OCR · A-Level · Biology

    Course version: H420 · Version from Sept 2025

    (d) the process of primary succession in the development of an ecosystem

    Full specification wording

    Ecosystems: (d) the process of primary succession in the development of an ecosystem

    Primary succession is the colonisation of bare, previously uncolonised ground where no soil or seed bank exists, for example after a volcanic eruption or glacial retreat.

    Checked against the OCR H420 specification (opens in a new tab) on .

    • Point 4 of 6
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Primary succession is the colonisation of bare, previously uncolonised ground where no soil or seed bank exists, for example after a volcanic eruption or glacial retreat. Pioneer species such as lichens and mosses arrive by wind or water dispersal, tolerate harsh conditions and begin weathering rock. Their death and decay, together with trapped particles, slowly build a thin soil layer and add organic matter and nutrients. As soil deepens and water retention improves, larger plants such as grasses and shrubs can establish, outcompeting pioneers. Each stage is a seral stage; the community changes until a stable climax community forms, determined by climate. Biodiversity and soil depth generally increase, while pioneer species decline. The process is directional and slow, and it differs from secondary succession, which starts on existing soil.

    What earns the marks

    • Primary succession begins on bare, previously uncolonised ground with no pre-existing soil or seed bank.
    • Pioneer species such as lichens and mosses colonise, tolerate harsh conditions and begin to weather rock.
    • Death and decay of pioneers, plus trapped particles, build a thin soil layer and add organic matter and nutrients.
    • As soil deepens and retains more water, larger plants such as grasses and shrubs establish and outcompete pioneers.
    • Each community is a seral stage, and succession continues until a stable climax community determined by climate forms.
    • Biodiversity and soil depth generally increase through succession, while pioneer species decline.
    • Primary succession differs from secondary succession, which begins on already-formed soil.

    Common mistakes and how to put them right

    • Thinking primary succession starts on bare rock that already has soil.

      Put it right by stressing that no soil or seed bank exists at the start.

    • Confusing primary with secondary succession.

      Put it right by noting secondary succession begins on existing soil after a disturbance.

    • Assuming the climax community is always the same everywhere.

      Put it right by explaining it is determined by the prevailing climate.

    • Believing pioneers are outcompeted immediately.

      Put it right by describing the gradual build-up of soil before larger plants establish.

    Exam tips

    • Link each stage to a change in an abiotic factor, such as soil depth or water retention, to show the mechanism.
    • Use the term seral stage and climax community precisely when describing the sequence.
    • Contrast primary with secondary succession explicitly when a question asks you to distinguish them.

    Try the lesson

    The start of the Ecosystems: (d) the process of primary succession in the development of an ecosystem lesson, made from checked teaching notes for this specification point.

    Have a go first

    What name is given to the first organisms to colonise an area of bare rock at the start of primary succession?

    Listen

    Primary succession is the colonisation of bare, previously uncolonised ground where no soil or seed bank exists, for example after a volcanic eruption or glacial retreat. Pioneer species such as lichens and mosses arrive by wind or water dispersal, tolerate harsh conditions and begin weathering rock. Their death and decay, together with trapped particles, slowly build a thin soil layer and add organic matter and nutrients. As soil deepens and water retention improves, larger plants such as grasses and shrubs can establish, outcompeting pioneers. Each stage is a seral stage; the community changes until a stable climax community forms, determined by climate. Biodiversity and soil depth generally increase, while pioneer species decline. The process is directional and slow, and it differs from secondary succession, which starts on existing soil. A strong answer earns marks for these points. Primary succession begins on bare, previously uncolonised ground with no pre-existing soil or seed bank. Pioneer species such as lichens and mosses colonise, tolerate harsh conditions and begin to weather rock. Death and decay of pioneers, plus trapped particles, build a thin soil layer and add organic matter and nutrients. As soil deepens and retains more water, larger plants such as grasses and shrubs establish and outcompete pioneers. A common slip: Thinking primary succession starts on bare rock that already has soil. Instead, stressing that no soil or seed bank exists at the start. Exam tip: Link each stage to a change in an abiotic factor, such as soil depth or water retention, to show the mechanism.

    See it

    A top-to-bottom flowchart of primary succession. The first box shows bare, uncolonised ground with no soil or seed bank. An arrow leads to pioneer species such as lichens and mosses colonising and weathering rock. The next box shows their death and decay plus trapped particles building a thin soil layer and adding organic matter and nutrients. This leads to soil deepening and water retention improving. Next, grasses and shrubs establish and outcompete pioneers. An arrow then leads to seral stages, where the community changes, and on to a stable climax community determined by climate. The final box notes that biodiversity and soil depth increase while pioneer species decline.
    The sequence of primary succession from bare ground through pioneer species and seral stages to a climate-determined climax community.

    Put it in order

    Put these steps in the order they happen.

    1. 1. Grasses and shrubs establish and outcompete pioneers
    2. 2. Biodiversity and soil depth increase; pioneer species decline
    3. 3. Seral stages: community changes
    4. 4. Bare, uncolonised ground: no soil or seed bank
    5. 5. Pioneer species (lichens, mosses) colonise and weather rock
    6. 6. Stable climax community determined by climate
    7. 7. Death and decay plus trapped particles build thin soil and add organic matter and nutrients
    8. 8. Soil deepens and water retention improves

    Key words

    Say what each word means before you turn the card.

    Colonisation of bare ground

    Try answering aloud or in your head before turning the card.

    Pioneer species and weathering

    Try answering aloud or in your head before turning the card.

    Soil formation and organic matter

    Try answering aloud or in your head before turning the card.

    Seral stages and climax community

    Try answering aloud or in your head before turning the card.

    See every definition
    Colonisation of bare ground
    The arrival of organisms on bare, previously uncolonised ground where no soil or seed bank exists, for example after a volcanic eruption or glacial retreat.
    Pioneer species and weathering
    Lichens and mosses that arrive by wind or water dispersal, tolerate harsh conditions and begin weathering rock.
    Soil formation and organic matter
    The death and decay of pioneers, together with trapped particles, slowly build a thin soil layer and add organic matter and nutrients.
    Seral stages and climax community
    Each stage is a seral stage, and the community changes until a stable climax community forms, determined by climate.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-to-bottom flowchart of primary succession. The first box shows bare, uncolonised ground with no soil or seed bank. An arrow leads to pioneer species such as lichens and mosses colonising and weathering rock. The next box shows their death and decay plus trapped particles building a thin soil layer and adding organic matter and nutrients. This leads to soil deepening and water retention improving. Next, grasses and shrubs establish and outcompete pioneers. An arrow then leads to seral stages, where the community changes, and on to a stable climax community determined by climate. The final box notes that biodiversity and soil depth increase while pioneer species decline.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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