7.4 Trophic Levels

• What are trophic levels used to describe?

  The position of an organism in a food chain, web or pyramid
• How are trophic levels represented?

  By numbers, starting at level 1 with plants and algae
• What is trophic level 1?

  Producers - plants and algae who produce their own organic nutrients and biomass using energy from sunlight
• What is trophic level 2?

  Primary consumers - herbivores which feed on producers
• What is trophic level 3?

  Secondary consumers - carnivores which eat herbivores
• What is trophic level 4?

  Carnivores that eat other carnivores
• How does energy flow through trophic levels?

  • Energy flows from the sun to the first trophic level in the form of light
  • Producers convert light energy to chemical energy during photosynthesis
  • The chemical energy is transferred to consumers as they consumer consumers
• What are apex predators?


  Carnivores with no predators, who are at the very top of the food chain
• How is the chemical energy stored within apex predators passed on?

  They are passed on to decomposers when apex predators die and are decomposed
• How do decomposers break down dead plant and animal material?

  • 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
• What is the purpose of decomposition?


  To break down dead plant and animal material and release organic nutrients back into the environment (soil) which are essential for the growth of plants (producers)
• What is biomass?


  • The mass of living tissues (or recently dead tissues not yet decomposed) that a plant or animal is made up of.
  • It is the 'dry mass' of the organism (how much the mass the plant or animal would have without including all the water in the organism)
• What happens to biomass as you move up through the trophic levels in a food chain or food web?

  It decreases
• What does a pyramid of biomass show?


  The relative biomass (mass of living or recently dead material) at each trophic level in a food chain
• What type of energy is biomass a store of?

  Chemical energy
• How is biomass transferred from one trophic level to the next?

  By the transfer of chemical energy:

  • When primary consumers consume producers, they break down the biomass of the producer (digestion) and use the chemical energy to increase or sustain their own biomass
  • This is repeated with secondary consumers and so on
• Approximately, what percentage of the biomass of each trophic level is passed on?

  10 %
• Why are food chains rarely made up of more than 6 trophic levels?

  Because the total amount of biomass available eventually becomes too small to support another trophic level
• Approximately what percentage of the incident energy from photosynthesis is passed on from producers?

  1 %
• Give 3 ways biomass is lost

  • Not all the ingested material is absorbed
  • Some is egested as faeces
  • Some absorbed material is lost as waste
  • Carbon dioxide and water are waste products of respiration
  • Water and urea are the waste products in urine
  • Some of the material is inedible (e.g. bones)
• How do you calculate percentage efficiency of biomass transfer from trophic levels?

  A) biomass in higher trophic level

  B) biomass in lower trophic level

  
• Why is dry biomass more useful than wet biomass?

  The moisture content in organisms can vary widely and produce inaccurate results