Food Chains

A food chain is a simple diagram showing who eats whom in an ecosystem. It starts with a producer (a plant or algae that makes its own food from sunlight) and ends with a top predator that nothing else eats. Each arrow in a food chain shows the flow of energy from one organism to the next. Food chains are one of the basic ideas in ecology: they explain how energy from the Sun ends up feeding every animal on Earth, including you.

  • Always starts withA producerAlmost always a plant or algae
  • Always ends withA top predatorLike a lion, shark or eagle
  • Number of stepsUsually 3 to 5Rarely more, energy runs out fast
  • Energy lost per stepapprox. 90%Only 10% passes up
  • Source of energyThe SunAlmost always
  • Each step calledA trophic levelFrom "trophe", Greek for food

A simple food chain

Here is a typical land food chain:

Sun → grass → rabbit → fox

The arrows mean "is eaten by" (energy flows in the direction of the arrow). The Sun gives its energy to the grass through photosynthesis. The rabbit gets some of that energy by eating the grass. The fox gets some of that energy by eating the rabbit. Each arrow loses energy, but the chain works.

A typical ocean food chain might be:

Sun → phytoplankton → zooplankton → small fish → tuna → shark

The links in the chain

Each step in a food chain is called a trophic level, from the Greek word for "food".

  • First trophic level (producers): plants and algae. They make food from sunlight, water and carbon dioxide.
  • Second trophic level (primary consumers): herbivores that eat the producers. Rabbits, cows, caterpillars, krill.
  • Third trophic level (secondary consumers): small carnivores that eat the herbivores. Foxes, frogs, small birds.
  • Fourth trophic level (tertiary consumers): bigger predators that eat the secondary consumers. Hawks, snakes, tuna.
  • Top trophic level (apex predators): predators with no natural predators of their own. Lions, sharks, eagles, polar bears.

The 10% rule

Energy is not transferred efficiently between trophic levels. Roughly 90% of the energy is lost at each step, as heat from breathing, movement and waste. Only about 10% makes it to the next level.

So if a meadow of grass contains 10,000 units of stored energy, only about 1,000 units reach the rabbits, only 100 units reach the foxes, and only 10 units would reach a hypothetical fifth-level predator. By the fifth or sixth step, there is barely any energy left. This is why food chains usually only have 3 to 5 steps, and why top predators are always rarer than the animals they eat.

The four basic shapes

Food chains can take several common shapes depending on where the energy starts.

  • Grazing food chain: starts with living plants. Most familiar (grass → rabbit → fox).
  • Detritus food chain: starts with dead organic matter. (Dead leaves → woodlice → shrew → owl).
  • Parasitic food chain: tiny parasites get bigger as you go up. (Plant → insect parasitised by wasp → wasp parasitised by smaller wasp). Uncommon shape but ecologically important.
  • Chemical-energy food chain: starts not with sunlight but with chemicals from hot deep-sea vents. (Hydrogen sulphide → chemosynthetic bacteria → tube worms → deep-sea crabs).
Fact The food chain that starts at deep-sea hydrothermal vents does not depend on the Sun at all. Around vents on the ocean floor, sometimes 3 km below the surface, boiling water full of chemicals pours out of cracks in the seabed. Special bacteria use those chemicals to make food (a process called chemosynthesis), feeding entire communities of tube worms, crabs, blind shrimp and giant clams that have never seen the Sun. Many scientists think life on Earth may have first appeared in places like this.

Decomposers: the recyclers

Every food chain eventually feeds into a vital extra group: the decomposers (bacteria, fungi and some animals like worms and woodlice). When any organism in the food chain dies, decomposers break it down and return its nutrients to the soil. Without decomposers, the world would slowly fill up with dead bodies and run out of nutrients for new plants. Decomposers close the loop and keep the whole ecosystem running.

Why food chains matter

Real ecosystems are far more complex than a single straight food chain (most animals eat more than one thing, and most are eaten by more than one predator). But food chains are still incredibly useful because they show:

  • How energy flows through an ecosystem.
  • Why removing one species can affect everything above and below it.
  • Why top predators are rare.
  • How pollution can build up in top predators (a process called biomagnification).
Did you know? Toxic chemicals tend to build up as they move up a food chain, an effect called biomagnification. A small amount of pollution in plankton becomes a slightly bigger amount in small fish, a bigger amount in tuna, and a much bigger amount in killer whales or polar bears at the top. This is one of the reasons doctors warn pregnant women not to eat too much tuna or swordfish: the mercury that builds up in those large fish can be harmful in big doses.
Deeper dive: how a single food chain can collapse a whole ecosystem

Food chains may look like neat lines, but in nature they connect into much bigger webs (see food webs). When a single link in a chain is broken, the effects can ripple in both directions and sometimes collapse a whole ecosystem.

A famous example is the collapse of the Atlantic cod fishery off Newfoundland, Canada, in the early 1990s. Cod had been the top fish predator of the region for centuries, feeding on smaller fish and shrimp. After decades of intense over-fishing, cod numbers crashed in 1992 to less than 1% of their historical level. The Canadian government banned all cod fishing, hoping the population would bounce back within a few years.

It did not. The smaller fish and shrimp that cod used to eat had exploded in numbers, and they ate the cod's baby food (zooplankton) so efficiently that cod numbers struggled to recover. The whole food web had rearranged itself into a new state with no top predator. Even more than 30 years later, cod numbers in the area are still far below historical levels, and the local fishing communities have not recovered economically.

This kind of cascading collapse is a major worry for modern fisheries. Many of the world's fish populations are under similar pressure, and ecologists work hard to understand how to manage them sustainably before the food chain breaks down completely.

For the fuller picture, see food webs. For the groups in a chain, see producers, consumers and decomposers.