Trophic levels and food-chain arrows
A producer (green plant or alga) makes its own food by photosynthesis and starts every chain. A primary consumer eats producers; a secondary eats primary consumers; a tertiary eats secondary consumers. Decomposers (bacteria, fungi) break down dead matter, releasing nutrients. Each feeding stage is a trophic level; a food web links many chains. Key rule: arrows follow ENERGY FLOW — from the eaten to the eater (), never towards the food.
Pyramids of numbers, biomass and energy
Three ecological pyramids summarise a chain, drawn with producers at the base and each bar's width showing the quantity. Numbers = how many organisms; biomass = mass of living material; energy = amount of energy. A numbers pyramid can be INVERTED because it ignores size — one large oak supports many primary consumers, so the base is narrow. A biomass pyramid uses mass, so is usually upright. An energy pyramid is ALWAYS upright, as energy is lost at every transfer.
Energy transfer and the 10% rule
About 10% of the energy at one trophic level passes to the next; the other ~90% is lost — as HEAT from respiration, in MOVEMENT, in EGESTION (undigested food) and in EXCRETION (urea). So food chains are short (rarely over four or five levels): too little energy remains for another. Eating producers (crops) is more efficient for humans than eating animals fed on them. To explain why one chain transfers less, list the loss routes AND link them to the trophic-level count.
Drawn from real examiner reports.
Food-chain arrows point prey to predator
Each arrow shows the transfer of energy from one organism to the next, so it points FROM the eaten TO the eater (). Examiners repeatedly see arrows pointing the WRONG way, towards the food. Drawing the arrow to mean "is eaten by" loses the mark — it must follow the direction of energy flow, producer to top consumer.
Trace the longest chain, not a pyramid
When asked to write a food chain FROM a food web, some candidates draw a pyramid of numbers instead of a chain, or pick a chain that is not the LONGEST available. "Draw the longest food chain" means trace the path with the most trophic levels, drawing the arrows from producer towards the top consumer. Read the command word carefully and count the links before choosing.
List loss routes AND link to chain length
To explain why a longer chain delivers less energy, link two things: (1) the loss routes at each transfer — heat from respiration, movement, egestion, excretion — since only ~10% passes on; AND (2) a longer chain has an EXTRA trophic level, so one more transfer loses ~90%. Listing routes without the link, or the link without the routes, caps the mark.
Define producer and decomposer precisely
A producer must be defined as an organism that makes its OWN food (glucose) by PHOTOSYNTHESIS — "a plant" or "the bottom of the chain" is not a definition. A decomposer is a bacterium or fungus that breaks down dead organisms and waste, releasing nutrients — not just "something that rots things". Both marks depend on stating the mechanism, not giving a rough label.
Primary vs secondary consumer
A PRIMARY consumer eats producers (the herbivore, 2nd trophic level); a SECONDARY consumer eats primary consumers (3rd level). These two are routinely swapped. Count the arrows from the producer: one arrow along = primary, two = secondary, three = tertiary. Fix the trophic level first, then name the consumer.
Give a direction, not "affects"
In data/effect questions give a DIRECTION: "reduced light means LESS photosynthesis, so producers grow LESS, so there is LESS food for primary consumers, so their numbers FALL". Words like "affects" or "changes" score nothing — say whether each quantity goes up or down and follow the effect along the chain, level by level.
Keep the three loss routes distinct
The loss routes earn separate marks, so do not merge them: respiration releases energy lost as HEAT; egestion is the loss of UNDIGESTED food (faeces) that never entered cells; excretion is the loss of metabolic waste (urea). "Energy is lost as waste" is too vague — name the specific route, because each named route is its own mark.
Energy maths: multiply by 0.1 per step, or use %
For energy maths, only ~10% passes to the next level: multiply by 0.1 (or divide by 10) once per transfer — count the arrows for how many times — or find a percentage as energy passed on ÷ energy available . Always MANIPULATE the data; never just quote a table number.
Read a web: label levels, arrows, longest chain
For food-web questions, label each organism's trophic level, then draw arrows FROM the eaten TO the eater (energy flow). For "the longest food chain", trace the path with the most links. An organism can sit at more than one trophic level — read the whole web first.
Explain: loss routes plus a direction of change
For effect/explain questions, name the loss routes (respiration, movement, egestion, excretion) AND link them to the number of trophic levels, and state a DIRECTION ("less"/"more"/"falls"), not the vague "affects". Both mechanism and direction are needed for full marks.
Explain pyramid shape from what each bar shows
To explain a pyramid's shape, say what each bar measures: numbers counts organisms and IGNORES size, so one big producer can make it inverted; biomass and energy both fall at each level, so those pyramids stay upright. Match the explanation to the pyramid TYPE named.
The trophic levels (feeding stages) — statement 4.6:
Food chains and food webs — statement 4.7:
(here grass is the producer, rabbit the primary consumer, fox the secondary consumer).
The three pyramids — statement 4.7:
| Pyramid | What each bar's width shows | Can it be inverted? |
|---|---|---|
| Pyramid of numbers | The number of organisms at each trophic level | Yes — it ignores size (e.g. one oak tree feeds many insects, so the base is narrow) |
| Pyramid of biomass | The mass of living material at each trophic level | Rarely — using mass corrects for organism size, so it is usually a true pyramid |
| Pyramid of energy | The amount of energy at each trophic level | No — energy is lost at every step, so it is always upright |
All three are drawn with the producers at the base.
Energy transfer and the ~10% rule — statements 4.8-4.9:
Only about 10% of the energy at one trophic level is passed on to the next. The other ~90% is lost by:
Also, not every part of an organism at one level is eaten by the next.
Define a decomposer and name the two groups of organisms that are decomposers.
A food chain in a garden is:
(a) Name the producer in this food chain. (1 mark) (b) Name the primary consumer and the secondary consumer. (2 marks) (c) State what the arrows in the food chain represent. (1 mark)
