Succession (DP IB Environmental Systems & Societies (ESS))

Succession

• Ecosystems are dynamic, meaning that they are constantly changing

• 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

  • When glaciers retreat, they also leave bare rock or moraines

• Primary succession does not only occur on bare rock—any barren terrain that is slowly being colonised by living species is undergoing 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

    • They are also able to tolerate the salty environment i.e. the high concentrations of sodium and calcium ions caused by sea spray

• Secondary succession is a very similar process but happens on bare soil where there has been a pre-existing community, such as:

  • An agricultural field that has stopped being used

  • A forest area after an intense forest fire

The stages of succession

• A seral community (also known as a sere) is a temporary and intermediate stage in the ecological succession of an ecosystem

  • Each seral community, in succession, causes changes in environmental conditions

  • These changes allow the next community to replace it (e.g. through competition) until a stable climax community is reached

• First, seeds and spores carried by the wind land on the exposed rock and start 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 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

Changes occurring during succession

• As the the structure and species composition of an ecosystem changes during succession, so do the patterns of energy flow, productivity, species diversity and nutrient cycling that ecosystem

1. Energy flow:

   • During the early stages of succession, the energy flow in the ecosystem is relatively low

   • This is because there are only a few species present and most of the energy is used to build biomass

   • As the ecosystem becomes more complex, energy flow increases

2. Productivity:

   • During the early stages of succession, gross productivity and net productivity are low because there are only a few species present

   • This means the ecosystem’s overall gain in energy and biomass per unit area per unit time is relatively small

   • As the ecosystem becomes more complex, gross productivity and net productivity increase

3. Species diversity:

   • Diversity refers to the number of species present in an ecosystem

   • During the early stages of succession, diversity is low because there are only a few species present

   • As the ecosystem becomes more complex, diversity increases because there are more niches available

   • This means more species are able to coexist within the same habitats in the ecosystem

4. Nutrient cycling:

   • Nutrient cycling refers to the movement of nutrients through an ecosystem

   • During the early stages of succession, nutrient cycling is relatively simple

   • This is because there are only a few species present and abiotic processes dominate nutrient cycling

   • As the ecosystem becomes more complex, nutrient cycling becomes more complex

   • This is because there are more species present and each species has unique nutrient requirements and cycling processes