Main Difference

Mature forest contains variety of plants. Sometimes entire forest can be wiped out by natural destructive process for example fire. The day after fire the forest looks abandoned. However over time the forest will grow back. First low growing grass develops, and then bushes followed by small trees. Thus the predictable change that comes in a community is called succession. There are two type of succession, primary and secondary succession. Primary ecological succession the ecosystem must be built after being completely destroyed to the point where even the soil is gone. Where as in secondary ecological succession the ecosystem must built when the soil is still intact.

Primary Succession

The main feature of primary ecological succession is soil formation. Primary succession can occur in newly formed organic island or a rock exposed from a receiving glacier. Over time the pioneer specie populates the area. The pioneer organisms are responsible to colonize the area first. These pioneer organisms can be mosses and lichens. They are spore bearing organism. These organisms follow seed dispersal, which are lighter and easily carried by air. These pioneer organisms help built the greenery plus helps in soil formation specially lichens.  Lichens secrete toxins into the rocks and break them down into the soil. Hence it is the first step towards soil formation. On the other hand the wearing down of rock by water and wind also contributes in soil production. The third type is when mosses and lichens die, their biomass degrades into soil. Over time this barren will become mature adult forest. Based on primary physical source the succession is of two types, autogenic succession and allogenic succession. Autogenic is when community itself changes its environment for example fallen leaves. While allogenic refers to the change brought up by the response to the change in external environment. An example can be the formation of wood land after a volcanic eruption.

Secondary Succession

Secondary ecological succession can occur after a small forest fire or tornado or when a law company clears a little piece of land. The destruction can be small scale or large scale forming a gap. In the gap the light intensity is high. As a result, soil moisture and relative humidity are low while its temperature and nutrients are increased. The normal system is disturbed but in all this the soil remains intact. As the soil is there it does not need to go through all the process as primary does to form soil. No pioneer organisms are required for it. Some organisms are already present in the soil. It starts from the middle of succession and then grows from there on wards to the final climax community. First low growing plants will populate the area. Then trees will appear and develop. It produces mature forest faster than primary succession.

Key Differences

• As everything is destroyed in fire so primary succession take longer time to develop a mature forest about thousand years, while in secondary succession it take hundred to two hundred years as there is no complete destruction.
• Primary occurs in barren land while secondary occurs where it has been denuded recently.
• In primary soil is absent from the beginning while is present in the secondary.
• In primary no humus while in secondary humus is present.
• Intermediary seral communities are many in primary while are few in secondary.
• Reproductive structures of any previous structure in primary is absent while present in secondary.

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