Primary & Secondary Succession (Cambridge (CIE) AS Environmental Management)

Primary & Secondary Succession

• Ecosystems are dynamic, meaning that they are constantly changing

• Sometimes, ecosystems change from being very simple to being relatively complex:

  • This process is known as succession

  • During succession, the biotic conditions (i.e. the living factors) and the abiotic conditions (i.e. the non-living factors) change over time

• Primary succession is the process that occurs when newly formed or newly exposed land (with no species present) is gradually colonised (inhabited) by an increasing number of species

• This new uninhabited land can be created in several ways. For example:

  • The magma from erupting volcanoes cools and often leads to the formation of new rock surfaces or even new rocky islands in the sea

  • Another way new land can be exposed is by sea-level dropping or the drying up of a lake, leaving areas of bare rock

• Primary succession does not only occur on bare rock. Any barren terrain that is slowly being colonised by living species is undergoing primary succession. For example:

  • Sand dunes in coastal areas (marram grasses are the pioneer species in these environments as they have deep roots to access water that other plants can't reach and are able to tolerate the salty environment (i.e. the high concentrations of sodium and calcium ions caused by sea spray)

Primary Succession Occurs in a Series of Stages

• 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

  • As these pioneer species die and decompose, the dead organic matter (humus) forms a basic soil

• Seeds of small plants and grasses, sometimes also carried in the wind or sometimes transported other ways (e.g. in bird faeces) land on this basic soil and begin to grow (these smaller plants are adapted to survive in shallow, relatively nutrient-poor soils):

  • As these small plants and shrubs die and decompose, the new soil becomes deeper and more nutrient-rich

  • The roots of these small plants and shrubs also form a network that helps to hold the soil in place and prevent it from being washed away

• Larger plants and shrubs, as well as small trees, that require deeper, more nutrient-rich soil, can now begin to grow:

  • These larger plants and small trees also require more water, which can be stored in deeper soils

• Finally, the soil is sufficiently deep, contains enough nutrients and can hold enough water to support the growth of large trees:

  • These final species to colonise the new land become the dominant species of the now relatively complex ecosystem

  • The final community formed, containing all the different plant and animal species that have now colonised the new land, is known as the climax community

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, for example:

  • 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, for example:

  • 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 plants, 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 that has been cleared of vegetation and animal species e.g. after a wild fire or after heavy deforestation has occurred:

  • Secondary succession is very similar to primary succession except that soil is already present so the process begins at a slightly later stage

Comparison of Pioneer and Climax Communities