Trophic levels & Pyramids of biomass & Transfer of biomass (all biology only)

Lesson 7.11 · AQA 8461

Trophic Levels, Pyramids of Biomass & Transfer of Biomass

Spec 4.7.4.1, 4.7.4.2, 4.7.4.3 · Paper 2 · ~10 min

Biology only

What you'll be able to do

By the end of this lesson, you'll be able to:

Objective 1 4.7.4.1

Describe the four trophic levels in a food chain — and explain how decomposers fit into the ecosystem.

Objective 2 4.7.4.2

Construct accurate pyramids of biomass from given data, with proportional bar widths and Level 1 at the bottom.

Objective 3 4.7.4.3

Explain biomass losses between trophic levels, and calculate transfer efficiency as a percentage or fraction.

The four trophic levels

Trophic levels are numbered starting at Level 1, with the position of each organism determined by how far along the food chain it sits.

LevelNameDescription

1ProducersPlants and algae — make their own food by photosynthesis.

2Primary consumersHerbivores — eat plants/algae.

3Secondary consumersCarnivores that eat herbivores.

4Tertiary consumersCarnivores that eat other carnivores.

Apex predator

A carnivore with no predators of its own. Sits at the top of its food chain — may be at Level 3, 4 or higher depending on the chain.

Decomposers

Decomposers (mostly bacteria and fungi) break down dead plant and animal material — but they feed differently from consumers.

Spec language

Decomposers secrete enzymes into the environment to break down dead material. The small soluble food molecules then diffuse into the microorganism.

A food chain in action

Each arrow points in the direction of biomass and energy flow — from the organism being eaten to the one eating it.

Note

Decomposers don't have a single trophic level — they feed on dead material from all levels, recycling minerals back to producers (covered in Lessons 7.06 and 7.07).

What a pyramid of biomass shows

A pyramid of biomass shows the relative amount of biomass at each trophic level of a food chain. The biomass is the total mass of living material.

Read the diagram correctly

This is schematic: it shows the characteristic pyramid shape. The biomass values drop roughly 10× per level (the 10% rule), so a true-to-scale drawing of these figures is impossible on one page — see the next slide on choosing a scale.

Three rules

Trophic Level 1 sits at the bottom (the producers).

Bars are centred on a vertical line and drawn symmetrically.

Bar widths are proportional to the biomass — not arbitrary.

Constructing a pyramid

You're given biomass values. To get the marks for an accurate pyramid:

Method

Choose a scale that lets the largest bar fit on the paper. Use the biomass at Level 1 to set this.

Calculate each bar width in proportion to its biomass.

Centre each bar on a vertical line; draw it symmetrically.

Label each level (organism name) and add a scale or axis label (e.g. biomass / kg).

Why scale matters

If your largest bar is, say, 8 cm long for 10,000 g/m², then the Level 2 bar (1,000 g/m²) is 0.8 cm, and Level 3 (100 g/m²) is just 0.08 cm — too small to draw at this scale. AQA accepts pyramids drawn so each level is visible provided the construction rule is consistently applied.

Common mark loss

Drawing all bars the same width, or putting Level 1 at the top. Both score zero on the construction marks.

Where the marks come from (typical 3-mark construct)

Past mark schemes for "complete the pyramid" questions allocate marks roughly as:

1 mark — suitable scale (largest bar uses most of the available width)

1 mark — axis label (e.g. "biomass / kg") and bars labelled with the organism

1 mark — correct values plotted, bars centred and symmetrical

Where biomass starts — the 1% rule

Every food chain begins with producers. They capture light energy and convert it to biomass through photosynthesis.

~1%

of incident light energy is transferred by producers (plants/algae) into biomass via photosynthesis

~99%

of light energy is reflected, missed by chlorophyll, or transmitted through leaves

Spec wording

"Producers are mostly plants and algae which transfer about 1% of the incident energy from light for photosynthesis."

In aquatic chains

The producers are usually algae and phytoplankton — they perform exactly the same role as land plants.

Between trophic levels — the 10% rule

Once biomass is in the food chain, only a small fraction passes from each level to the next.

~10%

of biomass is transferred from each trophic level to the level above it

~90%

is lost — egested as faeces, and as CO₂/water in respiration and water/urea in urine

Where the biomass at one level goes

Schematic proportions — respiration is typically the largest loss (large amounts of glucose used in respiration). Only the small dark-green portion is passed to the next level.

Worked through a chain

Start with 10,000 g of producer biomass per m². At each step we lose ~90%:

10,000 g → 1,000 g → 100 g → 10 g

producer · primary · secondary · tertiary

Why biomass is lost between levels

The AQA spec lists three categories of loss. Use the spec wording.

Reason 1 — Egestion

Not all the ingested material is absorbed — some is egested as faeces. Things like cellulose in plant cell walls pass straight through animals that can't digest them.

Reason 2 — Respiration

Absorbed material is lost as waste — carbon dioxide and water are released during respiration. Large amounts of glucose are used in respiration to release energy for life processes.

Reason 3 — Excretion in urine

Absorbed material is also lost as water and urea in urine — the waste products of protein breakdown excreted by the kidneys.

Examiner alert

"Energy is lost" alone is not enough. Name the specific route — egestion as faeces, CO₂/water in respiration, water/urea in urine.

What the 10% rule means for ecosystems

The 10% rule explains a key feature of food chains — and you may be asked to explain how biomass transfer affects the number of organisms at each trophic level.

Consequence 1 — Numbers decrease up the chain

Less biomass at each level means fewer organisms can be supported at higher levels. There's enough grass for many rabbits, but only enough rabbits for a few foxes.

Consequence 2 — Food chains are short

Most food chains have only 3–5 trophic levels. The numbers make this clear — starting from 10,000 g/m² of producer biomass and applying the 10% rule:

L1: 10,000 g → L2: 1,000 g → L3: 100 g → L4: 10 g → L5: 1 g

A Level 5 predator has only 0.01% of the original producer biomass to feed on — not enough to support a viable population.

Linked exam idea (food security)

Eating lower in the food chain (e.g. grain rather than animals fed on grain) means more food is produced per unit of land — because no biomass is lost at the animal step. Comes back in Lesson 7.12.

Calculating transfer efficiency

You need to calculate biomass transfer efficiency as a percentage or fraction. Same formula either way.

Biomass transfer efficiency efficiency (%) = (biomass at higher level ÷ biomass at lower level) × 100

The same answer can be written as % · fraction · decimal · ratio . For 800 kg / 8,000 kg: 10% · 1/10 · 0.1 · 1 : 10 .

Worked example

A field of grass has a biomass of 8,000 kg. Sheep grazing the field gain a total of 800 kg of body mass over a year.1Identify higher level (sheep, 800 kg) and lower level (grass, 8,000 kg).2Apply formula: (800 ÷ 8,000) × 1003= 10% — right in the typical range for a natural ecosystem.4As a fraction of mass: 800 / 8,000 = 1/10 — the same answer, just unscaled.

Typical ranges

Natural ecosystems: ~10%. Intensive farming (e.g. grain-fed poultry) where animals expend less energy moving or staying warm: 30–50%+. Always work from the data in the question — don't default to "10%" if the numbers say otherwise.

Direction matters

It's higher ÷ lower, never the other way round. Doing 8,000 ÷ 800 gives 10 (or 1,000%) — a nonsense answer that loses every mark on the question.

Explain biomass loss

In a food chain, only about 10% of the biomass at one trophic level is transferred to the level above.

Explain why so much biomass is lost between trophic levels. 3 marks

1

Not all ingested material is absorbed — some is egested as faeces. (1)

2

Absorbed material is lost as waste — carbon dioxide and water in respiration (large amounts of glucose are used in respiration). (1)

3

Absorbed material is also lost as water and urea in urine. (1)

Three separate routes = three marks. A vague "energy/biomass is lost" with no named route scores zero. Name faeces, respiration products, and urine.

Calculate efficiency

A crop of clover has a biomass of 2,500 kg. Rabbits feeding on the clover have a total biomass of 375 kg.

Calculate the efficiency of biomass transfer from the clover to the rabbits. Give your answer as a percentage. 2 marks

1

efficiency = (biomass at higher level ÷ biomass at lower level) × 100 = (375 ÷ 2,500) × 100 (1)

2

= 15%(1)

15% is above the ~10% rule of thumb — and that's fine. The 10% figure is only "approximate"; real values vary. Always work from the data given, not the rule of thumb.

Multi-step calculation

A farmer raises 200 lambs on a pasture. Each lamb reaches a final mass of 25 kg. Grass yield on this pasture is 4,000 kg per hectare. Assume biomass transfer from grass to lamb is 10%.

Calculate the area of pasture, in m², needed to grow enough grass. Give your answer to 3 significant figures. (1 hectare = 10,000 m²)5 marks

1

Total lamb biomass = 200 × 25 = 5,000 kg

2

Grass biomass needed — apply 10% rule in reverse: 5,000 ÷ 0.10 = 50,000 kg

3

Hectares needed = 50,000 ÷ 4,000 = 12.5 hectares

4

Convert to m²: 12.5 × 10,000 = 125,000 m²

5

Final answer to 3 sig figs: 125,000 m² (or 1.25 × 10⁵ m²)

Marks are awarded for each step — show every line of working. Even if your final arithmetic slips, you can still score 3–4 out of 5 from correct setup.

How decomposers feed

A pile of dead leaves in a forest is slowly broken down by bacteria and fungi.

Describe how a decomposer such as a bacterium gets its nutrients from the dead leaves. 3 marks

1

The decomposer secretes enzymes out of the cell into the dead material. (1)

2

The enzymes break down large insoluble molecules into small soluble molecules. (1)

3

The small soluble molecules diffuse into the microorganism. (1)

This is called extracellular digestion — digestion outside the cell. All three steps (secrete · break down · diffuse in) are creditable. "Decomposers eat dead things" scores zero — the mechanism is what's marked.

Examiner alerts

Alert 1 — Use the spec language

Spec losses are egested as faeces, CO₂ and water in respiration, water and urea in urine. Vague answers like "lost as energy" lose the mark.

Alert 2 — Show working in calculations

Pattern 12 calculations carry up to 5 marks. Every step has a mark. Final answer alone earns full marks only if correct — but partial working earns partial credit if it's wrong.

Alert 3 — Mind the units and sig figs

The question tells you the units and sig figs required. Re-read the final sentence before writing the answer. "3 significant figures" and "m²" are part of the marking.

Alert 4 — Decomposers use enzymes outside the cell

If asked how decomposers feed, the marks are for enzymes secreted + small soluble molecules diffuse in. Saying "they eat dead things" scores zero.

Common errors — fix these now

✗ Weak

"Producers are at the top of the pyramid."Wrong orientation — loses construction marks.

✓ Strong

"Level 1 (producers) is at the bottom of the pyramid."Spec rule.

✗ Weak

"Energy is lost between levels."Vague — no specific route named.

✓ Strong

"Biomass is lost as faeces, as CO₂ and water in respiration, and as water and urea in urine."Spec list, three routes.

✗ Weak

"Efficiency = (8,000 ÷ 800) × 100 = 1,000%"Division upside-down.

✓ Strong

"Efficiency = (800 ÷ 8,000) × 100 = 10%"Higher level ÷ lower level.

✗ Weak

"Energy is lost as urine."Mixes the terms — energy isn't a substance "in" urine.

✓ Strong

"Biomass is lost as water and urea in urine; energy is lost as heat from respiration."Biomass = material routes (faeces, urine, CO₂/water). Energy = heat from respiration.

Recap — three things to remember

1 · The two key efficiencies

~1% of incident light energy is transferred by producers via photosynthesis

~10% of biomass passes from one trophic level to the next

2 · Three biomass losses (spec language)

Egestion — material not absorbed, lost as faeces

Respiration — CO₂ and water released (large amounts of glucose used)

Urine — water and urea excreted

3

The efficiency formula

efficiency (%) = (biomass at higher level ÷ biomass at lower level) × 100

Always higher ÷ lower. Show every step in multi-mark calculations.

Next lesson

7.12 — Food security & farming techniques. Biology only. We'll see how the 10% rule directly affects how humans produce food, and how factory farming pushes transfer efficiency higher.