9.7BTrophic levels
Biology only — not on the Combined Science specification
Trophic levels can be represented by numbers, starting at level 1 with plants and algae. Further levels are numbered according to how far the organism is along the food chain.
Decomposers sit outside the numbering but do essential work. They break down dead plant and animal matter by secreting enzymes into the environment; small soluble food molecules then diffuse into the microorganism.
That is digestion turned inside out — the enzymes go out to the food rather than the food coming into a gut — and it is the detail questions ask for.
- Trophic level
- a numbered position in a food chain, starting at level 1 with plants and algae, with further levels numbered by how far along the chain the organism feeds
- Decomposer
- an organism that breaks down dead plant and animal matter by secreting enzymes into the environment, after which small soluble food molecules diffuse into the microorganism
9.7BPyramids of biomass
Biology only — not on the Combined Science specification
Pyramids of biomass represent the relative amount of biomass in each level of a food chain. Trophic level 1 is at the bottom of the pyramid.
To construct one accurately from data: draw each bar centred on the same vertical axis, with its width in proportion to the biomass at that level. Choose a scale that makes the smallest bar visible, label each bar with its trophic level and organism, and give the units of biomass. Accuracy means the bars are to scale — a pyramid drawn by eye is not a pyramid of biomass.
- Pyramid of biomass
- a diagram representing the relative amount of biomass in each level of a food chain, with trophic level 1 at the bottom and each bar drawn to scale
9.7B–9.8BTransfer of biomass
Biology only — not on the Combined Science specification
Two figures to know exactly.
Producers are mostly plants and algae, which transfer about 1% of the incident energy from light for photosynthesis.
Only approximately 10% of the biomass from each trophic level is transferred to the level above it.
Losses of biomass are due to:
• not all the ingested material being absorbed — some is egested as faeces • some absorbed material being lost as waste, such as carbon dioxide and water in respiration, and water and urea in urine.
And large amounts of glucose are used in respiration, which is where most of the rest goes: it is transferred to the surroundings rather than built into the animal's body.
Biomass is a mass, so it is measured in units such as g/m² or kg — not in joules. Efficiency can be given as a percentage or as a fraction of mass, and questions ask for either.
Worked example
A field of grass has a biomass of 40,000 g/m². The rabbits feeding on it have a biomass of 3,600 g/m². Calculate the efficiency of biomass transfer, as a percentage and as a fraction.
- Efficiency = (biomass at the higher level ÷ biomass at the lower level) × 100.
- 3,600 ÷ 40,000 = 0.09.
- As a percentage: 0.09 × 100 = 9%.
- As a fraction of mass: 3,600/40,000 simplifies to 9/100.
Answer: 9%, or 9/100 of the mass — close to the approximate 10% figure, with the rest lost as faeces, urine, and carbon dioxide and water from respiration.
This is also why food chains are short and why there are fewer organisms at each level. With only about a tenth of the biomass passing up each step, there is very little left to support a fifth or sixth level — an apex predator would need an impossibly large area beneath it. Explaining the number of organisms at each level means saying that, not just observing that the pyramid narrows.
Common mistake
The 10% is biomass transferred, not "energy lost as heat" alone. The losses are specific and examinable: material egested as faeces because it was never absorbed, waste lost as carbon dioxide and water in respiration and as water and urea in urine, and glucose used up in respiration.
- Biomass
- the mass of living material, measured in units such as g/m² or kg; only about 10% of the biomass at each trophic level is transferred to the level above
Before you practise
- Level 1 is the producer; then primary, secondary and tertiary consumers; decomposers break down the dead material at every level.
- A pyramid of biomass is drawn to scale with the producer at the bottom, and is always a pyramid.
- Around 10% of the biomass is transferred to the next level, so food chains are rarely longer than four or five levels.
- Energy is lost in respiration, in movement and heat, and in material that is never eaten or is egested and excreted.
- efficiency = (biomass transferred ÷ biomass available) × 100.
- Read carefully whether the question is about biomass or about energy, and use the figures it gives.