Succession at Higher Level: The same ideas as SL, with different places: Krakatoa's recovery after 1883, Iversen's pollen from Danish bogs and Mount St Helens seen from space. At HL you then go further: what shapes the climax, and how people divert a succession.
Practise this as you read
- Explain each replacement: conditions change, then competition.
- Compare a climax and a pioneer community, point by point.
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In one sentence: succession is the replacement of one community by another in an area over time, until a stable climax community is reached.
The points to remember
- Succession = the replacement of one community by another in an area over time.
- It happens because biotic and abiotic conditions change.
- Each step is a seral stage (a sere); the whole series runs from pioneer to climax.
- Pioneer community: the first colonisers of bare ground: few, tough species.
- Climax community: the final, stable community, in equilibrium with the climate and soil.
- It is slow: it can take hundreds of years.
Remember it as: Think of a relay race: each community runs its stage, then hands over to the next.
Real example: in 1883 the volcano Krakatoa exploded and buried what was left of the islands in hot ash. Nothing survived. Within 3 years ferns and grasses had arrived; by the 1930s a tropical forest stood there. Each seral stage lasted a few decades.
Define it with both parts: Succession: the change in the community (the species present) of an area over time. A definition without time, or without the community, is only half an answer.
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Why does one community give way to another? Because each community changes the abiotic and biotic conditions of its own area, making it better for other species than for itself.
How one community replaces another
- A community changes the conditions of its area (soil, shade, nutrients, water).
- The new conditions let other species colonise.
- The newcomers outcompete and replace the first community.
- The cycle repeats, each community making way for the next, until the climax.
- Changes are abiotic (deeper soil, more nitrogen, more shade) and biotic (new species, competition).
Grasses arrive
- Wind-blown grass seeds cover the ash of Rakata island by about 1890.
They change the ground
- Roots bind the ash; dead grass adds humus, so the soil holds water.
Trees move in
- Birds and bats drop fig seeds; casuarina seeds arrive; they root in the new soil.
Trees win
- By the 1930s the trees shade the grasses out: a forest replaces the grassland.
Say what changed, then who won: Every step has two parts: the old community changes the conditions, then the new one outcompetes it. 'New species arrive' alone misses why the old ones leave.
Succession is too slow to watch, so how do we know it happened? One record is kept in peat.
Pollen in peat: the evidence
- Peat builds up in layers, year after year, in waterlogged bogs.
- It is wet and acidic, so pollen does not decay: it is preserved.
- Deeper = older. A core of peat is a record back through time.
- Each plant has its own pollen shape, so the plants of each period can be named.
- A change in pollen up the core shows one community replacing another: succession.
Real example: in 1941 the Danish scientist Johannes Iversen studied pollen in Danish bogs. He found birch, then mixed oak and lime forest after the ice. Then, about 5,800 years ago, tree pollen fell and weed pollen rose: the first farmers' clearings, which he called landnam.
Deeper means older: In a peat core, read from the bottom up to go forward in time. A pollen type that replaces another higher up the core is a community replacing another.
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Change over time can also be mapped. Old maps, air photos and satellite images of the same place show one community replacing another.
How to map change
- Compare maps, photos or satellite images of one area from different dates.
- Or compare sites of different, known ages side by side (young lava, old lava).
- Say what changed, from what to what, and give the numbers or dates.
- Name each community: 'birch woodland became heath', not 'the land got worse'.
Real example: on 18 May 1980 Mount St Helens erupted and flattened about 600 km² of forest. Satellite images taken every year since then map the green returning: first specks, then lupin meadows, then young conifer forest.
Name what changed into what: 'The vegetation changed to a desert' names nothing. Write 'birch woodland fell from about 25% to 1.5% of the land and was replaced by heath and bare ground'.
Zonation and succession both describe communities changing, so they are easy to mix up. The difference is space against time.
Zonation or succession?
- Zonation = change in communities across space, along an environmental gradient.
- Succession = change in the community of one area over time.
- Zonation: walk up a mountain today. Succession: stay in one spot for 200 years.
- In a comparison, always finish with the contrast: distance (space) v time.
Zonation
- Change across space
- Along a gradient: tide, height, distance
- Kilimanjaro: rainforest, heath, then alpine desert uphill
Succession
- Change over time
- In one place, stage after stage
- Krakatoa: ash, then grass, then forest
Real example: walking up Mount Kilimanjaro you pass zones in one day, because temperature falls with height. On Krakatoa, the same ground changed from ash to forest over 50 years: succession.
Finish with the contrast: Describing each one is not enough. Say the difference directly: zonation is a change over space (distance along a gradient); succession is a change over time.
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The papers often ask how a climax community differs from a pioneer community. Each point is a comparison: say which has more, or what each has. Pioneers are r-selected; climax species are K-selected.
How a climax differs from a pioneer community
- Climax has more species and more habitats and niches.
- Climax has longer food chains and more branched food webs.
- Climax has deeper soil, more nutrient cycling and a larger decomposer community.
- Climax has more biomass, gross productivity and community respiration.
- Climax has K-selected species and trees; pioneers are small, fast-growing r-selected species.
- Climax is more resilient (more stable) because it is more complex.
| Pioneer community | Climax community | |
|---|---|---|
| Species and niches | Few species, few niches | Many species, many niches |
| Food webs | Short chains, simple | Long chains, many branches |
| Soil | Thin or none, few nutrients | Deep, fertile, fast cycling |
| Biomass and productivity | Low biomass, low gross productivity | High biomass and gross productivity |
| Typical plants | Lichens, mosses, grasses | Trees and woody shrubs |
| Strategy | r-selected: small, fast, many seeds | K-selected: large, slow, long-lived |
| Stability | Easily disturbed | More resilient |
Real example: on Krakatoa in 1886 the pioneers were a crust of cyanobacteria and a few ferns; today's forest has hundreds of plant species, tall trees, and fruit bats, birds and reptiles in a web of food chains.
Compare, every time: 'Trees' alone is not a difference. Write 'the climax has trees, while the pioneer community has lichens and mosses'. The opposite way round (pioneer has fewer species) counts too.
How this comes up: Paper 2, Section B, part (a): identify four differences between a climax and a pioneer community [4].
On Rakata, the island left by the 1883 eruption of Krakatoa, the pioneer community of 1886 was a crust of cyanobacteria and ferns. By the 1990s the island held a closed tropical forest.
Identify four differences between a climax community and a pioneer community.
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