4bFeeding relationships

Every organism in a feeding relationship sits at a trophic level — a feeding level, counted from the start of the food chain.

The levels

Decomposers (bacteria and fungi) feed on dead organisms and waste (faeces, urine, fallen leaves). They break down these materials and release mineral ions, such as nitrates, back into the soil where plants can absorb them. Decomposers are not usually given a number — they act on every trophic level.

How to work out a trophic level

Count the arrows from the producer, don't guess from the animal's size or how fierce it looks.

grass → grasshopper → frog → snake → hawk

• grass = producer (trophic level 1)
• grasshopper = primary consumer (it eats the producer) — level 2
• frog = secondary consumer — level 3
• snake = tertiary consumer — level 4
• hawk = quaternary consumer — level 5 (the fourth consumer, but the fifth trophic level)

The trap is that the consumer number is always one less than the trophic level number, because the producer takes level 1.

A second chain to test yourself on:

algae → water flea → minnow → pike

• algae = producer, level 1
• water flea = primary consumer, level 2
• minnow = secondary consumer, level 3
• pike = tertiary consumer, level 4

Chains rarely run past four or five levels, because energy is lost at every transfer and there is not enough left to support another level.

The same species can occupy different trophic levels in different chains within a food web. A fox eating a rabbit is a secondary consumer; a fox eating blackberries is a primary consumer. So never learn an animal's level — work it out from the chain in front of you.

Common mistake: writing "secondary consumer" for the herbivore. A primary consumer is the first consumer, i.e. the animal that eats the plant — it is not the first animal to be eaten. Examiners report this swap every series.

Examiner tip. Count the arrows every time rather than relying on memory, and be careful with the two ways of naming a level: the hawk in the chain above is the quaternary consumer and the organism at the fifth trophic level — both are correct — what is wrong is calling it the fourth trophic level. Also call decomposers decomposers, not consumers: they feed on dead material and waste and are not given a trophic level number.

A food chain shows the feeding relationship between organisms and the direction in which energy and biomass are transferred.

lettuce → slug → thrush → sparrowhawk

Rules for drawing a food chain

1. Start with the producer on the left.
2. Arrows point from the organism being eaten to the organism that eats it — the arrow shows the direction of energy/biomass flow, not "who eats whom".
3. Use the names of organisms, not "herbivore" or "trophic level 2".
4. No numbers, no boxes, no bars — a food chain is names joined by arrows.
5. If the question asks for a chain with four organisms, or one containing the badger, deliver exactly that.

Common mistake: examiners report arrows drawn the wrong way round, students drawing a pyramid of numbers when a food chain was asked for, and picking a food chain from the web that does not match the one specified in the question. Check the arrow direction and reread the stem before you move on.

Food webs

A food web is a set of interconnected food chains in a habitat. It is more realistic than a single chain because most consumers eat more than one type of food, and most organisms are eaten by more than one predator.

From a food web you may be asked to:

• extract a food chain — follow one path of arrows all the way from a producer
• name the trophic level of an organism in a particular chain
• predict the effect of a change in one population

Predicting changes in a food web

If a predator is removed (e.g. wolves are culled):

• its prey population increases because fewer are eaten
• the prey's food source decreases because more is being eaten
• other species that share that food source may then decrease

Work through the arrows one step at a time and say increase or decrease, then give the reason ("because…").

Common mistake: a question asking for the effect on one named population is not asking for a tour of the whole ecosystem. Candidates lose marks by describing every knock-on effect of a change in the wolf population when the question named a single species. Answer the question asked, and only that.

Examiner tip. Read every word of the stem. A common error is spotting 'food chain' and rushing to write, missing 'containing four organisms' or 'starting with algae'. Also, always double-check arrow direction: prey → predator.

A pyramid is a set of horizontal bars drawn to scale, one per trophic level, with the producer at the bottom.

Why pyramids of number can be the wrong shape

Because size is ignored — one organism counts as one, whether it is an oak tree or a greenfly.

• One oak tree supports thousands of insects → the producer bar is tiny, so the pyramid is inverted at the bottom.
• Parasites (e.g. fleas on a fox) are small and numerous → the top bar is wider than the one below.

Why pyramids of biomass are nearly always the right shape

Biomass accounts for size. Because biomass is lost at every transfer (see the 10% rule), each level has less living material than the one below. Biomass is measured as dry mass, which means organisms must be killed and dried — a disadvantage of this method.

Pyramids of energy transfer

These are always true pyramids because energy is transferred to the next level, and some energy is always lost at each stage (respiration, heat, faeces, urine). Energy cannot be created, so a higher level can never contain more energy than the one below.

Drawing tips

• Bars must be horizontal, symmetrical about a central line, and drawn to scale using a sensible scale you state.
• Label each bar with the organism's name.
• Producer at the bottom.

Percentage calculations

Efficiency of transfer = (energy in next level ÷ energy in previous level) × 100

Percentage change = (change ÷ original value) × 100

Common mistake: dividing by the wrong number. The denominator is the value you are comparing with — usually the earlier/lower trophic level, or the group named in the question. Write out the fraction before you touch the calculator, and give the answer to the number of decimal places or whole number asked for.

Examiner tip. In pyramid calculations the single commonest error is dividing by the wrong denominator, followed by not rounding as instructed (e.g. 'to the nearest whole number') and misreading graph or table values. Underline the value the question says to compare with.

Feeding transfers both energy and substances from one trophic level to the next.

Where it all starts

Producers absorb light energy and use photosynthesis to make glucose. Only about 1–2% of the light falling on a leaf is captured — the rest misses the leaf, is reflected, or is the wrong wavelength.

Producers also absorb mineral ions from the soil, including nitrate ions, which they combine with glucose to make amino acids and then proteins.

What is passed on when an animal eats

When a consumer eats, the food is digested into small soluble molecules, absorbed, and used to build the consumer's own body. So what moves along the chain is:

• energy (stored in chemical bonds in food molecules)
• carbohydrates, fats and proteins (as biomass)
• carbon and nitrogen atoms, locked up in those molecules
• water and mineral ions

Animals get their protein by eating protein and digesting it to amino acids. They do not absorb nitrates to make protein — only plants need nitrate ions for that.

Common mistake: writing that nitrates are transferred along the food chain so the animal can make protein. This muddles the mineral requirements of plants with those of animals. The correct answer is that protein/amino acids are transferred.

Decomposers return the substances to the environment: they break down dead organisms and waste, releasing carbon dioxide and mineral ions such as nitrates back for producers to use again. Substances are recycled; energy is not — it is lost from the food chain.

Why only about 10% of energy reaches the next level

Of the energy in one trophic level, roughly 90% never becomes biomass in the level above:

Only the energy that is stored as new biomass (growth) is available to the next trophic level.

Consequences of the 10% rule

• Food chains are usually short (4–5 levels) — there is not enough energy left to support another level.
• There are usually fewer organisms, and less biomass, at each higher level.
• Growing crops to eat directly feeds more people than feeding those crops to animals, because one energy transfer is removed.

Examiner tip. Know the difference between the command words. Describe = say what happens ("energy decreases at each level"). Explain = give the reason ("because energy is lost through respiration, heat, faeces and urine"). Examiners report that many candidates describe when asked to explain and score nothing — if the question says 'explain', every point needs a 'because'.