5.3.6 Trophic Levels

• Net primary production (NPP) is the chemical energy that is stored in the plant’s biomass (after respiratory losses have been taken into account)
  • This means NPP is the energy that is available to a plant for growth and reproduction

• NPP also represents the energy that is available to organisms in higher trophic levels in the ecosystem
  • Trophic levels describe the position of an organism in a food chain, web or pyramid
  • Trophic levels can be represented by numbers or by the name of that trophic level e.g. plants and algae are in trophic level 1, otherwise known as producers
  • Further trophic levels are numbered subsequently according to how far the organism is along the food chain

• Like plants and other producers, these consumers store chemical energy in their biomass

Trophic levels table

Trophic levels for a simple food chain - the blue arrows show how the chemical energy originally produced by the primary producer (grass) is transferred to other organisms in the community

Trophic levels for a simple food web – note that some organisms can belong to more than one trophic level (such as the squirrel, fox and eagle in this food web)

Decomposers

• The net primary production (i.e. the chemical energy) of producers is also available to another group of organisms known as decomposers
• The two main groups of decomposers are bacteria and fungi
• Decomposers carry out a very important function in ecosystems – they break down dead plant and animal material (in the process gaining the chemical energy still stored in the dead matter)
• They do this by:
  • Secreting digestive enzymes onto the surface of the dead organism
  • These enzymes break down the dead matter into small soluble food molecules
  • These molecules are then absorbed by the decomposers

• This process of decomposition also helps to release organic nutrients back into the environment (eg. the soil) which are essential for the growth of plants and other producers