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Primary succession is a type of ecological succession that takes place in an environment that is recently formed and lacks habitable soil but then is colonized for the first time by living organisms.

• Primary succession is a series of events occurring in an order to develop a stable ecosystem.
• The new environments are usually formed due to natural events like volcano or glacier eruption that causes the lack of soil or the absence of living organisms.
• The groups of organisms that colonize these environments are termed ‘pioneer organisms’ and are usually composed of lichens, algae, and fungi.
• Primary succession begins with the weathering of rocks to form soil to inhabit the pioneer species.
• The initiation of primary succession usually occurs either due to a biological factor or an external factor.
• Once some amount of soil is formed, organisms like lichen that have few soil requirements, begin to grow in these areas. These organisms enter the new environment from a different environment.
• These species further assist the breakdown of mineral-based rocks formed of lava or glaciers into the soil to inhabit other species.
• The pioneer species continue to grow, reproduce, die, and decompose, creating pockets of soil where other species might grow and flourish.
• The decomposition of these organisms adds to the organic content of the soil, thus, contributing to soil formation.
• This process is repeated multiple times throughout the succession. In each new stage, new species move into the environment created by the preceding species, and some may even replace the previous species.
• Some examples of primary succession include the formation of a new ecosystem after a volcano, glacier outbursts, or a nuclear explosion.

Secondary succession is a type of ecological succession that occurs in an environment with an already established ecosystem that gets disruption due to some events like fire or hurricane and is then re-colonized by other organisms.

• Unlike primary succession, secondary succession begins in an environment with pre-existing soil.
• Secondary succession thus takes place in an environment where the initial succession has been disrupted, but some plants and animals might still exist.
• This type of succession usually follows disasters like a forest fire, hurricane, or harvesting, causing the existing ecosystem to be destroyed.
• The initiation of secondary succession always occurs due to some external factors.
• After a forest fire, the tall trees become destroyed, and the first plants that grow in such areas are usually annual plants.
• The following annual plants are grasses and low-lying plants and other pioneer species.
• The early colonizers in secondary succession are also termed ‘pioneer species’ like in primary succession.
• These pioneer species, unlike those in primary succession, mostly arise from the pre-existing groups of organisms of the community.
• Over time, changes in the environment caused due to the growth of grasses result in the growth of new groups of plants like shrubs and herbs.

Some examples of secondary succession include succession after fire, harvesting, logging, or abandonment of land or the renewal after a disease outbreak.

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