Food chains and webs

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Key points

  • All organisms in an ecosystem depend on each other.
  • Food chains show the flow of energy from one organism to another.
  • Food chains show the feeding relationships between organisms.
  • Food webs show how all the food chains in an ecosystem interact.

Food chains

Video - Food chains and ecosystems

Can you answer these questions based on the video?

1. What does a food chain start with?

2. What is shown by the arrows in a food web?

  1. A producer, which is often a plant.
  2. The flow of energy.


Ecology is the study of living organisms and the places that they live. An ecologist studies the number and distribution of living organisms in an . Knowing this is essential to allowing us to protect the organisms that need help to survive.

Trophic levels

A food chain is a list of organisms in a that shows their feeding relationship, i.e what eats what. The organisms are joined by arrows which show the transfer of energy in food between them. The stages in food chains are called trophic levels.

a food chain showing grass at the bottom, then rabbit in the middle, fox at the top
A simple food chain with three trophic levels

Food chains always start with a producer. This is usually a green plant or algae that completes to store energy from sunlight as glucose. Grass is the producer in the grass → rabbit → fox food chain. Photosynthesis provides the energy for most life on Earth.

A primary consumer eats a producer. The rabbit is the primary consumer in the example food chain. This is in turn eaten by a secondary consumer, which is the fox.

After this might be a tertiary consumer (which eats a secondary consumer) and possibly a quaternary consumer (which eats a tertiary consumer), but not in this example.

Animals that are hunted and eaten are prey, and these are consumed by predators. The final consumer at the top of the food chain is called a top (or apex) predator and is not eaten by anything else.

Nothing in this food chain eats the fox, which makes it the apex predator

What is the final consumer in this food chain?

grass → grasshopper → frog → snake → Hawk WITH ACCOMPANYING LABELS Producer → Primary consumer → Secondary consumer → Tertiary consumer → quaternary consumer and Top predator
A food chain with five trophic levels.

The hawk is the final consumer, or apex predator, in this food chain.

Volcanic vents

There are volcanic vents at the bottom of the oceans where it is so dark that no plant or algae could live. Places like this are the only food chains on Earth that don’t start with photosynthesis.

The producers are bacteria which feed directly on the chemicals released from the vents and use a variety of chemical reactions to make glucose. This process is called chemosynthesis.

A volcanic vent where no plants or algae can grow

Video - Where does energy come from?

Can you answer these questions based on the video?

1. Where does almost all the energy in food chains originally come from?

2. What is an animal that isn’t eaten by anything else called?

3. What do lots of food chains make?

4. What eventually happens to the top consumer?

  1. The Sun.
  2. An apex (or top) predator.
  3. A food web.
  4. They die and rot returning the nutrients to the soil or are consumed.

Food webs

Most of organisms that live in a habitat usually have more than one food source. They usually consume more than one organism from the trophic level below. This means that there are almost always more than one food chain and these are interlinked into a food web.

a food web showing what eats what and the transfer of energy between organisms
A food web

This food web is made up of lots of food chains, including:

  • grass → insect → vole → hawk
  • grass → insect → frog → fox
  • grass → insect → vole → fox

Some organisms, like the rabbit and slug, have just one consumer. Others, like the frog and vole, have two.

Which animal has three consumers?

The insect.

Toxic materials in the food chain

Toxic materials are poisonous. Some quickly break down into harmless substances in the . Others are persistent (they stay in the environment and do not break down). These substances in the food chain and damage the organisms in it, particularly in the predators at the end of the chain. This is because accumulating compounds cannot be excreted.

In the past, mercury compounds were used to make insecticides (substances that kill the insects that damage crops), and special paints that stop barnacles growing on the hulls of ships.

Unfortunately, when mercury gets into a food chain, it damages the and of mammals, including humans. The diagram shows how mercury can accumulate in the food chain.

a food chain showing plankton, small fish, large fish. the amount of mercury increases as the organisms get larger
Mercury accumulates as you move up the food chain.

In the sea, tiny animals and plants called plankton absorb the mercury compounds. When the plankton are eaten by small fish, the mercury they contain stays in the fish. As the fish need to eat a lot of plankton, the concentration of mercury in them becomes higher than its concentration in the plankton.

Larger fish eat the small fish, and larger ones still (such as tuna fish) eat them. This creates a high concentration of mercury in the tuna. People eating contaminated tuna may get mercury poisoning. Mercury is now banned from many chemical products and mercury use in industry is carefully regulated.

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