Producer, consumer, herbivore, carnivore, decomposer
Producer — makes its own organic food from light energy by photosynthesis. Consumer — gets energy by eating other organisms. Herbivore — a consumer that eats only plants. Carnivore — a consumer that eats only animals. Decomposer — a bacterium or fungus feeding on dead organisms and waste matter, releasing nutrients back to the environment. Trophic level — position in the chain: producer = 1, primary consumer = 2, secondary = 3.
Food web — many chains, one Sun
A food web is a network of interconnected food chains; most consumers eat, and are eaten by, more than one species. Construct one by joining chains that share species, so an organism can carry arrows in and out. Read one by tracing arrows — any organism with no arrows entering it is a producer. The energy in every chain originally comes from the Sun: producers only convert light energy into chemical energy stored in glucose, they do not create it.
Energy transfer (Extended) — roughly 10% per level
Only a small percentage of the energy — typically about 10% — taken in at one trophic level becomes new biomass in the level above. The rest is lost as heat from respiration (largest in mammals and birds), used for movement, lost in egested faeces and excreted waste, or left in uneaten parts such as bones and roots. So pyramids of biomass narrow at every step, and chains rarely exceed four or five trophic levels — beyond that too little energy remains.
Drawn from real examiner reports.
Food chain ≠ food web
A food chain is one linear sequence — grass → rabbit → fox. A food web is several chains interconnected, showing that most organisms have more than one food source and more than one predator. Asked to construct a web from a set of feeding relationships, drawing a single chain leaves most of those relationships unshown and loses the marks for them.
Arrows follow energy, not appetite
In grass → rabbit → fox each arrow means energy passes from the organism eaten to the organism eating it. Arrows drawn from predator back to prey, or pointing back toward the producer, score nothing. Before committing to an arrow, say the relationship aloud as "grass is eaten by rabbit" — the arrow points the same way as "is eaten by".
Flagged s23 P31 Q5a — arrows drawn backwards instead of along energy flow
Numbers pyramids can be inverted
(Extended) A pyramid of numbers counts individuals regardless of size, so it can be inverted — one oak tree supporting thousands of aphids gives a narrow base under a much wider bar. A pyramid of biomass totals the mass at each level and is almost always upright. Examiners report candidates cannot tell the two shapes apart, and draw them identically.
Flagged s23 P11/22 Q36-38 · w22 P42 Q1d · s22 P32 Q4b ii
"More accurate" earns no mark
(Extended) Asked for the advantage of a pyramid of biomass over a pyramid of numbers, "it is more accurate" or "it is better" scores zero. The mark scheme wants the mechanism: a pyramid of biomass takes the mass of each organism into account, so it is not distorted by large numbers of very small organisms the way a headcount is.
Flagged s23 P31 Q5d — "more accurate" alone was not credited
The Sun, not the producer, is the source
When a question asks for the original source of energy in a food chain, the answer is the Sun (light energy). "The producer" or "the plant" loses the mark — producers convert light energy into chemical energy in glucose by photosynthesis, they do not create it. The word "originally" in the question is the signal that "the Sun" is wanted.
Flagged s23 P11/23 Q35-36 — producers named as the original energy source
Decomposers are not carnivores
Decomposers feed on dead organisms and waste matter at every level of the web, not on living prey, so they are neither herbivores nor carnivores and do not occupy a normal trophic level. Candidates are reported calling them carnivores, or drawing them as an extra bar on top of a numbers or biomass pyramid as though they were the final consumer.
Flagged s23 P11 Q37 · P13 Q38 — decomposers called carnivores or placed at the top of a pyramid
Trace the arrows
Find the organisms with no arrows entering them — those are the producers. Arrows leaving an organism lead to what eats it; arrows entering it come from what it eats. Count the steps from the producer (level 1) to get any organism's trophic level.
Remove → direct effect → knock-on effect
For "effect of a change" questions use one chain: species removed → its predators lose a food source so their numbers fall → its prey are eaten less so their numbers rise → the change ripples onward. Always name the specific organisms in the given web, never "some animals".
Divide the upper level by the lower
(Extended) For percentage energy transfer, divide the energy at the upper trophic level by the energy at the level below it, then multiply by 100. An answer above 100% means the division was done the wrong way round — check before moving on.
Name the mechanism of energy loss
"Explain why energy transfer is inefficient" wants named mechanisms — heat from respiration, movement, egestion, excretion, uneaten parts. "Energy is used up" or "energy disappears" earns nothing. Expect one named mechanism per mark, so give as many as the tariff asks for.
Producer: an organism (usually a green plant or alga) that makes its own organic food using light energy (photosynthesis).
Consumer: an organism that obtains its energy by eating other organisms.
Herbivore: a consumer that only eats plants (or algae).
Carnivore: a consumer that only eats other animals.
Decomposer: a bacterium or fungus that feeds on dead organisms and waste matter, breaking down organic material and releasing nutrients back into the environment.
Food chain: a diagram showing the transfer of energy from one organism to the next, starting with a producer. Arrows point in the direction of energy flow (from the organism eaten to the organism that eats it).
Food web: a network of interconnected food chains in an ecosystem, showing that most organisms are eaten by, and eat, more than one species.
Trophic level: the position an organism occupies in a food chain (producer = level 1; primary consumer = level 2; secondary consumer = level 3; and so on).
(Extended) Pyramid of numbers: a diagram showing the number of individual organisms at each trophic level, drawn as bars stacked with the producer at the base. Can be inverted (e.g. one tree supporting many insects).
(Extended) Pyramid of biomass: a diagram showing the total mass of living material at each trophic level. Almost always upright because it is not distorted by large numbers of small organisms.
Define food chain, food web and trophic level.
(a) Define the term producer.
(b) Define the term decomposer.
(c) State the direction in which the arrow points in a food chain, e.g. between "grass" and "rabbit."
