What are the differences between primary and secondary ecological succession?

Primary ecological succession occurs on newly formed or exposed land, while secondary ecological succession occurs on previously established land that has been disturbed or damaged.

Primary ecological succession is the process of plant and animal colonization on a barren land surface, such as a volcanic island, a sand dune, or a glacier retreat. The first organisms to colonize the area are usually pioneer species, such as lichens, mosses, and ferns, that can tolerate harsh environmental conditions and can fix nitrogen from the air. As the pioneer species die and decompose, they create a thin layer of soil that can support more complex plants, such as shrubs and trees. Over time, the ecosystem becomes more diverse and stable, with a climax community that is adapted to the local climate and soil.

Secondary ecological succession is the process of plant and animal re-establishment on a disturbed or damaged land surface, such as a forest fire, a flood, or a human activity like logging or agriculture. Unlike primary succession, secondary succession starts with a soil seed bank and residual vegetation that survived the disturbance. The initial colonizers are usually fast-growing and opportunistic species, such as grasses and weeds, that can quickly cover the bare ground and compete for resources. As the ecosystem recovers, more shade-tolerant and long-lived species, such as shrubs and trees, replace the early colonizers. The final community may resemble the original community, but with some differences due to the nature and intensity of the disturbance.