There is plenty of grass but only a few foxes. Why?
Your guess
Pick one before you read on. It is fine to be wrong.
By the end you can
Explain why food chains rarely have more than five links
Draw a pyramid of numbers for a food chain
Every living thing needs energy to move, grow and stay alive. Plants take in energy from sunlight. Animals get theirs from the food they eat.
We measure energy in kilojoules, written kJ. Say the grass in a field holds 1000 kJ. A stack of bars that shows the energy at each link is a pyramid of energy. Watch one build up.
Draw it first
A pyramid of energy
1 The grass holds 1000 kJ of energy from the Sun.
2 An insect eats the grass. About a tenth passes on, so the insect gets 100 kJ.
3 A frog eats the insect. It gets about a tenth of that, 10 kJ.
4 A snake eats the frog. Only 1 kJ is left for the snake.
Where does the rest go? The animal uses most of its food to move and to keep warm. That energy ends as heat. Some of the food passes out as droppings. Some parts, such as bones, are never eaten. Only the food built into its body can pass on.
Of every 100 kJ an animal eats, about 10 kJ pass on to the next animal.
The tenth is a rule of thumb. The real share changes from one chain to another, but it is always a small part.
Now work it out. Each step keeps about a tenth, so divide by 10 to go one step up.
Worked example
The grass in a field holds 1000 kJ. About a tenth passes on at each step. How much energy reaches the frog, the third link in the chain?
The insect gets a tenth of the grass: 1000 ÷ 10 = 100 kJ.
The frog gets a tenth of that: 100 ÷ 10 = 10 kJ.
Answer: 10 kJ
Now follow the energy to the end of a chain of five. This is a pyramid of energy too. Each step has a tenth of the one before. The bars are as wide as the energy. The last two are too thin to see.
See it three ways
With things
Imagine a field of grass. A chain of five living things feeds from it, one after another.
As a picture
In words
The fifth living thing gets only a ten thousandth of the energy in the grass. That is too little to feed a sixth. So food chains rarely have more than five links.
In symbols
1000 ÷ 10 ÷ 10 ÷ 10 ÷ 10 = 0.1 kJ
A chain of five links is about as long as a chain gets.
Scientists also count living things. A pyramid of numbers has one bar for each link in the chain. The producer is at the bottom. Each bar is as wide as the number of living things in it.
Numbers of living things in a field. The producer is at the bottom.
A pyramid of numbers is not always the shape of a pyramid. One big tree can feed hundreds of insects. A few birds eat those insects. The bottom bar is then thin and the middle bar is wide. A pyramid of energy always narrows going up, because energy is lost at every step. A pyramid of numbers need not.
One tree, many insects, a few birds. This is not a pyramid.
Try it
Pass the beans
You need: 100 beans, seeds or small stones, three bowls or cups.
Put 100 beans in the first bowl. They stand for the energy in the grass.
Move only 10 beans to the second bowl, for the insect. Push the other 90 to one side. They are used or lost.
From the 10, move only 1 bean to the third bowl, for the frog. Push the other 9 aside.
Try to pass on a tenth of one bean to a fourth bowl for the snake.
What you will see: One bean is too little to feed a snake. In a real chain, too little energy is left after a few links. The next animals cannot live on it.
If you cannot try it, this is what happens.
A wrong idea many people have
The animal at the top of a chain has the most energy.
Why it feels right: It is big and strong, and it eats the most food.
The test that breaks it: Look at the energy bars. Each step up holds only a tenth of the step below. All the hawks together hold the least energy of any step.
What is true: Energy gets less at each step. The top has the least. That is why there are few big hunters and so much grass.
Key words
energy
what a living thing needs to move, grow and stay alive.
pyramid of energy
a stack of bars that shows the energy at each link. It always narrows going up.
pyramid of numbers
a picture with a bar for each link in a chain. Each bar is as wide as the number of living things.
Back to your guess
The answer is C: Only about a tenth of the energy passes up each step, so higher animals get less.
At each step of a food chain, most energy is used for living and ends as heat. About a tenth passes on. So each step up has less energy, and can feed fewer living things.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
About how much of the energy in food passes on to the next animal in a chain?
See the answer
About a tenth
An animal uses most of its food for living, and loses much as heat. About a tenth passes on to the next animal.
If you chose A: Animals use most of the energy to live, and lose much as heat. Far less than all passes on. Go to S3.2.4.
If you chose C: It is much less than half. A rough rule is about a tenth. Go to S3.2.4.
The grass in a field holds 5000 kJ. About a tenth passes on at each step. How much reaches the frog, the third link?
See the answer
50 kJ
The insect gets 5000 ÷ 10 = 500 kJ. The frog gets 500 ÷ 10 = 50 kJ.
If you chose B: That is the energy that reaches the insect, the second link. The frog is one step further. Go to S3.2.4.
If you chose C: That is the energy that reaches the snake, the fourth link. Stop at the frog, the third. Go to S3.2.4.
Why do food chains rarely have more than five links?
See the answer
There is too little energy left to feed another link.
At each step only about a tenth of the energy passes on. After four or five steps the amount is too small to feed more.
If you chose A: Size is not the reason. Chains of tiny animals are short too. The limit is energy. Go to S3.2.4.
If you chose C: Plants are not affected by how long the chain is. Energy is lost at each step, so there is too little left. Go to S3.2.4.
A chain has 500 grass plants, 50 insects and 5 birds. How should its pyramid of numbers be drawn?
See the answer
The grass at the bottom as the widest bar, the birds at the top as the narrowest
The producer is at the bottom. Each bar is as wide as its number: 500, then 50, then 5. So the bars get narrower going up.
If you chose A: The producer goes at the bottom. The grass has the biggest number, so its bar is the widest. Go to S3.2.4.
If you chose B: Each bar must be as wide as its number. The numbers differ, so the widths differ. Go to S3.2.4.
Look at the picture of one tree. Why is it not a pyramid?
See the answer
One big tree feeds a great many insects, so the middle bar is the widest.
A pyramid of numbers counts living things, not size. One tree is one living thing, but it feeds hundreds of insects.
If you chose A: Each bar is exactly as wide as its number. The picture is right. The numbers do not make a pyramid. Go to S3.2.4.
If you chose C: The tree is the producer, because it makes food from light. It stays at the bottom, even though it is a single living thing. Go to S3.2.4.
From an earlier chapter
In a food web, which way does the arrow between a rabbit and a fox point?
See the answer
From the rabbit to the fox
The rabbit is the food and the fox is the eater. An arrow points the way the food goes, to the fox.
If you chose B: That would mean the rabbit eats the fox. An arrow points to the eater, from the food. Go to S3.2.3.
If you chose C: A fox eats a rabbit, so they are joined by an arrow from the food to the eater. Go to S3.2.3.
Keep it
Copy this into your exercise book.
Energy goes up a food chain, but only about a tenth passes on at each step. The rest is used for living or lost as heat. A pyramid of energy always narrows going up. A pyramid of numbers need not.
Draw: Draw a pyramid of numbers for grass 200, rabbits 40 and foxes 5. Make each bar as wide as its number.
