Ecological Succession (HL) (HL IB Biology)

What is ecological succession?

• Ecosystems are dynamic, meaning that they are constantly changing
• This process of change in an ecosystem is known as ecological succession
• Succession can be defined as

Progressive change in the species that make up an ecological community over time

What can trigger succession?

• The process of succession can be triggered by abiotic and biotic factors
  • An abiotic factor could be volcanic activity that creates new land on which an ecosystem can develop, or a fire that destroys part of an existing ecosystem, leaving unfilled niches for the development of a new community
  • A biotic factor could be the death of an organism, followed by its decomposition, altering the soil and allowing a new series of species to survive

• Primary succession is succession 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

Changes during primary succession

• 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 new organisms 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

Succession diagram

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

The general principles of primary succession

• As succession progresses, the following can always be observed:
  
  • Larger plant species can be supported
  • There is an increase in the primary production of a community
  • Species diversity increases
  • The complexity of food webs increases
  • Increased nutrient cycling

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

• The succession process described above results in a stable climax community that, if left undisturbed, will remain unchanged over time
• In some ecosystems, however, this stable equilibrium state does not occur and there is a changing cycle of communities rather than a single, unchanging climax community; this is known as cyclical, or cyclic, succession
  • Note that these cycles occur over long time periods, e.g. the lifetime of an oak tree, which can be hundreds of years
• An example of cyclical succession can be seen in grazed woodlands, which can contain a changing mosaic of grassland, woodland, and scrub
  • This habitat can be described as wood pasture
• In a grazed wood pasture, the following cycle of events can be observed
  • Grazers, such as cattle, graze on pockets of open grassland within a woodland, where they consume tree seedlings and prevent the growth of trees
  • Fast-growing thorny species, such as brambles, may arise in some areas within the grassland, creating a mosaic of thorny scrub
  • Tree seedlings can survive within the thorny vegetation, which deters grazers
  • Trees grow up through the thorny scrub, which will eventually fail to get enough light through the tree canopy and die off
  • At the end of its life the tree dies and falls, creating an open area where grass can grow, and restarting the cycle

Cyclic succession diagram

Succession can occur in cycles over many hundreds of years, e.g. within a grazed woodland