AQA GCSE Biology (8461)

4.7.4 Trophic levels in an ecosystem (Bio only)

Trophic levels are the 'rungs of the ladder' in a food chain — and the central GCSE story is that the ladder gets thin very quickly. This page nails down what each level is called (producers, primary, secondary and tertiary consumers, apex predators), where decomposers fit in, and exactly why only about 10% of the biomass at one level survives to become biomass at the next. You will also practise the standard efficiency calculation: biomass transferred ÷ biomass available × 100. By the end you'll be able to identify trophic levels in any food chain, list the four marking phrases AQA wants for 'where biomass goes', and explain why food chains rarely have more than four levels.

From the 8461 specification

The trophic level of an organism is the position it occupies in a food chain. Level 1: Plants and algae make their own food and are called producers. Level 2: Herbivores that eat plants/algae are called primary consumers. Level 3: Carnivores that eat herbivores are called secondary consumers. Level 4: Carnivores that eat other carnivores are called tertiary consumers. Apex predators are carnivores with no predators. Decomposers break down dead plant and animal matter by secreting enzymes into the environment. Small soluble food molecules then diffuse into the microorganism. 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. Producers transfer about 1% of incident energy from light for photosynthesis. Approximately 10% of the biomass from each trophic level is transferred to the level above. Losses of biomass are due to: not all the ingested material is absorbed, some is egested as faeces; some absorbed material is lost as waste, such as carbon dioxide and water in respiration and water and urea in urine. Large amounts of glucose are used in respiration.

Why this matters

Energy enters living systems through a single front door: photosynthesis. Plants and algae use roughly 1% of the light that hits them to build glucose, and from there to build every other carbon-containing molecule in their bodies. Every animal, every fungus, every decomposer ultimately depends on this trickle of solar energy captured by producers. Because each consumer respires, moves, excretes, sheds and dies, the biomass available at each step shrinks dramatically. Ecologists capture this with the rule of thumb '10% transfers, 90% is lost' — a number that explains everything from why eagles are rare to why a hectare of wheat feeds more people than a hectare of beef cattle (which you'll meet in 4.7.5). The same losses apply at every step, so by the time you reach a fifth trophic level there is simply not enough biomass left to support a population. Decomposers — bacteria and fungi — recycle the bits that don't transfer, releasing mineral ions back to the soil and CO₂ back to the air so the cycle can begin again.

How to learn this topic

Build on what you already know

GCSE 4.4.1: photosynthesis — plants and algae build glucose from CO₂ and water using light energy.
GCSE 4.4.2: aerobic respiration — glucose + oxygen → CO₂ + water + ATP, releasing energy as heat.
GCSE 4.7.2: organisation of an ecosystem — producers, consumers, decomposers, food chains and food webs.
KS3: feeding relationships, energy transfer, the idea that 'food' is matter as well as energy.

Define trophic level as a numbered position in a food chain.
Name and exemplify levels 1 to 4 plus the apex predator label.
Place decomposers (bacteria + fungi) outside the levels but feeding from all of them.
Introduce the ~10% biomass transfer rule and the ~90% loss.
Itemise the losses: not consumed, faeces, respiration (heat), urine/urea.
Use the efficiency formula on a worked example.
Read and draw a pyramid of biomass to scale.
Explain why food chains are usually only 3-4 levels long.

Key terms

trophic level: The position of an organism in a food chain. Numbered upward from 1 (producers) to 2 (primary consumers), 3 (secondary consumers) and 4 (tertiary consumers). (Use the AQA word 'position'. Producers are level 1, NOT level 0.)
producer: A plant or alga (level 1) that makes its own biomass from inorganic CO₂ and water using light energy in photosynthesis. The entry point for energy and biomass into the ecosystem. (Marking phrase: 'producers transfer about 1% of light energy into biomass'.)
primary consumer: A herbivore (level 2) that eats producers. Examples: rabbit, cow, krill.
secondary consumer: A carnivore (level 3) that eats primary consumers. Examples: fox, blue tit, mackerel.
tertiary consumer: A carnivore (level 4) that eats other carnivores (secondary consumers). Examples: eagle, sparrowhawk, dolphin.
apex predator: A carnivore at the top of a food chain that has no natural predators. Often (but not always) the tertiary consumer. (AQA wording: 'carnivores with no predators'.)
decomposer: A microorganism — bacteria or fungus — that breaks down dead organic matter and waste by secreting enzymes into the environment and absorbing the small soluble molecules released. (Use the AQA wording: 'secrete enzymes', 'small soluble molecules diffuse into the microorganism'.)
biomass: The total mass of living material in an organism, population or trophic level. Measured in grams or kilograms. (Biomass DECREASES up the trophic levels — about a tenfold drop per step.)
pyramid of biomass: A scaled diagram with trophic level 1 at the bottom, in which the width of each horizontal bar is proportional to the biomass at that level. (Always draw to scale; producers at the bottom; bars narrow upwards.)
efficiency of biomass transfer: The percentage of biomass at one trophic level that is transferred to the next. Calculated as (biomass transferred ÷ biomass available) × 100. Typically about 10%. (Show the substitution AND the % — examiners want '10 ÷ 100 × 100 = 10%' written out.)
egestion: The removal of undigested material from the gut as faeces. Distinct from excretion (removal of metabolic waste like urea). (Faeces = egestion; urine = excretion. Don't muddle them.)
urea: Nitrogenous waste made in the liver from breaking down excess amino acids. Excreted in urine. Represents biomass lost from a consumer.

Notes

What a trophic level is

A trophic level is the position of an organism in a food chain — its 'feeding rung'. Levels are numbered upwards from the base. Two organisms are on the same level if they eat the same kind of food (plants, herbivores, etc.).

A food chain showing all four levels:

> grass → rabbit → fox → eagle

> L1 L2 L3 L4

The four trophic levels (AQA names)

Level 1 — producers. Plants and algae. They build their own biomass from inorganic CO₂ and water using photosynthesis. Producers transfer only about 1% of the light energy that hits them into chemical energy in glucose — but this is the entry point for energy into the whole ecosystem. Examples: grass, oak trees, seaweed, phytoplankton.

Level 2 — primary consumers. Herbivores — animals that eat producers. Examples: rabbit, cow, caterpillar, krill, zooplankton.

Level 3 — secondary consumers. Carnivores that eat primary consumers (herbivores). Examples: fox eating rabbit, blue tit eating caterpillar, mackerel eating krill.

Level 4 — tertiary consumers. Carnivores that eat other carnivores. Examples: eagle eating fox, sparrowhawk eating blue tit, dolphin eating mackerel.

Apex predators are carnivores at the top of a food chain with no natural predators. Often they are the tertiary consumer, but the term is about position, not number. Lions, killer whales, polar bears and golden eagles are all apex predators.

Where decomposers fit

Decomposers — bacteria and fungi — are microorganisms that break down dead organic matter and waste at every trophic level. They feed by secreting enzymes out of their cells onto the dead material; the enzymes digest it into small soluble molecules which then diffuse into the microorganism. Decomposers respire and release carbon dioxide to the atmosphere and mineral ions (nitrates, phosphates, potassium) back to the soil for producers to absorb. They are not part of the numbered trophic levels but they sit alongside every level, feeding on whatever dies or is egested.

Biomass and energy decrease up the levels

Biomass is the mass of living material in an organism, population or trophic level. The key AQA rule:

> Approximately 10% of the biomass at one trophic level is transferred to the level above. About 90% is lost.

### Where the lost biomass goes

When biomass is eaten by a consumer, only a small fraction ends up as new consumer tissue. The losses (rough proportions for a typical mammal):

~50% — not consumed. Many parts of a producer or prey are never eaten: roots, bark, bones, fur, feathers. Organisms that die before being eaten are not consumed either. (Decomposers feed on this part.)
~10% — faeces. Not all ingested material is absorbed. Undigested material is egested as faeces.
~30% — respiration. Most absorbed biomass is used in respiration to release energy for movement, growth and maintaining body temperature. The energy ultimately leaves the body as heat. Glucose, in particular, is used in respiration in large amounts. Carbon dioxide and water leave the body as waste.
~10% — urine/urea. Water and urea (nitrogenous waste from breaking down excess protein) leave the body in urine.
~10% — transferred. Only this remaining fraction is built into new consumer biomass and so passes to the next trophic level.

AQA's four marking phrases for 'why biomass decreases' are: not all ingested material is absorbed; biomass is lost in faeces; biomass is used in respiration; biomass is lost as waste (water/urea) in urine. Examiners also accept 'energy lost as heat'.

Efficiency of biomass transfer

The formula:

> efficiency (%) = (biomass transferred ÷ biomass available) × 100

Worked example: 100 kg of grass is eaten by rabbits and 10 kg becomes new rabbit biomass.

efficiency = (10 ÷ 100) × 100 = 10%.

A second worked example with a fish food chain: 500 g of phytoplankton produces 50 g of krill.

efficiency = (50 ÷ 500) × 100 = 10%.

Real ecosystems vary — small mammals lose more biomass to keeping warm (lower efficiency), insects keep more (higher efficiency). For GCSE just use 10% unless the question gives different numbers.

Pyramids of biomass

A pyramid of biomass is a diagram in which each trophic level is drawn as a horizontal bar whose width is proportional to the biomass at that level. Conventions:

Trophic level 1 (producers) is at the bottom.
The bars narrow as you go up because biomass falls roughly tenfold each level.
Bars should be drawn to scale (an exam question might ask you to draw the next bar at one-tenth the previous width).

A typical pyramid: producers 10 000 kg → primary consumers 1 000 kg → secondary consumers 100 kg → tertiary consumers 10 kg. The narrow top represents how little biomass remains to support apex predators.

Why food chains are usually 3-4 levels long

Because ~90% of biomass is lost at each step, the biomass available at level 5 would be only about 0.01% of the producer biomass. That is simply not enough to support a viable predator population. So food chains in nature very rarely exceed four trophic levels. If a fifth level exists at all, the animals at the top tend to be rare and have large home ranges.

Quick implications

Apex predators are always rare — there is not much biomass left for them.
Removing a producer (e.g. by deforestation) collapses every level above.
Eating producers directly (cereals, vegetables) accesses more biomass than eating meat — but the food-production implications belong to 4.7.5.
Decomposers ensure that the 90% 'lost' biomass at each level is not wasted: the carbon and minerals re-enter the cycle.

Exam tips

Memorise the four level names and their definitions verbatim. Producers (L1) photosynthesise; primary consumers (L2) are herbivores; secondary consumers (L3) are carnivores eating herbivores; tertiary consumers (L4) are carnivores eating other carnivores.
For 'why does biomass decrease up the levels' (4 marks) list these four: not all ingested material is absorbed (faeces); biomass used in respiration (heat); water and urea lost in urine; less biomass available to support the next level.
Apex predator = 'carnivore with no predators' — use that exact AQA phrase.
Decomposers are BACTERIA and FUNGI. The marking phrase is 'secrete enzymes' and 'small soluble molecules diffuse into the microorganism' (extracellular digestion).
For efficiency calculations, ALWAYS write the formula, substitute the numbers, then state the % answer. Example: '(10 ÷ 100) × 100 = 10%'. You usually pick up an extra mark for showing the working.
Draw pyramids of biomass with level 1 at the bottom and bars to scale — a tenfold reduction per level looks roughly right.
If asked why food chains rarely exceed 4 levels, say 'biomass available at higher levels is too small to support a viable population'.

Mark-scheme phrasing

Common misconceptions

Worked example

Question:

Answer:

Frequently asked questions

What exactly is a trophic level?

A trophic level is the POSITION an organism occupies in a food chain. Organisms are numbered upwards from level 1 (producers — plants and algae that photosynthesise), level 2 (primary consumers — herbivores), level 3 (secondary consumers — carnivores eating herbivores), and level 4 (tertiary consumers — carnivores eating other carnivores). Two organisms share a trophic level if they eat the same kind of food. The position numbers are about feeding role, not species — a cow and a caterpillar are both level 2 even though they're vastly different animals.

Why does only about 10% of biomass transfer between trophic levels?

When a consumer eats biomass from the level below, only a small slice ends up as new consumer tissue. Roughly half of the available biomass is never consumed at all (roots, bones, animals that die uneaten). Of what IS eaten, about 10% passes straight through and leaves as faeces because not all ingested material is absorbed by the gut. About 30% is used in respiration to release energy for movement, growth and keeping warm — that energy ultimately escapes as heat. Around 10% leaves in urine as water and urea. Only the remaining ~10% becomes new consumer biomass and is passed to the next level.

How do I calculate the efficiency of biomass transfer?

Use the formula: efficiency (%) = (biomass transferred ÷ biomass available) × 100. 'Biomass transferred' is the biomass present at the higher level; 'biomass available' is the biomass at the lower level. Worked example: if a field of grass has 2 000 kg of biomass and supports rabbits with 180 kg of biomass, efficiency = (180 ÷ 2 000) × 100 = 9%. Always show the formula, the substitution and the final % — examiners usually award a method mark even if you slip on the arithmetic.

Are apex predators the same as tertiary consumers?

Not always. Tertiary consumer is a NUMBERED trophic level (level 4) — a carnivore that eats other carnivores. Apex predator is a job description — a carnivore at the top of its food chain with no natural predators. They often coincide (eagles are both tertiary consumers and apex predators in many chains) but a top secondary consumer can also be an apex predator if nothing eats it, and level-4 animals can have predators in some food webs. Read the question carefully.

Where do decomposers fit on a pyramid of biomass?

They don't sit on the numbered pyramid at all. Decomposers — bacteria and fungi — feed on dead organic matter and waste from EVERY level: dead producers, dead consumers, faeces, urine. Examiners sometimes draw them beside the pyramid with arrows pointing in from each level. Their job is to break dead material down by secreting enzymes onto it; small soluble molecules diffuse into the decomposer. As they respire they release CO₂ back to the atmosphere, and they return mineral ions like nitrates and phosphates to the soil — so producers can carry on growing.

Why are food chains so short in nature?

Because biomass shrinks so fast. With ~10% transfer per level, 10 000 kg of producer biomass becomes 1 000 kg of primary consumer biomass, then 100 kg of secondary, then 10 kg of tertiary. At a hypothetical level 5 there would be just 1 kg — not enough to sustain a population of large carnivores. So food chains very rarely exceed four trophic levels in the wild. When they do, the top animals tend to be small or rare and to roam over huge home ranges to gather enough food.