Ecological succession is a series of progressive changes in the species that make up a community over time.  

• Primary succession occurs when new land is formed or bare rock is exposed, providing a habitat that can be colonized for the first time.  
• What happens to this land during primary succession? First, weathering and other natural forces break down the substrate, rock, enough for the establishment of certain hearty plants and lichens with few soil requirements, known as pioneer species.  
• Algae, fungi, bacteria, lichens colonize on the bare rocks and provide organic materials for further higher level of colonization, while the planktons i.e., phytoplankton’s, zooplanktons provide base at the shallow water for further higher levels of succession in hydro sere. 

Source: NCERT