A food chain follows energy from a producer to the animals that eat it. Real communities are tangled webs of chains, and pyramids let us compare the trophic levels at a glance. Pyramids of energy, and why so little energy reaches the top of a chain, are Extended content.
A food chain shows the transfer of energy from one organism to the next, beginning with a producer; a food web is a network of interconnected food chains. A trophic level is an organism's position in a food chain, food web or ecological pyramid: producers, then primary, secondary, tertiary and quaternary consumers. Pyramids of numbers and biomass compare the levels. Extended: only a small fraction of the energy at one trophic level reaches the next, so food chains usually have fewer than five trophic levels.
Fig. 1: the same oak tree food chain drawn as a pyramid of numbers (one large producer, so not a pyramid shape) and as a pyramid of biomass in g per m2 (a true pyramid)
Section 01
Chains, webs and the roles in them.
In the food web every arrow points from the organism that is eaten to the organism that eats it, which is the direction energy flows. Click each organism to see its role and a food chain that leads to it.
Term
Meaning
In the sim
Producer
makes its own organic nutrients, usually using energy from sunlight, through photosynthesis
grass, oak tree
Consumer
gets its energy by feeding on other organisms
every animal shown
Herbivore
an animal that gets its energy by eating plants
rabbit, mouse, caterpillar
Carnivore
an animal that gets its energy by eating other animals
blue tit, snake, fox, hawk
Decomposer
gets its energy from dead or waste organic material
bacteria and fungi (not shown)
Trophic levels: in oak → caterpillar → blue tit → hawk, the oak is the producer, the caterpillar the primary consumer, the blue tit the secondary consumer and the hawk the tertiary consumer. An animal one step higher still would be a quaternary consumer.
One animal, several levels: the hawk is a secondary consumer when it eats mice but a tertiary consumer when it eats blue tits or snakes.
Overharvesting: press Overharvest the rabbits. Fewer rabbits means more grass and fewer foxes; hungry foxes eat more mice, so mice fall too, which affects snakes and hawks.
Introduced species: press Introduce a foreign predator. The stoat has no natural predators here and eats blue tits, mice and rabbits. Fewer blue tits means more caterpillars, which can damage the oak trees.
Examiner trap
Arrows in a food chain show the direction of energy flow, so they point towards the eater: grass → rabbit means the rabbit eats the grass. Drawing them the other way loses the mark.
Section 02
Pyramids of numbers and biomass.
Each block is one trophic level with the producers at the bottom. Choose a food chain, then switch between numbers, biomass and energy. Watch what happens to the oak tree chain.
Pyramid
Grass chain (grass, rabbits, foxes)
Oak chain (oak, caterpillars, blue tits)
Numbers (individuals)
10 000, 200, 2
1, 50 000, 300
Biomass (g per m2)
2000, 200, 20
5000, 500, 50
Energy (kJ per m2 per year, Extended)
20 000, 2000, 200
20 000, 2000, 200
Pyramid of numbers: the number of organisms at each level. It ignores size, so one huge oak tree supporting thousands of caterpillars gives a shape that is not a pyramid.
Pyramid of biomass: the total mass of living material at each level. It takes account of the size of the organisms, so the oak chain becomes a true pyramid. This is its advantage over a pyramid of numbers.
Drawing: draw bars to scale, centred, producers at the bottom, and label each level with the organism or trophic level.
Examiner trap
A pyramid of biomass is better than numbers, but it is still a snapshot at one moment. A fast-growing producer that is eaten quickly (such as plankton) can have a small standing biomass, so even biomass pyramids are occasionally inverted.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
In the food web, which organism is a producer?
Quick check+10 XP
In the chain oak tree → caterpillar → blue tit → hawk, what is the trophic level of the hawk?
Quick check+10 XP
In the sim, the rabbits are overharvested. Which change follows?
Quick check+10 XP
Why is the pyramid of numbers for the oak chain not a pyramid shape?
Section 03
Why so little gets through.
Extended. Choose a route, then press Send 10 000 kJ of crop. Compare how much energy reaches humans when they eat the crop directly with when the crop is fed to cattle first.
Pyramid of energy: shows the energy passing through each trophic level per unit area per year (kJ per m2 per year). It is always pyramid shaped, because energy is lost at every level.
Advantages: it takes account of size and of how quickly organisms grow and reproduce over a whole year, and different organisms store different amounts of energy per gram. So it is the most accurate way to compare trophic levels or ecosystems.
Inefficient transfer: typically only about 10% of the energy at one level becomes new biomass in the next. The rest is lost as heat from respiration, used for movement, lost in faeces (undigested food) and urine (excretion), or is in parts that are not eaten, such as roots and bones.
Fewer than five levels: after four transfers only about 0.01% of the producer's energy is left, not enough to support another population of animals.
Crops versus livestock: eating the crop directly, humans receive about 1000 kJ from 10 000 kJ; via cattle only about 100 kJ. Removing the extra trophic level makes eating crops about ten times more energy efficient, so the same land feeds more people.
Examiner trap
Energy is not lost "in" the food chain because animals are wasteful: most of it leaves as heat from respiration, which every organism must carry out to stay alive.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
Which description of a food web is correct?
Exam style+20 XP
What is an advantage of a pyramid of biomass over a pyramid of numbers?
Exam style+20 XP
Extended: why do food chains usually have fewer than five trophic levels?
Exam style+20 XP
Extended: 10 000 kJ of maize is fed to cattle, and humans eat the beef. About how much energy reaches the humans?
Skill unlocked
Food chains, webs and pyramids, mastered.
This skill is now recorded on your profile. Keep the chain going.