Ecosystems and interdependence · System
Food chains and food webs
Nobody has ever found a food chain with ten links in it. There is a reason, it is arithmetic, and it decides the shape of every ecosystem on Earth.
Start here
Why is there no animal that eats the animal that eats the eagle?
Chains stop. Grass, rabbit, fox — and then nothing. Four or five links is about the limit anywhere in the world, on land or at sea, and the rule holds whether the organisms are enormous or microscopic.
What stops food chains getting longer?
There is not enough energy left. Only about a tenth of what one level holds passes to the next, so after four steps you are down to a ten-thousandth of what the plants captured. A predator at the sixth level would have to hunt an area the size of a county to feed itself, and no such animal exists — the arithmetic forbids it.
At the bench · climb the chain
Follow the energy up
level 1 of 4
Watch what arrives at each level — and where the rest of it went.
The chain
Grass
Producer
10,000 kJ
100% of the original
Ten thousand kilojoules captured from sunlight and built into plant material.
Rabbits
Primary consumer
1,000 kJ
10% of the original
Most of what a rabbit eats is respired to stay alive and stay warm, or passes straight through. Only a tenth becomes rabbit.
Foxes
Secondary consumer
100 kJ
1% of the original
A fox is a warm, active hunter, so its bill for simply existing is high. A tenth again reaches this level.
Golden eagle
Tertiary consumer
10 kJ
0.1% of the original
Ten kilojoules of the original ten thousand. This is why eagles hold territories of many square kilometres and why there are so few of them.
Oak leaves
Producer
10,000 kJ
100% of the original
One large oak supports the whole of the rest of this chain, and hundreds of other species besides.
Caterpillars
Primary consumer
1,000 kJ
10% of the original
Caterpillars are unusually efficient eaters — they do little but eat — but most of the leaf still goes to respiration and droppings.
Blue tits
Secondary consumer
100 kJ
1% of the original
A pair of blue tits needs something like a hundred caterpillars a day to raise a brood, which is why they time their nesting to the caterpillar peak.
Sparrowhawk
Tertiary consumer
10 kJ
0.1% of the original
Ten kilojoules. One sparrowhawk needs the small birds of a whole wood, and the wood can support one pair.
Phytoplankton
Producer
10,000 kJ
100% of the original
Microscopic algae doing the photosynthesis you met in Why almost all life depends on it — roughly half the world’s total.
Zooplankton
Primary consumer
1,000 kJ
10% of the original
Drifting animals grazing the algae, in numbers nobody can count.
Herring
Secondary consumer
100 kJ
1% of the original
A hundred kilojoules of the original ten thousand.
Seals
Tertiary consumer
10 kJ
0.1% of the original
Ten kilojoules. A seal eats several kilograms of fish a day and it is already scraping the bottom of the barrel.
Orca
Quaternary consumer
1 kJ
0.01% of the original
One kilojoule of the ten thousand — a ten-thousandth. This is about as far as any chain on Earth goes, and orcas hunt across whole oceans to make it work.
Ten thousand kilojoules entered at the bottom and 10 arrived here — 0.1% of it. Add one more level and there would be a tenth of that again, which is not enough to build an animal out of. That is the whole reason chains stop.
Everyone has a job title
A web is chains that share their members.
Bottom of every chain
Producers
Plants and algae. They do not eat — they build their own food, so they are the point where energy enters the living world.
Level two
Primary consumers
Herbivores: the animals that eat producers. The most numerous animals in almost every ecosystem, because this is where the food is.
Levels three and up
Secondary and tertiary consumers
Predators, and the predators of predators. Rarer at every step up, and each one needs a larger area to feed from.
Everywhere at once
Decomposers
Bacteria and fungi feeding on dead material and droppings from every level. They are not the end of the chain — they are underneath all of it, returning the minerals to the soil.
Key fact
The arrows in a food chain show the direction energy travels, so they point from the organism being eaten to the organism eating it. Only about a tenth of the energy at each level reaches the next, which is why chains are short and top predators are rare.
A food chain is one route through an ecosystem. A food web is all the routes at once, drawn on top of each other, because almost nothing eats only one thing and almost nothing is eaten by only one thing. The web is the truthful picture; the chain is a single thread pulled out of it so it can be talked about.
Think again
“The arrow points at what the animal eats.”
Draw it that way and every arrow in the diagram is backwards. The arrow is not saying eats; it is saying energy travels this way, so it always points from the eaten towards the eater — grass to rabbit, rabbit to fox. The fox does not send anything to the rabbit. Once you read arrows as energy rather than as appetite, several things become easy: you can see instantly which organisms are producers, because nothing points into them from another organism; you can count how many steps energy has taken from the Sun; and a food web stops looking like a tangle and starts looking like a map of where the energy goes. Examiners mark arrow direction, and it is the single most common lost mark in this topic.
“Ninety per cent of the energy is lost at each level.”
Lost is the wrong word, and you have already met why in Conservation of energy: energy is never destroyed, so it has to be somewhere. Follow it. Most of it was released by respiration and ended up warming the surroundings — a rabbit spends most of what it eats simply staying alive and staying warm. Some passes out undigested and goes to the decomposers, along with everything in the rabbit when it eventually dies. A little is in parts the fox does not eat. Nothing has gone missing; it has left this chain, which is a different claim. The reason the number matters is not bookkeeping but shape: it makes each level about a tenth the size of the one below, and after four or five levels there is nothing left to build an animal out of.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Read the arrows
A food chain is written: lettuce, slug, hedgehog. Which way do the arrows point, and why?
Rung 2 · The one that catches people
Only about 10% of the energy at one level reaches the next. What happened to the other 90%?
Rung 3 · Explain the shape
In almost every ecosystem there are far more producers than primary consumers, and far more primary consumers than top predators. Explain why, and say what would have to be true for an ecosystem to have more foxes than rabbits.
Rung 4 · Take it somewhere new
A country wants to feed more people from the same farmland. An adviser suggests converting land currently used to raise cattle over to growing wheat. Explain the energy argument in favour, and give one honest limitation of it.
Key note
Producers capture energy from sunlight; consumers get theirs by eating; decomposers release the rest back. Arrows point in the direction energy travels, from eaten to eater. About a tenth of the energy at each level reaches the next, the rest having been used in respiration or passed to decomposers, which is why food chains are short and a food web has far more organisms at the bottom than at the top.
Going further
The same arithmetic decides what a country can feed itself on. A hectare of wheat eaten as bread feeds people at the first consumer level; the same hectare grown as feed for cattle and eaten as beef feeds them at the second, with about a tenth as much arriving. That is not an argument that nobody should eat beef — cattle graze land that will not grow wheat, and grass is inedible to us — but it is why the two are not interchangeable, and why the question of how much land a diet requires has a hard number attached to it. The same rule explains something odder at sea: whales are enormous, and the largest of them feed on krill, only two levels up. An animal that size at the fifth level is impossible, so the biggest animals that have ever existed eat some of the smallest.
Before this lesson
Connects to
At GCSE this becomes
- Trophic levels, pyramids of biomass, and calculating the efficiency of energy transfer between levels.
Where to next
Ask Mr Badmus AI
Want to build a chain from your own local wildlife?
The tenth-of-the-energy rule is a teaching average. Real transfer efficiencies range from a few per cent to around twenty depending on the organisms and the ecosystem, and the figures on the bench are rounded to make the pattern readable rather than taken from a particular study.
Lesson content © MrBadmusAI.