AQA · A-Level · Biology

    Course version: 7402 · Version from Sept 2016

    Changes that organisms produce in their abiotic environment can result in…

    Full specification wording

    Populations in ecosystems (A-level only): Changes that organisms produce in their abiotic environment can result in a less hostile environment and change biodiversity.

    Abiotic conditions on newly exposed ground are hostile: there is no soil, so no store of water or mineral ions, temperatures swing widely, and exposure to wind and light is high.

    Checked against the AQA 7402 specification (opens in a new tab) on .

    • Point 11 of 14
    • 3 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Abiotic conditions on newly exposed ground are hostile: there is no soil, so no store of water or mineral ions, temperatures swing widely, and exposure to wind and light is high. Colonising organisms change this. Dead remains decomposed by saprobionts add humus, which holds water and releases mineral ions. Plant cover shades the surface, narrowing the temperature range and reducing evaporation. Roots bind the substrate and reduce erosion; on a dune, marram grass traps sand and adds humus. Each change makes the abiotic environment less hostile. More species can tolerate the conditions, so biodiversity rises, measured as species richness and an index of diversity.

    What earns the marks

    • Naming an abiotic factor that becomes less hostile, such as soil water content, mineral ion concentration, temperature range, wind exposure or pH.
    • The mechanism, such as dead organisms being decomposed by saprobionts to form humus that retains water and releases mineral ions.
    • Stating that more species are adapted to, or can tolerate, the new conditions.
    • Increased biodiversity expressed as species richness and/or an index of diversity; note that species richness alone counts species present but omits evenness, so an index of diversity gives a fuller comparison.
    • Recognising that diversity can fall again where a closed climax canopy shades out earlier species.

    Common mistakes and how to put them right

    • Writing that the environment improves without naming a single abiotic factor.

    • Treating biodiversity as species richness only, ignoring the number of individuals of each species.

      Put it right: Species richness omits evenness, so use an index of diversity for fuller comparisons.

    • Claiming diversity always reaches its maximum at the climax community.

    • Describing decomposition without naming saprobionts or humus.

    • Confusing a less hostile abiotic environment with reduced competition, when competition usually intensifies.

    Exam tips

    • Quote the index of diversity when data are supplied, and say what a higher value means about the community.
    • Give the abiotic factor and the direction of change, for example soil water content increases.
    • Link every abiotic change to the species that can now survive.

    Try the lesson

    The start of the Populations in ecosystems (A-level only): Changes that organisms produce in their abiotic environment can result in a less hostile environment and change biodiversity. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Pioneer mosses colonise bare rock. As they grow and decay, a thin layer of organic matter builds up on the surface. Which statement best explains the ecological effect of this change to the abiotic environment?

    Listen

    Abiotic conditions on newly exposed ground are hostile: there is no soil, so no store of water or mineral ions, temperatures swing widely, and exposure to wind and light is high. Colonising organisms change this. Dead remains decomposed by saprobionts add humus, which holds water and releases mineral ions. Plant cover shades the surface, narrowing the temperature range and reducing evaporation. Roots bind the substrate and reduce erosion; on a dune, marram grass traps sand and adds humus. Each change makes the abiotic environment less hostile. More species can tolerate the conditions, so biodiversity rises, measured as species richness and an index of diversity. A strong answer earns marks for these points. naming an abiotic factor that becomes less hostile, such as soil water content, mineral ion concentration, temperature range, wind exposure or pH. the mechanism, such as dead organisms being decomposed by saprobionts to form humus that retains water and releases mineral ions. stating that more species are adapted to, or can tolerate, the new conditions. increased biodiversity expressed as species richness and/or an index of diversity; note that species richness alone counts species present but omits evenness, so an index of diversity gives a fuller comparison. A common slip: treating biodiversity as species richness only, ignoring the number of individuals of each species. Instead, species richness omits evenness, so use an index of diversity for fuller comparisons. Exam tip: Quote the index of diversity when data are supplied, and say what a higher value means about the community.

    See it

    A top-down flowchart. The top node, colonising organisms on hostile ground, branches into three changes. First, dead remains decomposed by saprobionts lead to humus added, which holds water and releases mineral ions. Second, plant cover shades the surface, so the temperature range narrows and evaporation is reduced. Third, roots bind the substrate and reduce erosion. All three changes feed into one node: the abiotic environment becomes less hostile. That leads to more species being able to tolerate the conditions, then to biodiversity rising, measured as species richness and an index of diversity. A final node notes that diversity can fall again where a closed climax canopy shades out earlier species.
    It shows how colonising organisms make the abiotic environment less hostile, raising biodiversity, with diversity able to fall again at a closed climax canopy.

    Key words

    Say what each word means before you turn the card.

    humus

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

    species richness

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

    index of diversity

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

    See every definition
    humus
    Material formed when dead remains are decomposed by saprobionts, which holds water and releases mineral ions.
    species richness
    A measure of biodiversity that counts species present but omits evenness.
    index of diversity
    A measure of biodiversity that gives a fuller comparison than species richness alone.

    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-down flowchart. The top node, colonising organisms on hostile ground, branches into three changes. First, dead remains decomposed by saprobionts lead to humus added, which holds water and releases mineral ions. Second, plant cover shades the surface, so the temperature range narrows and evaporation is reduced. Third, roots bind the substrate and reduce erosion. All three changes feed into one node: the abiotic environment becomes less hostile. That leads to more species being able to tolerate the conditions, then to biodiversity rising, measured as species richness and an index of diversity. A final node notes that diversity can fall again where a closed climax canopy shades out earlier species.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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