Each stage prepares the next: Each seral community changes the conditions around it. That lets the next community move in and outcompete it, stage after stage, until a stable climax is reached.
The rule for every succession
- Each seral community changes the environmental conditions of its area.
- The new conditions suit other species better, so they can colonise.
- The newcomers outcompete the old community (for light, water, space) and replace it.
- The cycle repeats until a stable climax community is reached.
- Bare rock: mosses and lichens start soil formation, so larger plants can colonise.
Remember it as: Change it, lose it: each community changes the place, then loses it to the next.
Real example: Rangitoto erupted about 600 years ago and left bare black lava. Lichens and mosses weathered the rock and their remains made the first soil in cracks. That soil let ferns and then pohutukawa trees root: today the island holds the largest pohutukawa forest in the world.
Link every step: Write each step as a chain: 'mosses add humus, so the soil holds water, so grasses can root'. A list of plant names with no changes explains nothing.
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On sand dunes the stages lie side by side: the youngest dunes by the sea, the oldest inland. Walking inland is like walking forward in time, as humus builds up in the sand.
How each dune stage changes the sand
- Pioneer grasses tolerate salt and trap blown sand, building the dune.
- Marram binds the sand; its dead leaves add the first humus.
- Humus lets the soil hold water and nutrients; rain washes out the salt.
- The soil turns more acidic; heath, then shrubs and trees, can grow.
- Marram needs fresh sand: on stable dunes it is outcompeted and replaced.
Real example: at Studland the dunes have grown out to sea over about 400 years. Marram grass holds the young yellow dunes. Further inland, humus has turned the sand grey and acidic, and heather heath grows; the oldest ridges carry birch and pine.
Marram is not killed by the others: Marram grows best when fresh sand buries it. On stable, older dunes it gets no new sand, so it is outcompeted: the conditions it made now suit other plants better.
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A lake or pond can fill in and become dry land. Each community traps silt and adds dead matter, so the water gets shallower, stage by stage.
How a lake fills in
- Water plants trap silt; their dead remains settle on the bed.
- Reeds root in the shallows and build peat: the water gets shallower.
- The peat rises above the water: fen plants take over from the reeds.
- Trees such as alder and willow dry the ground and shade out the fen.
- Where the ground is dry enough, woodland is the climax.
Real example: the Norfolk Broads are medieval peat pits that flooded. Reeds, then fen and then alder carr have spread into them, and many Broads have shrunk or vanished. Some are kept open only by cutting the reeds.
Name the change, then the newcomer: 'The reeds build peat, so the ground rises above the water, so fen sedges can grow and outcompete the reeds.' Change, then colonise, then outcompete.
Why does a succession stop? Each stage keeps changing the conditions until one community is in balance with the climate and soil. That community can replace itself, so it stays: the climax.
Changes, competition, climax
- The changes are abiotic: deeper soil, more humus, nutrients and water; less salt; more shade.
- Each stage often makes conditions worse for itself and better for the next.
- Replacement is by competition: the better-suited newcomers win light, water and space.
- Change slows once the community is in balance with the climate and soil: a stable climax.
- Which climax is reached depends on the climate: oak woodland in lowland England, rainforest in Hawaii.
A seral stage
- Changes the conditions
- Makes the place better for others
- Is replaced through competition
The climax
- In balance with climate and soil
- Its own young can grow under it
- Stable: not replaced
Real example: in lowland England, a succession left alone ends in oak woodland. The Studland dunes and the drying Broads are both heading there; oak seedlings can grow in the shade of oaks, so the woodland renews itself.
Stable, not frozen: A climax still has births, deaths and small changes, but its species replace themselves, so the community as a whole stays the same unless a disturbance strikes.
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How this comes up: Paper 2, Section B, part (a): outline how one stage of a succession allows the next to colonise [4].
In September 2021 lava from the Cumbre Vieja volcano covered about 12 km² of La Palma, in the Canary Islands, Spain. Scientists expect lichens and mosses to be the first living things on the new rock.
Outline how pioneer lichens and mosses on bare rock allow larger plants to colonise.
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