Hello Sudesh!

I am trying to explain the answer to your question using points.

1)Primary succession in ecological view is the development of new habitats in barren and lifeless areas that are unaffected and untouched by any pre-existing life forms. On the other hand, secondary succession is the development of habitat that follows after the destruction of an already existing habitat and ecosystem.

2) There is no initial vegetation in the places that experience primary succession. But initial vegetation is always present in places where secondary succession occurs.

3) Primary succession is stimulated by an external agent or a biological factor(for eg, lichens on barren rock are responsible for weathering of those rocks to form fertile soil). In case of secondary succession, only an external factor initiates the process.

4) Soil is absent before the beginning of primary succession but soil is always present before secondary succession.

5) The species which initiate the primary succession process are always from an outside ecosystem. These species are called pioneer species(eg, lichens). The species which initiate secondary succession are from the same ecosystem.

6) Primary succession is a very long process while secondary succession has a shorter duration.

7) Some examples of primary succession include the formation of a new ecosystem after a volcano, glacier outbursts, or a nuclear explosion. Some examples of secondary succession include succession after fire, harvesting, logging, or abandonment of land or the renewal after a disease outbreak.

Hope this answer helps!