pioneer species but with organisms from subsequent successional stages. A forest cleared by a fire