---
title: "Food Chains"
book: "Primary & Middle School Biology"
subject: biology
language: en
chapter: 19
exercises: 10
source: https://one-course.com/books/biology/1/en/chapter/19-food-chains
---

# Chapter 19 — Food 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](https://one-course.com/books/biology/1/en/chapter/13-caring-for-nature#def-g2-caring-for-nature-environment)’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](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living) 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](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living) 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](#def-g3-food-chains-chain) the grasshopper”. Writing the arrows backward — toward the meal — is the commonest mistake in [food chains](#def-g3-food-chains-chain); 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](#def-g3-food-chains-chain) is always a [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) (or another [living thing](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living) that makes its own food). [Plants](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) alone feed without eating anyone — from light, water and air, as [Definition 3.1](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) said. Every [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-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](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) $\rightarrow$ tadpole $\rightarrow$ dragonfly larva $\rightarrow$ stickleback $\rightarrow$ heron. In the forest: oak [leaves](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) $\rightarrow$ [caterpillar](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#ex-g2-animals-grow-up-butterfly) $\rightarrow$ 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.](https://one-course.com/images/onecourse/chapters/biology-1/g3-food-chains/img-48a89061087f.jpg)

*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).**

1. Choose an [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) and ask: what does it eat? Write the food to its left, arrow pointing at the eater;
2. ask the same question of the food, and continue leftward until you reach a [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) ;
3. ask: who eats my starting [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) ? Extend rightward while there is an eater;
4. check every arrow reads [“is eaten by”](#def-g3-food-chains-chain) — food flowing to the right.

## 19.2 When a link changes

**Proposition 19.6 (Links depend on each other).**

In a [food chain](#def-g3-food-chains-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](https://one-course.com/books/biology/1/en/chapter/10-what-animals-eat#ch-g2-what-animals-eat) 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](#def-g3-food-chains-chain) mean? Which way does it point?

**Solution of Exercise 19.1.**

“[Is eaten by](#def-g3-food-chains-chain)”. It points from the eaten to the eater — following the food.

**Exercise 19.2 ★.**

What kind of [living thing](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living) is always the first link? Why?

**Solution of Exercise 19.2.**

A [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) (or another [living thing](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living) that makes its own food). Because [plants](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) alone feed without eating anyone — from light, water and air; every [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-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 of Exercise 19.3.**

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

**Exercise 19.4 ★.**

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

**Solution of Exercise 19.4.**

The arrows are backward: grass $\rightarrow$ rabbit $\rightarrow$ fox — grass [is eaten by](#def-g3-food-chains-chain) the rabbit, which [is eaten by](#def-g3-food-chains-chain) the fox.

**Exercise 19.5 ★.**

Build a chain from these links: blue tit, oak [leaves](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts), sparrowhawk, [caterpillar](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#ex-g2-animals-grow-up-butterfly).

**Solution of Exercise 19.5.**

Oak [leaves](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) $\rightarrow$ [caterpillar](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#ex-g2-animals-grow-up-butterfly) $\rightarrow$ blue tit $\rightarrow$ sparrowhawk.

**Exercise 19.6 ★.**

In the garden chain salad $\rightarrow$ snail $\rightarrow$ hedgehog, who [is eaten by](#def-g3-food-chains-chain) the snail? Who eats the snail?

**Solution of Exercise 19.6.**

The salad [is eaten by](#def-g3-food-chains-chain) the snail; the hedgehog eats the snail.

**Exercise 19.7 ★.**

Use [Method 19.5](#met-g3-food-chains-build) to make a chain of at least three links ending with the heron of the pond.

**Solution of Exercise 19.7.**

For example: water [plants](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) $\rightarrow$ tadpole $\rightarrow$ stickleback $\rightarrow$ heron.

**Exercise 19.8 ★.**

In the dry summer of [Example 19.7](#ex-g3-food-chains-dry), why does the buzzard suffer although it never eats grass?

**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](#prop-g3-food-chains-depend).

**Solution of Exercise 19.9.**

Poisoning the snails starves their eater: the hedgehog [leaves](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) 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 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.