Food chain — energy transfer that starts at a producer
A food chain shows energy transferred from one organism to the next, beginning with a producer — an organism (green plant or alga) making its own organic molecules from light energy by photosynthesis. A consumer gets energy by feeding on other organisms, numbered by distance from it: a primary consumer feeds on the producer, a secondary on the primary, a tertiary on the secondary. A herbivore eats only plants, a carnivore only animals.
Arrows, decomposers and food webs
Each arrow means "energy is transferred to" and points from the organism eaten to the organism that eats it — the direction of energy flow, not of biting. A decomposer (fungi, bacteria) gets energy from dead or waste organic matter (dead leaves, faeces), breaking it down and releasing nutrients back. A food web is a network of interconnected chains showing all feeding relationships in a habitat; most organisms have several food sources and predators.
(Extended) Trophic level — and why transfer is inefficient
A trophic level is an organism's position in a food chain, set by the number of energy transfers needed to reach it: producer 1st, primary consumer 2nd, secondary 3rd, tertiary 4th, quaternary 5th. Only about 10% of the energy in one level becomes biomass in the next. The rest goes in respiration (heat), egestion and excretion, or is never eaten (roots, bones). Because so much is lost at every transfer, chains rarely exceed five trophic levels.
Drawn from real examiner reports.
The Sun is not the producer
The Sun is the ultimate source of energy entering a food chain, but it is not an organism, so it is never a step in the chain. Asked for the source of energy, answer "the Sun" (or light from the Sun); asked for the first organism, name a green plant or alga as the producer. "Light" on its own is not accepted as the source, and "Sun → grass → rabbit" is a marked error.
Flagged Jun 2023 P11 Q12, P42 Q7iii · Nov 2023 P32 Q1c, P41 Q10c
Food-chain arrows drawn the wrong way
Candidates draw or read the arrow from predator to prey, as if it showed who eats whom. It shows the direction of energy flow — from the organism eaten TO the organism eating it. In "grass → rabbit → fox", energy flows from the grass into the rabbit because the rabbit eats the grass. Check that every arrowhead points towards the consumer, one feeding step up the chain.
Flagged Nov 2022 P31 Q10ai
(Extended) Naming heat loss only
Asked why transfer between trophic levels is inefficient, most candidates give only "energy lost as heat in respiration". Energy also leaves in egestion (undigested material passed out as faeces, never absorbed) and in excretion (metabolic wastes such as urea, not built into tissue). Only energy built into the organism's own biomass can be passed on.
Flagged Jun 2023 P42 Q7a · Nov 2023 P11 Q12
Herbivore ≠ decomposer
A herbivore is a living animal that eats only living plants. A decomposer — fungi and bacteria — gets its energy from dead or waste organic matter, never by eating a living organism. Do not place a decomposer in a food web as though it were an ordinary consumer feeding on live prey; show it acting on dead or waste material instead.
Food chain ≠ food web
A food chain is one simplified line of feeding relationships. A food web is the whole network of interconnected chains in a habitat, showing that most organisms have more than one food source and more than one predator. Answering a food-web question with a single chain loses the marks that come from the extra feeding links.
Vague answers on human impact
For over-harvesting or an introduced species, "it will affect the ecosystem" scores nothing. Trace the specific arrows from the named organism: state which population increases (a predator or competitor removed) and which decreases (a food source lost, or a new predator or competitor added), and name the feeding link that causes each change.
Start every chain at the producer
Find the producer first — a named plant or alga, never the Sun — and place it at the start. Then work outward one feeding step at a time, naming each organism primary, secondary, tertiary or (Extended) quaternary consumer by counting the energy transfers back to the producer.
Sweep back and check the arrowheads
Draw the chain first, then sweep back along it and confirm every arrowhead points towards the organism doing the eating. Reversed arrows are the commonest way marks are lost on construction questions, and one wrong arrow can cost the whole arrow mark.
(Extended) Give all three energy-loss routes
A 3-mark "explain why transfer is inefficient" question expects three separate routes: respiration (heat lost to the surroundings), egestion (faeces) and excretion (urea). A single vague "energy is lost as heat" earns one mark only, so name at least two — ideally all three.
(Extended) Count transfers for crop-vs-meat
Eating crop plants directly is one transfer (producer to human); eating animals fed on them adds a second. Energy is lost at each transfer, so fewer transfers leave more of the original energy available. State the number of transfers, rather than only "meat has less energy".
Food chain — shows the transfer of energy from one organism to the next, beginning with a producer.
Producer — an organism (a green plant or alga) that makes its own organic molecules using light energy (by photosynthesis).
Consumer — an organism that gets its energy by feeding on other organisms.
| Term | Definition |
|---|---|
| Primary consumer | An organism that feeds directly on the producer |
| Secondary consumer | An organism that feeds on a primary consumer |
| Tertiary consumer | An organism that feeds on a secondary consumer |
| Herbivore | An animal that gets its energy by eating only plants |
| Carnivore | An animal that gets its energy by eating other animals |
| Decomposer | An organism that gets its energy from dead or waste organic matter |
Food web — a network of interconnected food chains, showing all the feeding relationships within a habitat.
Trophic level (Extended) — the position an organism occupies in a food chain, defined by the number of energy transfers needed to reach it. Producer = 1st trophic level; primary consumer = 2nd; secondary consumer = 3rd; tertiary consumer = 4th; quaternary consumer = 5th.
Example food chain: grass grasshopper frog snake hawk
| Organism | Trophic level | Feeding role |
|---|---|---|
| Grass | 1st | Producer |
| Grasshopper | 2nd | Primary consumer, herbivore |
| Frog | 3rd | Secondary consumer, carnivore |
| Snake | 4th | Tertiary consumer, carnivore |
| Hawk | 5th | Quaternary consumer, carnivore |
Every arrow () means "energy is transferred to" and points from the organism eaten towards the organism that eats it.
Define producer and consumer.
Define the terms producer and primary consumer. (2 marks)
