Spatial & Temporal Changes in Communities (DP IB Environmental Systems & Societies (ESS))

Zonation & Succession

Zonation

• Zonation in ecology refers to the gradual change in the composition of species and communities across a landscape, based on a gradient of environmental factors such as:
  

  • Altitude

  • Latitude

  • Temperature

  • Moisture

  • Light

• As these factors change, the species present in an ecosystem also change, leading to distinct zones or bands of organisms that can be observed in the ecosystem

  • This process occurs due to the interactions between the physical environment and the biological components of an ecosystem

• An example of zonation can be observed in a rocky intertidal zone, where the physical and biological characteristics of the ecosystem change gradually from the high tide mark to the low tide mark

  • At the highest point (sometimes referred to as the spray zone), the zone is usually dry and dominated by lichen and other hardy plants that can withstand long periods of exposure to air and sunlight

  • In the high tide zone, the environment becomes more hospitable for other organisms such as barnacles, mussels, chitons, limpets and sea snails that can attach themselves to the rocks and withstand waves

  • Further down towards the low tide zone, the environment becomes even more favourable for marine organisms such as sea stars, anemones, and sea urchins that require the constant presence of water

An example of zonation in a rocky intertidal zone

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

• 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
    

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

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

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, diversity, and mineral cycling that ecosystem

• 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

• Gross and Net Productivity:
  

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

• 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, and more species are able to coexist within the same habitats in the ecosystem

• Mineral Cycling:
  

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

  • During the early stages of succession, mineral cycling is relatively simple as there are only a few species present and nutrient cycling is largely driven by abiotic processes

  • As the ecosystem becomes more complex, nutrient cycling becomes more complex because there are more species present and each species has unique nutrient requirements and cycling processes