Trophic Levels and Biomass Transfer

A trophic level is a feeding stage in a food chain. Every organism occupies a level based on how it gets its energy, and the levels are numbered starting from 1.

The number rises as you move along the chain, away from the original light source captured by plants.

Level	Name	What it does	Example
1	Producer	Makes its own food by photosynthesis	Grass, algae, oak tree
2	Primary consumer	Herbivore — eats producers	Rabbit, caterpillar
3	Secondary consumer	Carnivore — eats herbivores	Fox, ladybird
4	Tertiary consumer	Carnivore — eats other carnivores	Eagle, sparrowhawk

A producer is an organism (a plant or alga) that makes its own food using light energy. Producers are always trophic level 1.

A carnivore at the top of a chain with no predators of its own is called an apex predator — for example, a lion or a sparrowhawk.

Each organism is grouped by how it obtains food, not just by what it eats.

• Producers (level 1) — plants and algae. They use light energy in photosynthesis to make glucose, building their own biomass from carbon dioxide and water.
• Consumers (levels 2 and above) — animals that cannot make their own food, so they eat other organisms.
  • Primary consumers are herbivores (eat producers).
  • Secondary consumers are carnivores that eat herbivores.
  • Tertiary consumers are carnivores that eat other carnivores.

Decomposers sit outside this numbered ladder but are vital to the ecosystem.

A decomposer (for example a bacterium or fungus) breaks down dead plant and animal matter by secreting enzymes into the environment. The small soluble molecules produced then diffuse back into the decomposer.

When decomposers break down dead material they release mineral ions back into the soil. Plants reabsorb these ions, so decomposers recycle the nutrients that producers depend on.

Biomass is the mass of living material in an organism (or in a whole trophic level), usually measured as dry mass.

A pyramid of biomass is a diagram showing the relative amount of biomass at each trophic level of a single food chain. It is drawn as a stack of horizontal bars, to scale, with each bar's width proportional to the biomass at that level.

In a pyramid of biomass, trophic level 1 (the producers) is always drawn at the bottom, and each level above it is narrower.

Because biomass falls sharply at each step up the chain, the bars get narrower as you go up, giving the classic pyramid shape:

Unlike a pyramid of numbers, a pyramid of biomass is almost always a true pyramid shape — even one large producer, such as an oak tree, still holds far more biomass than all the organisms feeding on it.

Pyramids must be drawn to scale, so you first choose a scale, then convert each biomass figure into a bar width.

Worked example — draw a pyramid for this food chain. Biomass is the dry mass per square metre:

Trophic level	Organism	Biomass (g/m²)
1	Grass	10000
2	Rabbits	1000
3	Foxes	100

Step 1 — choose a scale. Let 1 cm represent 2000 g/m².

Step 2 — convert each biomass to a bar width using $\text{width}=\frac{\text{biomass}}{2000}$:

Level	Biomass (g/m²)	Bar width (cm)
1 (grass)	10000	$10000÷2000=5.0$
2 (rabbits)	1000	$1000÷2000=0.5$
3 (foxes)	100	$100÷2000=0.05$

Step 3 — draw the bars centred on top of each other, widest (level 1) at the bottom:

Always state the scale next to the pyramid (for example "1 cm = 2000 g/m²"). A scaled pyramid with no key cannot be read correctly.

Two key figures describe how energy and biomass move through a chain.

At the start of the chain, producers capture only a small fraction of the light that reaches them.

Producers (mostly plants and algae) transfer about 1% of the incident energy from light into biomass through photosynthesis. The other ~99% is reflected, passes straight through the leaf, or is the wrong wavelength to be used.

Between trophic levels, only a small fraction of biomass passes upward.

Only approximately 10% of the biomass at one trophic level is transferred to the level above it.

Worked example — applying the 10% rule down a chain. Suppose the producers contain 10000 kJ of energy in their biomass. Taking 10% at each step:

Trophic level	Energy in biomass	How it is found
Producers (L1)	10000 kJ	starting value
Primary consumers (L2)	1000 kJ	10% of 10000
Secondary consumers (L3)	100 kJ	10% of 1000
Tertiary consumers (L4)	10 kJ	10% of 100

Each arrow up the chain multiplies the biomass by $0.1$. After three transfers only $10000\times 0.1\times 0.1\times 0.1=10$ kJ remains — just 0.1% of what the producers started with.

Roughly 90% of biomass is not passed to the next level. There are three reasons, and you should be able to explain all of them.

1. Not all of an organism is eaten. Predators leave bones, fur, roots or woody stems. That biomass never enters the next level (decomposers break it down instead).
2. Not all eaten material is absorbed. Some ingested food passes through the gut without being absorbed and is egested as faeces.
3. Absorbed material is used and lost as waste. Large amounts of glucose are used in respiration, releasing energy for movement, growth and keeping warm. The waste products leave the body:
   • carbon dioxide and water lost in respiration;
   • water and urea lost in urine (excretion).

Three losses to memorise: not all eaten, egestion (faeces), and respiration / excretion (urine). Warm-blooded animals lose especially large amounts to respiration because they use energy to maintain body temperature.

Only the biomass left over after all these losses — about a tenth — is built into the consumer's body and becomes available to the next level.

The efficiency of transfer between two levels is the percentage of available biomass that actually becomes biomass in the level above.

$\text{efficiency}=\frac{\text{biomass transferred to next level}}{\text{biomass available in this level}}\times 100$

Worked example 1. A field of grass has a biomass of 20000 kJ. The rabbits feeding on it build 1600 kJ of biomass. Find the efficiency.

$\text{efficiency}=\frac{1600}{20000}\times 100=8\%$

Worked example 2. Of 500 g of plant biomass available, 45 g is transferred into caterpillars.

$\text{efficiency}=\frac{45}{500}\times 100=9\%$

Both results are close to the ~10% figure, as expected.

Why this limits food chains. Because so little biomass passes up each step, there is not enough left to support many levels. After four or five transfers the remaining biomass is too small to sustain another population, which is why food chains rarely have more than 4 or 5 trophic levels, and why top predators are few in number. It also explains the pyramid shape: each level supports a smaller mass — and usually a smaller number — of organisms than the one below it.

(Extra context — not required by AQA 8461.) This is also why a given area of farmland feeds far more people growing crops directly than rearing animals on those crops: each trophic level added wastes ~90% of the biomass.

1. Saying producers transfer 10% of light energy

The 1% figure is for producers capturing incident light. The 10% figure is for biomass passing between trophic levels. Mixing these up is a frequent error — keep them separate.

2. Forgetting respiration as a loss

Many answers only mention faeces. Respiration is usually the largest loss: glucose is broken down to release energy, and carbon dioxide and water are lost. Always include it.

3. Confusing egestion with excretion

Egestion is the removal of undigested food as faeces (it was never absorbed). Excretion is the removal of metabolic waste the body produced, such as urea in urine. They are different losses.

4. Drawing a pyramid of biomass without a scale

The bars must be drawn to scale and the scale stated. An unlabelled sketch with arbitrary bar widths cannot be marked as an accurate pyramid of biomass.

5. Putting the wrong level at the bottom

Level 1 — the producers — is always the bottom, widest bar. Building the pyramid upside down loses marks even if the figures are right.

6. Slips in the efficiency formula

Divide biomass transferred by biomass available, then multiply by 100. Dividing the wrong way round (available ÷ transferred) gives a percentage above 100%, which is impossible for a transfer.