Food chains and food webs

Who eats whom

In a field, a grasshopper eats grass. A frog eats the grasshopper, and a snake eats the frog. Food passes from one living thing to the next.

A series of living things in which each one feeds on the one before it is a food chain.

The arrows point from the food to the eater. They show the way food, and the energy in it, moves along the chain.

Here are a few more food chains.

• In a forest: grass → deer → tiger.
• In a pond: water plants → small fish → big fish → heron.
• In a field: grass → grasshopper → frog → snake → eagle.

Trophic levels

Each step of a food chain is a trophic level. The word "trophic" means to do with food.

• The first trophic level is always the producers, the green plants.
• The second level is the herbivores, or primary consumers.
• The third level is the small carnivores, or secondary consumers.
• The fourth level is the large carnivores, or tertiary consumers.

Every food chain starts with a producer, because only producers can make food from sunlight. Decomposers act on dead matter from every level.

Food webs

A real animal seldom eats only one kind of food. A frog eats grasshoppers and other insects. A snake eats frogs and mice. A hawk eats mice, snakes and small birds.

So the food chains of an ecosystem cross and link with each other. The network of linked food chains is a food web.

A food web is steadier than a single chain. If one kind of food runs short, an animal can often eat another. In a single chain, losing one link would starve everything above it.

Fewer animals at each level

A field of grass can feed many rabbits, but only a few foxes. Each rabbit eats many plants, and each fox needs many rabbits. So the number of living things usually falls at each higher trophic level.

Try it below. Change the grass, or what each animal needs, and see how many rabbits and foxes the field can support.

Try it in Python

The program counts how many rabbits and foxes a field of $8000$ grass plants can support.

Practise with new numbers

Common mistakes

• "The arrow means 'eats'." The arrow points from the food to the eater. It shows the flow of food and energy.
• "A food chain can start with an animal." Every chain starts with a producer.
• "A food web is one long food chain." A food web is many chains linked together.
• "There are more tigers than deer, since tigers are bigger." Higher levels usually hold fewer animals.

Remember these

• A food chain shows who eats whom, starting with a producer.
• Each step is a trophic level: producers, herbivores, then carnivores.
• Food chains link into a food web, because most animals eat more than one kind of food.
• A food web is steadier than a single chain.
• The number of living things usually falls at each higher level.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. In the chain grass → goat → lion, which is the producer and which is at the third trophic level?

2. Why does every food chain begin with a green plant?

3. In a pond chain, water plants → mosquito larvae → fish → kingfisher, what are the fish?

4. Why is a food web steadier than a single food chain?

5. A field supports thousands of grasshoppers but only a few snakes. Give the reason.