SuccessionEdexcel International A Level Biology: Revision notes

Succession is the gradual, directional change in the species that make up a community over time. It begins with the colonisation of an area by pioneer species and ends with a stable climax community. Each stage in the sequence is called a seral stage (a sere is the whole sequence).

Primary succession starts on newly formed or exposed land with no soil, for example bare rock after a volcanic eruption or a retreating glacier, or new sand dunes. Secondary succession starts where soil is already present, for example on abandoned farmland or after a forest fire, so it proceeds faster.

1. Colonisation: bare rock has no soil, little water, and extreme temperatures. Pioneer species such as lichens and cyanobacteria, which can survive these conditions, arrive as spores or seeds by wind and are the first colonisers.
2. Soil formation: pioneers weather the rock and, when they die and decompose, add organic matter (humus), forming a thin soil that holds water and nutrients.
3. Mosses, grasses and herbs colonise. They out-compete the pioneers, and add more organic matter. Nitrogen-fixing plants, such as lichens with cyanobacteria, alder and Dryas, increase the nitrate content.
4. Shrubs and small trees colonise and shade the ground. Light-demanding species are replaced.
5. Climax community: the final stage, such as woodland, in which the dominant species cannot be replaced.

Abiotic changes as succession proceeds: soil becomes deeper, with more organic matter, water and nitrate; extremes of temperature and wind speed decrease; pH may fall; light at ground level decreases under taller plants.

Biotic changes: the number of species (biodiversity) increases, though it may fall slightly at climax when one species dominates; biomass and productivity increase; food webs become more complex with more niches; the plants become larger and longer lived. Each seral community modifies the environment so that it favours other species, which out-compete the earlier ones.

The climax community is the stable end point of succession. Its species and biomass stay approximately the same, in dynamic equilibrium: births balance deaths, and photosynthesis balances respiration. It is determined by the climate (and soil) of the region, such as oak woodland in lowland UK or spruce forest in subarctic Alaska.

No other species can out-compete the dominant species under the prevailing conditions, so no further change takes place unless there is a disturbance such as fire or felling.