Succession (AQA A Level Biology): Revision Note

Primary succession

• Succession is the gradual change in an ecosystem over time, from a simple to a more complex structure

• It involves shifts in both biotic and abiotic conditions

  • This makes the environment less hostile and more suitable for new species

  • At the same time, it may become less suitable for previous species

  • As a result, biodiversity changes continually

• Primary succession occurs on newly formed or exposed land with no initial life (e.g. bare rock from cooled lava or dried-up lakebeds)

The stages of primary succession

1. Firstly, seeds and spores that are carried by the wind land on the exposed rock and begin to grow

   • These first species to colonise the new land (often moss and lichens) are known as pioneer species

   • Their death and decay form basic soil (humus)

2. Seeds of small plants and grasses, carried in the wind, in bird faeces etc, land on this basic soil and begin to grow

   • Their death and decay further increases the depth and nutrient content of the soil

   • Their roots help to hold the soil in place and prevent it from being washed away

3. Larger plants, shrubs and small trees can now begin to grow in the less hostile conditions (deeper soil, more nutrients and more water)

4. Finally, the soil is able to support growth of large trees

   • The final, dominant species form part of a climax community– a stable, complex ecosystem with a variety of plant and animal species

Management of succession in conservation

Human activities and succession

• Succession is the natural process where ecosystems change over time, often leading to a climax community

• Human activities such as mowing and grazing interrupt succession, maintaining ecosystems in earlier stages

  • Mowing: prevents shrubs and trees from establishing; only grasses persist

  • Grazing: livestock eat new shoots, halting succession and maintaining grass-dominated areas

Managing succession for conservation

• Conservation often involves halting succession to protect species diversity

  • This could be necessary where Intermediate stages of succession (e.g. grassland, heathland) support many plant and animal species not found in climax communities

    • This is because dominant species in climax communities can outcompete other species, or changes in abiotic conditions lead to an environment not suited to some species

  • Intermediate habitats can be also important for rare or threatened species, including pollinators like bees

• For example, the Scottish moorlands

  • Naturally would progress to spruce forest via succession

  • Moorlands support unique biodiversity not found in spruce forests

  • Management involves maintaining both climax forests and earlier moorland to maximise species diversity

Methods to prevent succession

• There are a few different ways that succession can be deliberately prevented for conservation purposes. For example:

  • Grazing: This involves introducing animals to eat tree/shrub shoots, halting succession

  • Managed burning: Where controlled fires are used to remove woody plants, allowing species like heather to regrow and resetting succession