What happens to the rabbits if all the foxes go away?
Your guess
Pick one before you read on. It is fine to be wrong.
By the end you can
Join food chains into a food web
Predict what happens to a food web if one animal disappears
A food chain shows one path of food. But most animals eat more than one thing. A fox eats rabbits and also mice. A hawk eats snakes and also rabbits.
So the chains cross. Watch two chains join up.
Draw it first
Join food chains into a web
1 Start with one chain. Grass is eaten by a rabbit. The rabbit is eaten by a fox.
2 Add a second chain. Grass is eaten by a mouse, the mouse by a snake, and the snake by a hawk.
3 Add the links that cross. A bush feeds the mouse with its berries. The fox eats mice. The hawk eats rabbits.
Now the chains are joined. This is a food web. Look at the arrows. Each one still points from the food to the animal that eats it.
The plants are at the bottom. They are the producers, because they make food from light. Algae in a pond do the same. Every other living thing in the web eats something.
One chain is only one path through the web.
A predator is an animal that hunts and eats other animals. Its prey is what it eats. In the web, the fox is a predator, and the rabbit and the mouse are its prey. The rabbit is also prey for the hawk.
The food web. Follow any arrow from the food to its eater.
The table lists who eats each living thing. It comes from the same web, so it never disagrees with the picture.
Each food and the animals that eat it.
Now ask a question. What if all the foxes went away? Disease, hunting or drought could do it. Take the fox out of the web.
See it three ways
With things
Write each living thing on a scrap of paper. Lay them out like the web. Join them with string. Pull away the fox.
As a picture
In words
The rabbit and the mouse lose a predator. The rabbit is still eaten by the hawk. The mouse is still eaten by the snake. But fewer of them are eaten, so their numbers can rise.
In symbols
The fox ate 2 kinds of animal, so 2 kinds lose a predator.
A population is all the animals of one kind in a place. With fewer foxes, the rabbit population can grow. More rabbits eat more grass. Then the grass runs low. Look at the graph.
A made-up example. The foxes went before year 1.
At first the rabbits rise. Then the grass is eaten down. Many rabbits go hungry, and the number falls. The hawk, which eats rabbits, also has less food. One change spreads along the arrows.
Try it
Be the web
You need: a few scraps of paper or flat stones, a pencil, a piece of string.
Write algae, snail, insect, fish, frog and heron on six scraps. Lay them on the floor with algae at the bottom.
Lay a piece of string from each food to the animal that eats it. Algae is eaten by the snail and the insect.
Fish eat snails and insects. Frogs eat insects. Herons eat fish and frogs.
Pull the snail scrap away. See which strings go loose, and what each animal still has to eat.
What you will see: The fish lose one food but still have the insect. The algae lose one eater, so the insect may eat more of it. Nothing is left without food, but every animal is touched.
If you cannot try it, this is what happens.
A wrong idea many people have
If a predator disappears, its prey is better off.
Why it feels right: Nothing is hunting them any more.
The test that breaks it: Look at the graph. Without foxes the rabbits rise, but then they eat the grass, and many go hungry.
What is true: Everything in a web is joined. Taking one living thing away changes the others, often in ways that hurt in the end.
Key words
food web
food chains joined together, showing what eats what in a place.
predator
an animal that hunts and eats other animals.
prey
an animal that a predator eats.
population
all the living things of one kind in a place.
producers
living things that make food from light, such as plants and algae. Most food chains start with one.
Back to your guess
The answer is C: At first there are more rabbits. Later they may eat most of the grass, and many go hungry.
Fewer rabbits are eaten, so their number can rise. But more rabbits eat more grass. When the grass runs low, many rabbits go hungry. Everything in a web is joined.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
Two chains are drawn below. One living thing is in both chains. Which one?
See the answer
grass
Both chains start with grass. A web joins chains at the living things they share.
If you chose B: The rabbit is only in the first chain. The second chain has a mouse instead. Go to S3.2.3.
If you chose C: The snake is only in the second chain. The first chain ends with a fox. Go to S3.2.3.
In a food web, what does an arrow point to?
See the answer
The animal that eats
An arrow shows the way the food goes: from the food into the animal that eats it.
If you chose A: It is the other way round. An arrow follows the food, so it points to the one that eats. Go to S3.2.3.
If you chose C: Size does not matter. The arrow shows where the food and its energy go, and that is to the eater. Go to S3.2.3.
All the foxes in the web disappear. Which animals lose a predator?
See the answer
The rabbit and the mouse, 2 animals
The arrows into the fox come from the rabbit and the mouse. They are the fox's prey, so they lose a predator.
If you chose A: The fox does not eat these. Look at the arrows that end at the fox. They come from rabbit and mouse. Go to S3.2.3.
If you chose C: A fox does not eat plants, so plants do not lose a predator. Plants are eaten by the rabbit and the mouse. Go to S3.2.3.
The snake disappears. What happens to the hawk?
See the answer
It loses the snake but still has 1 food, the rabbit.
The hawk eats two things: the snake and the rabbit. Without the snake it still has the rabbit, so it is not left without food.
If you chose B: The hawk also eats rabbits. One food is gone, but another is left. Go to S3.2.3.
If you chose C: The snake was one of its foods, so the hawk has less to eat, not more. It may hunt more rabbits. Go to S3.2.3.
Use the graph. Why do the rabbits fall in number after year 4?
See the answer
The rabbits ate the grass down, so many went hungry.
With no foxes the rabbits rose, and a large population ate most of the grass. With too little food, the number fell.
If you chose B: The story says the foxes had gone. Nothing in the graph says they returned. Go to S3.2.3.
If you chose C: Rabbits do not all die at four years. The fall is because there is too little food for so many. Go to S3.2.3.
From an earlier chapter
The fox, the snake and the hawk are all vertebrates. What do they have in common?
See the answer
A backbone
Every vertebrate has a backbone. Fur, scales and wings differ from one kind to another.
If you chose A: A snake has scales and a hawk has feathers. Only some vertebrates have fur. Go to S3.2.1.
If you chose B: A hawk has wings but a fox and a snake do not. Look for what every vertebrate has. Go to S3.2.1.
Keep it
Copy this into your exercise book.
A food web is food chains joined together. Arrows point to the eater. Take one living thing away and the others change. A population is all the animals of one kind in a place.
Draw: Draw a web with grass, rabbit, mouse and fox. Draw arrows from food to eater. Cross out the fox and say what changes.
