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The first organism that colonise a bare area are called pioneers. After having adjusted to the environment, they begin to multiply. Normally pioneer species show hugh rate of growth but short life span. Initially there is no competition for pioneers but with time the competition results in the decrease in their number also. With tine the number of pioneers decreases due to the changes in environment and competition. As the pioneers doe and decay they add organic matter moisture and nitrogen to soil. In this enriched soil next groubof communities thrive well. They constitute one seral stage in succession . By their activities they modify the environment the changed environment becomes unsuitable for their growth and a new group of plants and animals invade the environment, forming the next seral stage. This way an orderly progression goes on with progressive replacement of one community by another till a climax or stable stage is reached. The intermediate stages from pioneer to climax community are called seral stages