The gradual process by which the ecosystem changes and develops over a period of time is called ecological succession. This is an important part of environmental science, as over time, some species in any ecosystem reduce in number, while some even become extinct. Meanwhile, some species may proliferate in the same period and completely change the balance of the ecosystem. Causes There are certain environmental conditions which are suitable for the growth and development of every species in this world. The one that evolves the best and manages to reproduces the most, becomes more abundant in number. When the prevalent environmental conditions change, consequently the web of life built around the particular environment begins to change. The best-known example of this process is the extinction of dinosaurs. Once upon a time they ruled the earth, but when the environmental conditions of the earth changed radically, they could not adapt to it, and went extinct. In their place, new animals and birds came, who were able to adjust to the new environment. Stages of Ecological Succession Now let us learn about the various stages involved in such modification of an ecosystem. It occurs in two important stages; the primary and the secondary. Primary succession occurs when plants and animals move to areas that had been uninhabitable before. Examples of such areas include an area which had been covered by lava, or which had no fertile land for trees could grow. But slowly, as the environment changes, the place becomes more suitable for living beings to survive. This process is called primary succession. In secondary succession, existing life forms and food webs are simply replaced by other species. The extinction of dinosaurs was such an event. This type of succession is more common of the two. Nudation This stage is the one in which due to improving environmental conditions, such as the correct temperature, the correct amount of water, and also with the presence of oxygen in the air, life starts emerging. This process occurred while life was first arising on earth. As conditions on earth slowly became more inhabitable to life as we know it, basic plants, such as lichens, started to emerge. Invasion In this stage, the new life forms spread far and wide. Carrying on with the example of the emergence of life on earth, when the rudimentary plants died, they decomposed and formed the top layer of the soil. Due to this, more complex plants started growing in their place. This second stage also consisted of the emergence of herbs and shrubs. These plants produced seeds, which was an important change that was needed in order to spread life far across. Although these plants did not survive for long, they left behind their successors. This type of species is what scientists refer to as 'opportunist species'. Slowly the organisms, plants as well as animals, started increasing in number due to the improving environmental conditions. Once the population of the organism started growing, the next stage was reached and that was the stage of competition. Competition This stage is the practical demonstration of the 'survival of the fittest' rule. The organisms who were able to survive the environmental changes, and those who were strong enough to fight and survive for their place in the food web, started growing in number. Those who couldn't adapt to changes soon died out. Reaction Stage In this stage, a modification of the environment takes place. This modification is influenced by the survivors of the earlier stage. Climax Community This is the final stage. This is a stage of constant species composition. It marks the last stage of ecological succession. Once this stage is reached, it usually remains unchanged for the next hundred years. Now you know what ecological succession is, and what its various stages are.