4.7.4 Trophic levels in an ecosystem (biology only)

4.7.4.1 Trophic levels

Exchange in Organisms

• There is a food chain in ecosystems.
• This consists of:
  • producers
  • primary consumers
  • secondary consumers
  • tertiary consumers
• These are known as trophic levels.

Producers

• Producers provide energy to the ecosystem by photosynthesising to produce food.
  • They are general algae and plants.
  • This is the first trophic level.

Primary Consumers

• Primary consumers are herbivores.
  • These are organisms that eat only plants.
  • Examples include deer and cows.
  • This is the second trophic level.

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Secondary Consumers

• Secondary consumers get their energy from primary consumers.
  • Therefore, organisms at this level of organisation are called carnivores.
  • These types of organisms eat only animals.
  • This is the third trophic level.

Tertiary Consumers

• Tertiary consumers eat the secondary consumers.
  • Organisms at this trophic level are also carnivores.
  • This is the fourth trophic level.
  • Organisms at this level are the apex predators.
• They are prey to no one.

Efficiency of Food Chains

• Only around 10% of energy is transferred along each trophic level.
  • This means that food chains are highly inefficient.

Decomposers

• Decomposers break down organic matter.
  • They do this by secreting enzymes into the environment.
  • Soluble food molecule then diffuse into the decomposer organism.

4.7.4.2 Pyramids of biomass

Biomass

• Biomass is tissue that is living or recently dead.

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Pyramids of Biomass

• You can use pyramids of biomass to map out the trophic levels in a food chain.
  • The biomass should decrease as you go up trophic levels.
  • When you construct these graphs, there are a few rules:

1. The producer is at the bottom of the pyramid, at trophic level 1

2. You must construct the pyramid to scale.

3. The bars must be equally spaced from the midpoint

4. The bars must be touching

4.7.4.3 Transfer of biomass

Biomass

• Pyramids of biomass are used to show the transfer of biomass.
• In this pyramid, we measure the length of the bars to work out the biomass that is transferred.

Lost Biomass

• Pyramids of Biomass
  • Only 10% of biomass is transferred.
  • 90% is lost through life processes.
  • This includes excretion of undigested material as faeces, respiration giving out carbon dioxide and loss of urea in urine.
  • Respiration uses a large amount of glucose and so lots of biomass is lost here.

Lost Biomass and Trophic Levels

Pyramids of Biomass

• The inefficiency of lost biomass means there are rarely more than 6 trophic levels per chain.
  • The energy transferred after this would be too negligible for an organism to be able to survive.

Producers and Biomass

• Producers form biomass through photosynthesis.
  • They harness ~1% of the energy from the sun’s rays.
  • The other 99% is lost in global warming, reflects off the plants or doesn’t reach them.

Biomass Transfer Calculations

• Efficiency of biomass transfer calculations can be done in order to measure the proportion of biomass transferred from one trophic level to another.
  • It is a classic percentage calculation, so you compare the two.
  • You divide the biomass of the higher trophic level by the biomass of the lower trophic level, then multiply this by 100.
  • This gives you a percentage.

• So if we had a trophic level 1 with a biomass of 20kg and a trophic level 2 with a biomass of 15kg, the efficiency of biomass would be 15/20 multiplied by 100.
  • This would give us 75%.