Primary & Middle School Biology · Grades 1–9

19Food Chains

Grass grows in the meadow. A grasshopper eats the grass. A lizard catches the grasshopper. A buzzard, circling high, takes the lizard. Four meals — and suddenly the buzzard’s afternoon depends on how green the grass grew. Nature’s meals link up into chains, and learning to read them is learning how a whole meadow holds together.

19.1 Reading and writing a food chain

Definition 19.1 (Food chain)

A food chain is a line of living things in which each one is eaten by the next. It is written with arrows, each arrow meaning “is eaten by”:

\text{grass} \longrightarrow \text{grasshopper} \longrightarrow \text{lizard} \longrightarrow \text{buzzard}.

Each living thing is a link of the chain.

Remark 19.2 (Mind the arrow’s direction)

The arrow points from the eaten to the eater — it follows the food. “Grass $\rightarrow$ grasshopper” reads “grass is eaten by the grasshopper”. Writing the arrows backward — toward the meal — is the commonest mistake in food chains; remember that the arrow shows where the food goes.

Proposition 19.3 (Every chain starts with a plant)

The first link of a food chain is always a plant (or another living thing that makes its own food). Plants alone feed without eating anyone — from light, water and air, as Definition 3.1 said. Every animal link lives, directly or through others, on that first green link.

Proof. Admitted at this level. ∎

Example 19.4 (Chains from three habitats)

In the pond: water plants $\rightarrow$ tadpole $\rightarrow$ dragonfly larva $\rightarrow$ stickleback $\rightarrow$ heron. In the forest: oak leaves $\rightarrow$ caterpillar $\rightarrow$ blue tit $\rightarrow$ sparrowhawk. In the garden: salad $\rightarrow$ snail $\rightarrow$ hedgehog. First link: always green.

The meadow chain’s last link on patrol: the buzzard’s afternoon depends, three links down, on how green the grass grew.

Method 19.5 (Building a food chain)

Choose an animal and ask: what does it eat? Write the food to its left, arrow pointing at the eater;
ask the same question of the food, and continue leftward until you reach a plant;
ask: who eats my starting animal? Extend rightward while there is an eater;
check every arrow reads “is eaten by” — food flowing to the right.

19.2 When a link changes

Proposition 19.6 (Links depend on each other)

In a food chain, every link depends on the ones before it, and feeds the ones after it. If one link becomes scarce, the next links go hungry — and the link before it, no longer eaten so much, may multiply. A change in one link travels along the whole chain.

Proof. Admitted at this level. ∎

Example 19.7 (The dry summer)

A drought browns the meadow grass. Grasshoppers, short of food, grow few; the lizards that hunted them go hungry and feed the buzzard poorly in their turn. One brown first link, and the whole chain above it thins — the buzzard never ate grass, yet it feels the drought.

Example 19.8 (Removing the hunter)

Suppose the buzzards vanish from the valley. Lizards, less hunted, multiply; grasshoppers are then eaten harder and dwindle; the grass, less nibbled, may even thicken. Removing the last link also travels down the chain — in the other direction.

Remark 19.9 (Real meadows are webs)

Our chain is a single thread, but the buzzard also takes mice, the lizard also eats flies, and the grass feeds rabbits too. Many chains crossing make a net — the food web that a later year will study (Chapter 10 already hinted at it with the fox, the rabbit and the grass). Chains first: they are the threads the net is woven from.

19.3 Exercises

Exercise 19.1 ★

What does the arrow of a food chain mean? Which way does it point?

Solution

Solution of Exercise 19.1.

“Is eaten by”. It points from the eaten to the eater — following the food.

Exercise 19.2 ★

What kind of living thing is always the first link? Why?

Solution

Solution of Exercise 19.2.

A plant (or another living thing that makes its own food). Because plants alone feed without eating anyone — from light, water and air; every animal link lives off that first green link, directly or through others.

Exercise 19.3 ★

Write the meadow chain of this chapter with its four links and its arrows.

Solution

Solution of Exercise 19.3.

Grass $\rightarrow$ grasshopper $\rightarrow$ lizard $\rightarrow$ buzzard.

Exercise 19.4 ★

Correct this chain: fox $\rightarrow$ rabbit $\rightarrow$ grass.

Solution

Solution of Exercise 19.4.

The arrows are backward: grass $\rightarrow$ rabbit $\rightarrow$ fox — grass is eaten by the rabbit, which is eaten by the fox.

Exercise 19.5 ★

Build a chain from these links: blue tit, oak leaves, sparrowhawk, caterpillar.

Solution

Solution of Exercise 19.5.

Oak leaves $\rightarrow$ caterpillar $\rightarrow$ blue tit $\rightarrow$ sparrowhawk.

Exercise 19.6 ★

In the garden chain salad $\rightarrow$ snail $\rightarrow$ hedgehog, who is eaten by the snail? Who eats the snail?

Solution

Solution of Exercise 19.6.

The salad is eaten by the snail; the hedgehog eats the snail.

Exercise 19.7 ★

Use Method 19.5 to make a chain of at least three links ending with the heron of the pond.

Solution

Solution of Exercise 19.7.

For example: water plants $\rightarrow$ tadpole $\rightarrow$ stickleback $\rightarrow$ heron.

Exercise 19.8 ★

In the dry summer of Example 19.7, why does the buzzard suffer although it never eats grass?

Solution

Solution of Exercise 19.8.

Because its food’s food’s food is grass: fewer grasshoppers on the browned grass means fewer lizards, and fewer lizards means lean hunting for the buzzard. A change in the first link travels up the whole chain.

Exercise 19.9 ★★

Gardeners who poison every snail sometimes lose their hedgehog — and then meet more slugs and snails than before. Explain both halves with Proposition 19.6.

Solution

Solution of Exercise 19.9.

Poisoning the snails starves their eater: the hedgehog leaves or dies. Later, with the hedgehog — the snails’ main enemy — gone, the surviving slugs and snails multiply harder than before: removing links travels both up the chain (hungry hedgehog) and back down it (unhunted snails).

Exercise 19.10 ★★

“The lizard depends on the grass.” True or false? Defend your answer with the meadow chain — and say in which direction a grass failure travels.

Solution

Solution of Exercise 19.10.

True — through the middle of the chain. The lizard eats grasshoppers, and grasshoppers eat grass: if the grass fails, the failure travels up, link by link, from grass to grasshopper to lizard. The lizard never touches grass, and depends on it all the same.

Terms defined in this chapter

Food chain

See all 479 terms in the glossary