Ecological Succession (Edexcel International A Level Biology)

• Ecosystems are dynamic, meaning that they are constantly changing
• The process of ecosystem change over time is known as succession
  
  • During succession, the biotic and abiotic conditions change 
• Primary succession is the process that occurs when newly formed or newly exposed land is inhabited by an increasing number of species
  • Newly formed land can be created by e.g.
    • The magma from erupting volcanoes cooling and forming new rock surfaces or new rocky islands in the sea
  • Newly exposed land can form by e.g.
    • A landslide that exposes bare rock
    • A glacier that retreats to reveal bare rock
• The arrival of organisms on bare land is known as colonisation, and the bare land is said to be colonised

Primary succession occurs in a series of stages

• Primary succession can occur on any type of bare land, including sand dunes at the edge of the ocean, and on exposed rock
• Primary succession on bare rock involves the following stages
  • Seeds and spores that are carried by the wind land on exposed rock and begin to grow
    • The first species to colonise the new land, often mosses and lichens, are known as pioneer species
    • Pioneer species can germinate easily and withstand harsh conditions such as low nutrient and water availability 
  • As pioneer species die and decompose, the dead organic matter forms soil
  • Seeds of small plants and grasses land on this soil and begin to grow
    • The plants at this early stage of succession are adapted to survive in shallow, nutrient-poor soils
  • The roots of these small plants form a network that helps to hold the soil in place and prevent it from being washed away
  • As these small plants die and decompose, the soil becomes deeper and more nutrient-rich 
  • Larger plants and shrubs, as well as small trees can now begin to grow
    • These larger plants and small trees also require more water, which can be stored in deeper soils
  • Over time the soil becomes sufficiently deep, contains enough nutrients, and can hold enough water to support the growth of large trees
  • The final species to colonise the new land become the dominant species of the now complex ecosystem
  • The final community formed, containing all the different plant and animal species that have now colonised the land, is known as the climax community
    • The type of climax community that forms depends on the location of the original bare land; in the tropics the climax community would be a rain forest, while in temperate regions it might be deciduous woodland
    • A climax community is not always the most biodiverse stage of succession, but it is a stable community

Primary succession is the process of ecosystem change over time, beginning with newly formed or newly exposed land

Succession changes the biotic and abiotic conditions

• At each stage in succession the newly arriving species change the local environment so that it becomes more suitable for other species that have not yet colonised the new land e.g.
  • Pioneer species such as lichens help to slowly break apart the top surface of bare rock; this fragmented rock, along with the dead organic matter left behind when the lichens die and are broken down, forms a basic soil 
  • Species such as grasses grow roots that stabilise the soil, enabling it to hold more moisture and nutrients
• Often the new colonising species then change the environment in such a way that it becomes less suitable for the previous species e.g.
  • Lichens cannot grow on soil so they disappear from the ecosystem once soil begins to form; the new species change the environment in such a way that it becomes less suitable for the lichens
    • Pioneer species may not be found in a climax community as they will be out-competed for light and other resources by the species that arrive during the later stages of succession
    • Pioneer species are well adapted for harsh conditions but are often poor competitors
  • As soil deepens and trees are able to grow, they may block out the light to shrubs and other smaller plant, out-competing them and causing them to die

Secondary succession

• There is also a type of succession called secondary succession which takes place on previously occupied land e.g. after a wild fire or deforestation
  • Secondary succession is very similar to primary succession except that soil is already present so the process begins at a later stage

Humans can prevent succession

• Human activities often prevent or interrupt the process of succession
• This stops a climax community from developing e.g.
  • Regular mowing prevents woody plants from establishing themselves in a lawn
  • The grazing activity of livestock such as sheep and cattle prevent new plants from establishing
• Climax communities that develop as a result of human intervention are known as a plagioclimax; these communities are stable but would not have occurred without human intervention, e.g. heathland