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Ecological succession is the gradual change in the structure and composition of a biological community over time. This process is often triggered by environmental disturbances.

Ecologists divide succession into two main types that follow a disturbance: primary succession and secondary succession.

Primary succession begins when living organisms colonize bare rock left behind by retreating glaciers or formed after volcanic eruptions. These environments contain no visible life and no developed soil.

The first organisms that establish under these extreme conditions are pioneer species, which can tolerate the harsh conditions created by environmental disturbances.

For example, lichens are common pioneer species. They are symbiotic associations between a fungus and an alga or a cyanobacterium.

The fungus attaches to the rock and absorbs minerals released during weathering, while the alga performs photosynthesis. Over time, lichens speed up rock breakdown by releasing acids and physically penetrating the surface.

As lichens grow and decompose, their organic remains combine with rock fragments to form a thin soil layer. As this soil deepens and becomes nutrient-rich, additional plant species establish in a predictable sequence.

Mosses often appear first, followed by grasses, then shrubs, shade-intolerant trees such as pines, and finally shade-tolerant trees such as oaks.

Over time, species composition becomes more stable and self-sustaining. This relatively stable stage is called a climax community. This state is not permanent, as community composition continues to shift in response to ongoing environmental variation. It persists until a new disturbance alters the environment. Primary succession may take hundreds of years to reach a climax community.

Secondary succession happens when a disturbance, such as a fire, removes existing vegetation but leaves the soil largely intact. Because the soil, seeds, and underground structures remain, recolonization happens more rapidly.

Pioneer species such as herbaceous perennials and grasses reinhabit the soil first, followed by trees. For example, oak and hickory forests often reach a stable climax community within about 150 years.

The climax community formed through secondary succession may differ from the original community.