Succession

About this topic

Succession is the gradual change in the species composition of a community over time. Primary succession starts on bare rock with no soil: lichens and mosses (pioneer species) break rock and build soil, followed by grasses, shrubs and trees. Secondary succession follows disturbance, such as fire or farming abandonment, where soil is already present, so it is faster.

At each stage, organisms change the environment so that new species can arrive. Dead pioneers add organic matter and make soil deeper and richer, retaining water. Eventually a stable climax community forms, such as an oak woodland in the UK, and it remains until disturbed. Species diversity usually rises during succession, and the community becomes more complex.

A real example is the volcanic island of Surtsey, which appeared off Iceland in 1963 and has since been colonised first by lichens, then plants, insects and seabirds. Another is a ploughed field left untouched: weeds appear in the first year, then shrubs, then young trees.

Succession explains why land managers sometimes mow or graze meadows to stop them turning to woodland. Students often think succession is quick or that the climax community never changes. In fact, primary succession can take hundreds of years, and climate or disturbance can reset it.