2.7 Ecological Succession

Primary vs. Secondary Succession

Ecological succession is the gradual, predictable change in species composition of a community over time. Primary succession begins in an area with no soil, such as bare rock after a volcanic eruption or glacial retreat, starting with pioneer species like lichens and mosses. Secondary succession occurs where soil already exists but a disturbance (fire, flood, farming) removed most organisms, so recovery happens faster.

Pioneer Species and Soil Building

Pioneer species are the first organisms to colonize disturbed or barren land. They are typically hardy, fast-growing, and tolerant of harsh conditions (e.g., lichens, mosses). Through weathering and decomposition, they build soil and add organic matter, enabling grasses, shrubs, and eventually trees to establish. Each stage changes conditions enough to favor the next set of species.

Climax Community and Species Diversity

Succession tends toward a relatively stable climax community, where species composition changes slowly if at all, given stable conditions. As succession proceeds, species diversity and ecosystem complexity generally increase, and food webs become more intricate. However, natural disturbances can reset succession at any stage, maintaining a mosaic of different community stages across a landscape.

Key terms

Ecological succession

The predictable, gradual change in species composition of a community over time.

Primary succession

Succession beginning in an area with no soil, such as bare rock after a volcanic eruption or glacier retreat.

Secondary succession

Succession that occurs where soil already exists after a disturbance (e.g., fire, flood, farming); faster than primary succession.

Pioneer species

Hardy, fast-growing first colonizers of barren land (e.g., lichens, mosses) that help build soil.

Climax community

A relatively stable community with slow species change that forms the end stage of succession under stable conditions.

Try an AP-style question

A lava flow cools into bare rock. What type of succession begins?

1. Secondary succession
2. Primary succession
3. Climax succession
4. Tertiary succession