Ecological Succession

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Ecological Succession


  • Ecosystems are made up of biotic and abiotic factors in a particular area within which there are a number of communities of organisms.
  • Ecosystems constantly change, sometimes slowly and sometimes very rapidly.
  • Succession is the term used to describe these changes, over time, in the species that occur in a particular area.
  • There are two types of succession; primary succession and secondary succession.
  • Primary succession is the progressive colonisation of bare rock or other barren terrain by living organisms.
  • This bare rock or other barren terrain can occur as a result of:
  • A glacier retreating and deposting rock.
  • Sand being piled into dunes by wind or sea.
  • Volcanoes erupting and depositing lava.
  • Lakes or ponds being created by land subsiding.
  • Silt and mud being depositing at river estuaries.
  • Secondary succession is the recolonisation of an area aferter an earlier community has been removed or destroyed, e.g. clearance of land for agricultural purposes or a forest fire.

Primary Succession

  • The first stage of primary succession is the colonisation by organisms called pioneer species.
  • Pioneer species often have features that suit them to colonisation; these may include:
  • The production of vast quantities of wind-dispersed seeds or spores, so they can easily reach isolated situations such as volcanic islands.
  • Rapid germination of seeds.
  • The ability to photosynthesise, as light is normally available by other 'food' is not. They therefore are not dependent on animal species.
  • The ability to fix nitrogen from the atmosphere because, even if there is soil, it has few or no nutrients.
  • Tolerance to extreme conditions.
  • Succession takes place in a series of stages, which at each stange a change in environment can be identified, especially in the soil, so that it becomes more suitable for other species.
  • These other species may then out-compete the species in the exisiting community and so a new community is formed.

Primary succession occurs as follows:

  • The abiotic conditions are hostile, so only a pioneer species can colonise and grow because, they are specialised to cope with the harsh conditions.
  • The pioneer species change the abiotic conditions - they die and saprobiotic microorganisms decompose their dead organic material to form humus, which then forms a basic soil.
  • This makes conditions less hostile, which allows new organisms to move in and grown.
  • As these new organisms die they are decomposed, adding more organic material to the soil, making it deep and contain more nutrients.
  • At each stage, different plants and animals that are


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