Every time an organism eats another, energy is transferred — but most is lost as heat. That single fact explains why ecosystems support far more plants than lions, and why food chains rarely exceed five links. This guide walks through food webs and energy flow from producer to top predator, with the calculations your GCSE examiner expects.
What is the difference between a food chain and a food web?
A food chain shows a single pathway of energy transfer:
grass → rabbit → fox
An arrow means "is eaten by" or "energy flows to". The organism at the start is always a producer (a plant or other photosynthesising organism); the organisms that follow are consumers.
A food web is a more realistic picture — it shows all the feeding relationships in an ecosystem simultaneously. Most organisms eat more than one thing and are eaten by more than one predator. A food web is a network of overlapping food chains.
Food webs are important because they show interdependence: if one species is removed or declines, the effects ripple through the web. For example, if rabbits in a meadow ecosystem decline due to disease:
- Grasses increase (fewer herbivores eating them)
- Foxes and birds of prey may decline (less food) or switch to other prey
- Populations of the rabbit's other prey competitors (e.g. voles) may initially rise, then fall as predator pressure increases on them
What are trophic levels?
A trophic level is a position in a food chain based on how many energy transfers separate the organism from the sun's energy.
| Trophic level | Description | Examples |
|---|---|---|
| 1 | Producers — photosynthesisers | Grass, oak trees, algae, phytoplankton |
| 2 | Primary consumers — herbivores | Rabbits, caterpillars, zooplankton |
| 3 | Secondary consumers — eat primary consumers | Foxes, thrushes, small fish |
| 4 | Tertiary consumers — eat secondary consumers | Eagles, large sharks, orcas |
| 5 | Apex predators (if present) | Very few ecosystems reach level 5 |
Decomposers (bacteria and fungi) occupy no single trophic level — they obtain energy from dead organic matter at every level. They are essential for returning nutrients to the soil for producers.
How is energy lost between trophic levels?
At each trophic level, the organism receives energy from its food. However, a large fraction of that energy never passes to the next level. Energy is lost as:
- Heat from respiration — organisms respire continuously; most of the energy released is lost to the environment as heat.
- Excretion — urea, carbon dioxide, water vapour, and other waste products carry some chemical energy out of the organism.
- Undigested material in faeces — not all of a prey organism is digestible; bone, fibre, and hair pass through the gut and are lost.
Only the energy stored in the organism's own biomass (its body mass) is available to the next trophic level.
The 10% rule: On average, only about 10% of the energy at one trophic level is transferred to the next. The remaining 90% is lost by the routes above.
This is an approximation — real ecosystems vary considerably (some transfers are closer to 5%, others closer to 20%), but 10% is the standard assumption for GCSE calculations.
How do you calculate energy transfer efficiency?
Formula:
$$\text{Efficiency} = \frac{\text{energy available at higher trophic level}}{\text{energy available at lower trophic level}} \times 100%$$
Worked example:
A meadow food chain: grass → voles → barn owls
- Grass captures 50,000 kJ/m²/year from sunlight through photosynthesis.
- Voles eat grass and store 5,000 kJ/m²/year in their biomass.
- Barn owls eat voles and store 500 kJ/m²/year in their biomass.
Efficiency of grass → voles: (5,000 ÷ 50,000) × 100 = 10%
Efficiency of voles → barn owls: (500 ÷ 5,000) × 100 = 10%
Now suppose a student asks: "How much energy from the grass ends up stored in barn owl biomass?"
After two transfers at 10% each: 50,000 × 0.10 × 0.10 = 500 kJ — just 1% of the original energy.
This is why apex predators are always rare: the energy available to them is a tiny fraction of what plants captured in the first place.
What is a pyramid of biomass?
A pyramid of biomass shows the total dry mass of organisms at each trophic level. Because energy is lost at each transfer, there is less biomass at each successive level. Pyramids of biomass are therefore almost always pyramid-shaped — wider at the base (producers) and narrowing towards the top (top predators).
| Trophic level | Dry biomass (kg/m²) |
|---|---|
| Grass (producers) | 500 |
| Voles (primary consumers) | 25 |
| Barn owls (secondary consumers) | 1 |
Note: pyramids of numbers are not always pyramid-shaped. A single oak tree can support thousands of caterpillars, making the number at trophic level 2 greater than at level 1 — giving an inverted shape. Biomass corrects for this by measuring mass rather than number.
Why does energy loss have practical consequences for humans?
The energy loss at each trophic level has a direct bearing on food production efficiency. Eating plants directly (trophic level 2) means you receive 10 times more of the sun's energy per hectare than if you ate animals that had eaten those plants (trophic level 3).
This is why:
- Feeding grain directly to humans supports more people per hectare than converting it to beef.
- Intensive livestock farming often involves keeping animals in confined conditions to reduce energy lost through movement and heat loss — maximising the proportion of energy that becomes body tissue (usable food).
GCSE questions often ask you to "explain, using ideas about energy transfer, why there are fewer carnivores than herbivores in an ecosystem". Your answer should reference the 10% transfer efficiency and the loss of energy through respiration, excretion, and undigested waste.
How do you read a food web question in the exam?
When asked to trace the effect of a change in population:
- Identify the species affected — is it prey, predator, or both?
- Trace upwards — what predators will be affected?
- Trace downwards — what prey will be affected (predation pressure changes)?
- Consider alternative prey/predators — can other species compensate?
- Describe the effect, then say whether it stabilises — ecosystems have feedback mechanisms.
Use the phrase "consequently" to link effects in a chain. Avoid just saying a population "increases" or "decreases" — say why and say how the ecosystem responds.
Frequently asked questions
Why do food chains rarely have more than five trophic levels?
At each step, approximately 90% of the energy is lost. After five trophic levels, only about 0.001% of the original energy remains (0.1⁵ × 100% = 0.001%). The amount of energy available is simply too small to support a viable population of predators at a sixth level. Real populations need sufficient energy not just to exist but to reproduce, find mates, defend territories, and escape their own predators — all energetically costly.
What is the difference between a pyramid of biomass and a pyramid of numbers?
A pyramid of numbers shows how many individual organisms there are at each trophic level. It can be non-pyramid-shaped: one large tree can support thousands of insects (an inverted base), or a single host animal can carry millions of parasites (an inverted top). A pyramid of biomass shows the total dry mass at each level and corrects for body size — it is almost always pyramid-shaped, because the total mass of material available decreases with each energy transfer.
Why is dry mass used rather than wet mass in biomass pyramids?
Wet mass includes water, which varies enormously between organisms (a jellyfish is about 96% water; a tree trunk is much less). Water content is not a meaningful measure of energy stored in biological molecules. Dry mass — mass after all water has been removed by heating — represents the actual organic material (carbohydrates, fats, proteins) that contains the stored energy. It gives a much fairer comparison of the biological material at each trophic level.
How do decomposers fit into energy flow in an ecosystem?
Decomposers (bacteria and fungi) break down dead organic matter at every trophic level — dead producers, dead herbivores, dead carnivores, and the waste products (faeces, urea) of all organisms. They do not sit within the main pyramid but form a parallel pathway. The energy they release through their own respiration is lost as heat; the inorganic nutrients (nitrates, phosphates, carbon dioxide) are returned to the environment for producers to use. Without decomposers, nutrients would become locked in dead tissue and ecosystems would eventually run out of the raw materials needed for growth.
Professor Darwin at aitutors.me can walk you through energy-transfer calculations step by step, help you trace food web changes, and practise the long-answer questions that test your ecological reasoning.