Succession

Primary succession

Primary succession starts on ground with no soil, such as bare rock, a lava flow or sand dunes.

1. Pioneer species (such as lichens and mosses, or marram grass on dunes) colonise. They tolerate harsh conditions: little water, few nutrients, extreme temperatures.

2. They weather rock and, when they die, decompose to form humus, building a thin soil that holds water and minerals.

3. Conditions become less hostile, so new species (grasses, then shrubs) can grow. They often outcompete the earlier species.

4. Each stage changes the environment, making it more suitable for the next. These stages are seral stages.

The climax community

Eventually a stable climax community forms, in balance with the climate. In much of the UK this is deciduous woodland.

During succession:

• soil depth, organic matter, water retention and nutrient content increase
• biomass increases
• biodiversity generally increases, though it may fall slightly at the climax when a few dominant species outcompete others

Secondary succession and deflection

Secondary succession happens where a community has been removed but soil remains, for example after a fire, flood or when farmland is abandoned. It is faster than primary succession because soil and seeds are already present.

Human activities such as grazing, mowing or burning can stop succession at an earlier stage, giving a plagioclimax (for example grazed grassland or managed heather moorland).

Worked example

Explain how lichens make it possible for mosses to grow on bare rock.

1. Lichens tolerate the harsh, dry, nutrient-poor rock.
2. They weather the rock surface and, when they die, decompose to form humus.
3. This produces a thin soil that holds water and minerals, which mosses need.

Answer: Lichens change the conditions by forming the first soil, making the habitat suitable for mosses.

Check yourself

1. Which feature helps a pioneer species colonise bare rock?
   • It can survive with little water and few nutrients
   • It needs deep, fertile soil
   • It grows only in shade
   • It must have a large root system in soil
   Show answer

   It can survive with little water and few nutrients. Pioneers tolerate harsh conditions.

2. How do pioneer species allow other species to colonise?
   • They release oxygen that kills competitors
   • They remove all water from the rock
   • They die and decompose, adding humus and forming soil
   • They prevent any other species growing
   Show answer

   They die and decompose, adding humus and forming soil. Humus builds soil that holds water and nutrients.

3. What is a climax community?
   • A stable final community in balance with the climate
   • The first species to colonise an area
   • A community that changes every year
   • A community with no producers
   Show answer

   A stable final community in balance with the climate. Climax = stable end point of succession.

4. Why is secondary succession faster than primary succession?
   • There are no plants at all at the start
   • Pioneer species must first form soil
   • The climate is always warmer
   • Soil and seeds are already present
   Show answer

   Soil and seeds are already present. With soil already there, plants establish quickly.

5. Sheep grazing stops a grassland from becoming woodland. What is this stable community called?
   • A pioneer community
   • A plagioclimax
   • A climax community
   • A seral community with no grass
   Show answer

   A plagioclimax. Human activity deflects succession, giving a plagioclimax.

6. Which change usually happens during succession?
   • Soil becomes thinner
   • Biomass decreases
   • Water retention by soil falls
   • Soil depth and biomass increase
   Show answer

   Soil depth and biomass increase. Each stage adds organic matter and supports larger plants.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)

Last updated 7 November 2026. Spotted a mistake? Tell us.