Cyclical succession, climax communities and arrested successionSpec D4.2.14, D4.2.15
In short
In some ecosystems there is a cycle of communities rather than a single unchanging climax community, as in heather heathland. Given specific environmental conditions, succession tends to lead to a particular climax community. Human influences can prevent it from developing: grazing by farm livestock and drainage of wetlands cause arrested succession.
Cyclical succession
In some ecosystems there is a cycle of communities rather than a single unchanging climax community. In heathland dominated by heather (Calluna vulgaris), patches go through a repeating cycle.
- Pioneer phase: young heather plants establish in a gap.
- Building phase: heather forms a dense canopy and dominates the patch; few other plants grow beneath it.
- Mature phase: the plants lose vigour and the canopy starts to open in the centre.
- Degenerate phase: old woody stems collapse outwards, leaving gaps colonised by lichens, mosses and other plants such as bearberry, then new heather seedlings, and the cycle restarts.
One full cycle takes roughly 30–40 years, so a heathland is a mosaic of patches at different phases while the heathland as a whole stays the same. This model was first described for heather in Scotland; it does not apply everywhere, for example not on wet bogs.
Climax communities and arrested succession
Given any specific environmental conditions, succession tends to lead to a particular type of climax community, a relatively stable community determined mainly by climate and soil, often forest where rainfall allows. Human influences can stop it from developing: arrested succession.
- Grazing by farm livestock: sheep and cattle eat tree seedlings, so grassland or moorland persists where woodland would grow.
- Drainage of wetlands: lowering the water table removes waterlogged conditions, so wetland and peat-forming communities are lost and the land is used for farming; the natural sequence cannot continue to its climax.
A community kept by human activity at a stage short of the natural climax is called a plagioclimax.
Quick check
Name the four requirements for ecosystem stability.
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Supply of energy, recycling of nutrients, genetic diversity, climatic variables within tolerance levels.
When is harvesting sustainable?
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When the rate of harvesting is lower than the rate of replacement.
What does BOD stand for?
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Biochemical oxygen demand.
Why are sealed glass vessels preferred for mesocosms?
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Matter cannot enter or leave, but energy transfer is still possible.
HL only Give two human causes of arrested succession.
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Grazing by farm livestock and drainage of wetlands.
Written and checked against the IB Biology HL specification · Updated October 2026