Ecosystems. Attnom Academy

S3.2.1 Chapter 1

Sorting living things

A mushroom grows out of the soil, but it is not green. Is it a plant?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A No. It is a fungus, which is a group of its own.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Yes. It grows in the soil and stays in one place, like a plant.
You picked B. Read on. At the end, the chapter comes back to your guess.
C No. It is an animal, because it is not green.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Sort living things into plants, animals, fungi and microbes
Sort animals into vertebrates and invertebrates, with examples

All the living things in one place, and the way they depend on each other, make an ecosystem. A pond is one. A forest is another. To study an ecosystem, you first need names for what lives in it.

There are millions of kinds of living things. Nobody can learn them one by one. So we sort them into groups.

A group tells you what to expect. If you know a thing is in a group, you already know a lot about it. Watch the four big groups build up.

Draw it first

The four groups of living things

1 Plants stand in one place. Grass, ferns and trees are plants.
2 Animals move about to find food. An ant, a fish and a goat are animals.
3 Fungi are mushrooms, moulds and puffballs. They are not plants.
4 Microbes are living things too small to see without a microscope.

Plants make their own food from light. They have green leaves for this. Animals cannot make food, so they eat other living things.

A plant. The green leaves catch light to make food.

Fungi (one is a fungus) have no leaves and no green. They cannot make food from light. They soak up food from the things they grow on, such as dead wood or old bread.

See it three ways

As a picture

In words

Plants make food from light. Animals eat food. A mushroom does neither, so it fits in neither circle. It is a fungus.

Microbes are the smallest living things. They are all around you, in soil, water and air. One drop of pond water holds thousands. Some microbes help us and some make us ill. Some fungi, such as yeast, are microbes too. Algae, which grow in ponds and the sea, make food from light like plants.

How each group gets food.

Now look inside the animal group. Some animals have a backbone, a row of small bones down the back. We call them vertebrates. People, fish, frogs, snakes, birds and goats are vertebrates.

The spine is the backbone. A person is a vertebrate.

Animals with no backbone are invertebrates. Ants, spiders, snails, worms and crabs are invertebrates. Most animals on Earth are invertebrates.

Animals with a backbone, and animals without one.

Try it

Find and sort

You need: a pencil, a sheet of paper, a small place outside, such as a yard or a path edge.

Be safe: Do not touch or taste a mushroom. Stay away from nests and from animals that sting or bite. Wash your hands when you finish.

Draw three columns on the paper. Write plants, animals and fungi at the top.
Look around the place for ten minutes. Write the name or draw each living thing in its column.
Do not pick, dig or catch anything. Look, count, and leave it as you found it.
Count each column.

What you will see: Most of what you find is plants. You find fewer animals, because many hide or move away. You may find no fungi at all. Microbes are there too, but too small to see.

If you cannot try it, this is what happens.

A wrong idea many people have

A mushroom is a plant.

Why it feels right: It grows out of the soil and stays in one place.

The test that breaks it: Look for green leaves. A mushroom has none. It can grow in a dark, damp place where a plant would die for lack of light.

What is true: A mushroom is a fungus. Fungi are a group of their own. They cannot make food, so they soak it up from what they grow on.

Key words

ecosystem: all the living things in one place and the way they depend on each other.
fungi: living things such as mushrooms and moulds. They have no leaves and soak up food.
microbes: living things too small to see without a microscope.
backbone: the row of small bones down the back, also called the spine.
vertebrates: animals with a backbone.
invertebrates: animals with no backbone.

Back to your guess

The answer is A: No. It is a fungus, which is a group of its own.

A mushroom is a fungus. Fungi make no food from light and have no leaves, so they are not plants. They do not eat and move like animals either.

Keep it

Copy this into your exercise book.

Living things go in four groups: plants, animals, fungi and microbes. A fungus is not a plant. Animals with a backbone are vertebrates. Animals with none are invertebrates.

Draw: Draw four boxes and write the groups. Put two living things in each. Write vertebrate or invertebrate by each animal.

S3.2.2 Chapter 2

Using and making keys

I found a small creature under a stone. How can I find out its name?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A Answer yes or no questions, one at a time, until only one name is left.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Pick the clue that stands out most, such as its colour, and match it to a picture.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Look at how big it is. Big and small animals have different names.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Use a branching key to name an unknown leaf or insect
Make a key with yes or no questions for four animals

Naming a living thing is hard when there are so many. A key helps. It is a set of yes or no questions. Each answer leads you along a branch, and each branch ends at a name.

This key is for four small animals only. Watch it grow, one row at a time.

Draw it first

A key for four small animals

1 Start at the top with one question. Does the animal have wings?
2 Yes goes left to bee. No goes right, to the next question.
3 The next question is about legs. Yes goes to spider. No goes down.
4 The last question is about a shell. Yes is a snail. No is a worm.

Every animal in the key has one path to its name, and no other. To use the key, look at your animal and answer each question truthfully. Do not skip a question.

The clues the key uses. Each question asks about one column.

A key also works for plants. Look at a leaf. Is it shaped like a needle? Is it long and thin? Does it have many small leaflets? The heavy line shows the way to a fern.

A leaf key. The heavy line is the path to fern.

Now make your own key. Choose questions with a clear answer that you can see. Wings, legs, fins, feathers and a shell are good clues. Colour and size are poor clues. They change from one animal to the next of the same kind. A key like this one works only for the things in it.

A good question has a clear answer that is the same for everyone.

See it three ways

With things

Put four animals in a row. Ask a question that sorts out one of them. The rest stay in a group.

As a picture

In words

Each question splits the animals into two groups. The next question splits one group again. You stop when every group holds one animal.

In symbols

Four animals need three questions: 4 − 1 = 3. A key for n animals has n − 1 questions.

Try it

Make a key for four animals

You need: a pencil, a sheet of paper, four animals you know, such as a hen, a fish, a dog and a snake.

Write the four names. Find a question that is true of only one of them.
Draw the question in a box. Draw a branch to the left for yes, with that name at the end.
Draw a branch to the right for no. Write a new question that sorts out the next animal.
Keep going until every animal has its own end. Ask a friend to name an animal with your key.

What you will see: Three questions are enough for four animals. If two animals end in one place, you need one more question. If a question has no clear answer, change it.

If you cannot try it, this is what happens.

A wrong idea many people have

Questions about colour or size make a good key.

Why it feels right: Colour and size are the first things you see.

The test that breaks it: Look at the ants from one nest. Some are big and some are small. No one answer about size is true of every ant.

What is true: Choose clues that stay the same, such as the number of legs, wings or a shell. A key is only as good as its questions.

Key words

key: a set of yes or no questions that leads you to the name of a living thing.
branch: one path in a key, from one answer to the next question or to a name.

Back to your guess

The answer is A: Answer yes or no questions, one at a time, until only one name is left.

A key uses yes or no questions about clear clues. Each answer rules some names out, until one name is left. Colour and size are poor clues, because they vary in one kind of animal.

Keep it

Copy this into your exercise book.

A key is a set of yes or no questions. Each answer leads along a branch to a name. Choose clear clues such as legs, wings or a shell. A key for n animals has n − 1 questions.

Draw: Draw a key for a bee, a spider and a worm. Write each question in a box with yes on the left and no on the right.

S3.2.3 Chapter 3

Food webs

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.

Your guess

A Nothing changes. Foxes and rabbits do not affect each other.
You picked A. Read on. At the end, the chapter comes back to your guess.
B The rabbits are safe, so there will be more and more of them for ever.
You picked B. Read on. At the end, the chapter comes back to your guess.
C At first there are more rabbits. Later they may eat most of the grass, and many go hungry.
You picked C. Read on. At the end, the chapter comes back to your guess.

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.

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.

S3.2.4 Chapter 4

Energy through a food chain

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.

Your guess

A Foxes are big, so only a few can fit in a place.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Animals get less energy from the Sun than plants do.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Only about a tenth of the energy passes up each step, so higher animals get less.
You picked C. Read on. At the end, the chapter comes back to your guess.

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.

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.

S3.2.5 Chapter 5

Decomposers and recycling

What happens to a leaf that falls to the ground and nobody picks up?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A It stays there for ever, because nothing eats a dead leaf.
You picked A. Read on. At the end, the chapter comes back to your guess.
B It dries up and blows away into nothing.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Living things break it down. Some goes into the air as gas, and the nutrients go into the soil.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain how fungi, bacteria and worms break down dead things
Make a compost heap and record what rots first

A forest drops leaves every year. Animals die too. Yet the ground is never buried deep in dead things. Something is clearing them away. Watch the cycle build up.

Draw it first

Dead things are recycled

1 Plants make food from light. Animals eat the plants, or each other.
2 Plants and animals die, and leaves and waste fall. These are the remains.
3 Decomposers feed on the remains and break them down.
4 This puts nutrients into the soil. Plants take them up and grow.

Decomposers are living things that break down dead plants, dead animals and waste. Nutrients are the simple substances that plants need from the soil, such as those in a rotted leaf.

The decomposers are fungi and bacteria. A fungus sends fine threads into a dead thing and soaks up food from it. Bacteria are microbes. They live on every surface and feed on dead things too.

Worms and woodlice help in a different way. They are detritivores. They eat dead leaves and tear them into small pieces. Small pieces have more surface, so fungi and bacteria can reach more of the leaf.

Decomposers break things down. Detritivores eat dead things and make small pieces.

See it three ways

With things

Look under dead leaves on the ground. You may see a worm or a woodlouse eating them.

As a picture

In words

Not every food chain starts with a living plant. A dead leaf can feed a worm, and a worm can feed a bird.

Decomposers are living things, so they need the same as other living things: warmth, water and air. Food goes bad fast on a warm, damp day. It lasts longer when it is kept cold or dried.

Decay is fastest when it is warm and damp.

Compost is rotted plant waste. Gardeners and farmers make it in a heap. The decomposers turn the waste into dark, crumbly soil that is full of nutrients. You can make a small heap and watch.

Layers of plant waste and soil. Put in plant waste only, because meat and bones attract rats.

Try it

Make a compost pot

You need: a pot, tin or box with a few holes in the bottom, damp soil, a fruit peel, a leaf, a piece of paper and a small stick, gloves, or water and soap for your hands.

Be safe: Wear gloves, or wash your hands well afterwards. Do not put your face near mould. Do not eat or taste anything from the pot. Use plant waste only.

Half fill the pot with damp soil. Bury the peel, the leaf, the paper and the stick in four places. Mark each place with a pebble.
Keep the soil damp, not wet. Put the pot in a warm place out of the rain.
Every few days, dig gently at each place. Look, then cover it again.
Write down the day each thing has rotted. Put the numbers in a table.

What you will see: Most often the peel rots first, then the leaf, then the paper. The stick takes longest, because wood is tough. Decomposers work on soft things first.

If you cannot try it, this is what happens.

This ring is the cycle again. The same nutrients are used by plant after plant, because decomposers return them to the soil.

Decomposers close the ring. Plants that die go straight to the remains too.

A wrong idea many people have

Dead things just disappear.

Why it feels right: A fallen leaf is gone after a few weeks.

The test that breaks it: Turn over the leaves on the ground. Under the top ones they are in bits. Below that is dark, crumbly soil from the leaves that fell before.

What is true: Nothing disappears. Decomposers change dead things. Some of the matter goes into the air as gas. The nutrients go back into the soil, where plants use them.

Key words

decomposers: fungi and bacteria that break down dead things.
nutrients: simple substances that plants take from the soil to grow.
detritivores: animals such as worms and woodlice that eat dead leaves in small pieces.
compost: rotted plant waste that makes good soil.

Back to your guess

The answer is C: Living things break it down. Some goes into the air as gas, and the nutrients go into the soil.

Fungi and bacteria feed on the dead leaf and break it down. Some of it goes into the air as gas. The nutrients go back into the soil, and plants use them to grow.

Keep it

Copy this into your exercise book.

Decomposers are fungi and bacteria. They break down dead things and put nutrients back in the soil. Worms and woodlice tear dead leaves into small pieces. Decay is fastest when it is warm and damp.

Draw: Draw a ring of five boxes: plants, animals, remains, decomposers, soil. Draw arrows round the ring.

S3.2.6 Chapter 6

Competition and adaptation

How can anything live in a desert, where it is hot, dry and bare?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A They have features that save water and help them cope with the heat and the cold.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Desert plants and animals do not need water.
You picked B. Read on. At the end, the chapter comes back to your guess.
C They make their own water out of sand.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain what plants and animals compete for
Describe how living things are adapted to deserts, rainforests and polar seas

Many living things share a place. But the place has only so much light, water and food. When living things need the same thing and there is not enough, they compete. A mate is an animal of the same kind that it breeds with.

Draw it first

What living things compete for

1 Plants compete for light, water, space and nutrients from the soil.
2 Animals compete for food, drinking water, shelter and mates.

Now look at plants in a crowd. Both squares below are 1 m wide. They are drawn to the same size.

16 plants share one square metre.

4 plants share one square metre.

In the crowd, each plant gets less light and less water. A crowded plant stays thin and weak. The same is true for animals. Two foxes that hunt the same rabbits each find less food.

See it three ways

With things

Imagine 10 litres of rain falling on a patch of ground. 5 plants share it equally.

As a picture

In words

The same water is shared by more plants, so each plant gets less. Twice as many plants means half the water each.

In symbols

10 L ÷ 5 plants = 2 L each

Living things that cope well in a place are said to be adapted to it. An adaptation is a feature that helps a living thing live in its habitat. Each habitat needs different ones.

First, a desert. A desert is very dry. In many deserts it is hot by day and cold at night. Look at the temperature through one day.

An example. Many deserts swing from hot to cold like this.

Plants of dry places often store water in thick stems. Spines lose less water than wide leaves. Many desert animals rest in a cool burrow by day. The fennec fox of the hot desert has big ears that lose heat.

Hot desert: a thick stem and spines for plants, big ears for a fennec fox.

Next, a tropical rainforest. It is warm and wet all year. Rain is heavy, and the forest floor is dark. Rainforests also grow in cooler places, but this picture is of the tropics. Tall trees race for the light. Many leaves end in a drip tip that lets the rain run off. Tree frogs have sticky toes to grip branches.

Tropical rainforest: tall trunks and drip tips for plants, sticky toes for tree frogs.

Last, a polar sea. The water is near freezing, at about 0 °C or a little below. Seals and whales have a thick layer of fat under the skin called blubber. It keeps the heat in. A smooth body helps them swim fast.

White fur hides a polar bear on the ice. Polar bears live only in the far north. Emperor penguins live in the far south.

Polar sea: blubber and a smooth body for seals and whales, white fur for a polar bear.

Try it

Crowded seedlings

You need: two cups with soil, 8 bean seeds, water, a ruler.

Be safe: Wash your hands after touching soil. Do not eat the seeds or seedlings.

Plant 6 seeds in one cup, and 2 seeds in the other. Use the same soil.
Give both cups the same water, and put them in the same light.
Look every few days. After three weeks, count the leaves on every seedling, and note how many are alive.
Find the mean number of leaves on a seedling in each cup. Notice the colour and the height too.

What you will see: The crowded seedlings often grow tall and thin, because they stretch for light. But they have fewer leaves, look paler, and some may die. Count leaves and look at colour, not height alone.

If you cannot try it, this is what happens.

A wrong idea many people have

An animal grows a new adaptation when it needs one.

Why it feels right: A polar bear lives in the cold of the far north and has thick fur. It seems it grew the fur because it was cold.

The test that breaks it: A person who moves to a cold place does not grow fur. A dog with short fur has puppies with short fur, however cold the yard is.

What is true: Adaptations are features that parents pass to their young. Animals with useful features live longer and have more young. Over many years, more of them have the feature.

Key words

compete: to need the same thing when there is not enough for all.
adaptation: a feature that helps a living thing live in its habitat.
blubber: a thick layer of fat under the skin that keeps heat in.

Back to your guess

The answer is A: They have features that save water and help them cope with the heat and the cold.

Every living thing needs water. Desert living things have adaptations that save water. Thick stems store it. Other features help them cope with heat by day and cold at night.

Keep it

Copy this into your exercise book.

Living things compete for light, water, space and food. An adaptation is a feature that helps a living thing live in its place. Parents pass adaptations to their young.

Draw: Draw three boxes: desert, rainforest, polar sea. Write one adaptation in each box and what it does.

S3.2.7 Chapter 7

Counting living things

How can anyone count the daisies in a whole field?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A They cannot. A field has too many plants to count.
You picked A. Read on. At the end, the chapter comes back to your guess.
B They look at the field and guess a number.
You picked B. Read on. At the end, the chapter comes back to your guess.
C They count the plants inside a few small frames, then use the counts to estimate the whole field.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Estimate the number of plants in a field using a square frame
Explain why samples should be placed at random

Counting every plant in a big field would take days. So scientists count a small part and use it to estimate the whole. The small part is called a sample.

For plants, the sample is taken with a quadrat. A quadrat is a square frame made of sticks and string. Here the frame is 1 m on each side, so it covers 1 m².

Draw it first

Count plants with a frame

1 Measure the field and mark it in squares the size of the frame. This field is 6 m by 4 m, so it has 24 squares.
2 Put the frame down on 5 squares. Choose the squares at random.
3 Count the plants inside each frame, and write the count in the frame.

Look closely at one frame. Count only the plants inside it. A plant lying across the string counts if its middle is inside. Use the same rule every time.

7 plants are inside this frame. The plants outside are not counted.

The next picture shows five frames placed at random. Each number is the plants counted in that frame.

See it three ways

With things

Write the numbers 1 to 24 on scraps of paper, one for each square. Fold them and put them in a cup. Shake the cup and draw 5 scraps without looking. Put a frame on each square you draw.

As a picture

In words

Each square had the same chance of being drawn. Some frames fell on thick patches and some on thin ones. That is what a fair sample looks like. Throwing a pebble is not fair. Squares near you are hit more often, and far squares are never hit.

In symbols

9 + 4 + 5 + 4 + 3 = 25 plants in 5 frames

Now work out the mean. To find the mean, add the counts and divide by how many counts there are. The mean is the number of plants in a typical frame.

The counts from the five frames, with their total and mean.

Each frame covers one square. The field has 24 squares. If a typical square holds 5 plants, the field holds 5 times 24.

Worked example

Five frames hold 9, 4, 5, 4, 3 plants. The field is 6 m by 4 m, so it has 24 squares the size of the frame. Estimate the plants in the field.

Add the counts: 9 + 4 + 5 + 4 + 3 = 25.
Divide by the number of frames: 25 ÷ 5 = 5 plants in a typical frame.
Multiply by the squares in the field: 5 × 24 = 120.

Answer: 120 plants

This is an estimate, not an exact count. Say that every plant had been counted, and the total was 125. The estimate of 120 is close. A different set of squares would give a slightly different estimate.

More frames make a better estimate. The graph shows the estimate after one frame, two frames and so on. It comes closer to the real total as more frames are counted.

The estimate after each frame. The real total is 125.

Now, why at random? Random means chosen by chance. Suppose you put every frame near the thick left edge. You would count a lot of plants and think the whole field is thick.

All the frames sit near the thick left edge. The mean is 9, so the estimate is 216. That is far too high.

Try it

Count a patch

You need: four sticks of equal length, about 1 m each, string to tie them into a square, 24 scraps of paper and a cup, a pencil and paper, a grassy patch of known size, such as 6 m by 4 m.

Be safe: Look for glass, thorns and nests before you start, and keep away from any you find. Look, count and leave the plants as they are.

Tie the sticks into a square frame. Pick one kind of plant to count, such as clover or daisies.
Lay a 1 m stick along the long side and then the short side. Multiply the two counts to get the squares. Say it is 24. Number them 1 to 24 row by row from one corner. Write the numbers on scraps and put them in the cup.
Shake the cup and draw 5 scraps without looking. Put the frame on each square you draw, and do not choose the square yourself.
Count the plants of your kind inside the frame. Write the number. Do this 5 times.
Find the mean. Multiply it by the number of squares in the patch.

What you will see: Each frame gives a different count. The mean smooths them out. Another time, with other draws, the answer will change a little but stay close.

If you cannot try it, this is what happens.

A wrong idea many people have

Put the frames where the plants are thickest, so you do not miss them.

Why it feels right: You want a good count, and thick patches have plenty to count.

The test that breaks it: Look at the picture of frames near the left edge. The estimate is 216, but the real total is 125.

What is true: Choose the squares at random. Then thick and thin patches both get a fair chance. The mean is a fair guide to the whole field.

Key words

sample: a small part that you count or measure to learn about the whole.
quadrat: a square frame, here 1 m on each side, laid on the ground to count plants.
estimate: a careful guess worked out from a sample.
mean: the total of the counts divided by how many counts there are.
random: chosen by chance, so that every place has the same chance.

Back to your guess

The answer is C: They count the plants inside a few small frames, then use the counts to estimate the whole field.

Scientists count the plants in a few square frames and work out the mean. Then they multiply by the number of frames that fit in the field. It is an estimate, and a fair one if the frames are placed at random.

Keep it

Copy this into your exercise book.

Count plants in a few square frames put down at random. Find the mean count. Multiply by the number of frames that fit in the field. Random choice makes the sample fair.

Draw: Draw a field as a grid of squares. Mark five squares with frames and write a count in each.

S3.2.8 Chapter 8

People and ecosystems

Can one person really change a whole forest, or a river?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A No. Ecosystems are far too big for people to change.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Only very large machines can change an ecosystem.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Yes. Farming, cutting trees and pollution all change the living things in a place.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain how farming, cutting trees and pollution change ecosystems
Plan a small action to protect an ecosystem near you

People live in ecosystems too. We grow food, cut wood and make waste. Each of these changes the living things around us. Start with a small forest web.

Draw it first

A forest food web

1 A tree feeds an insect. The insect feeds a bird.
2 A bush feeds a mouse with its berries. The mouse feeds the same bird.
3 The insect also eats from the bush. The chains are now joined.

Now see three ways that people change a web like this. The first is farming.

To farm, people clear land and plant one crop, such as wheat. A meadow or a forest holds many kinds of plant. A field of one crop holds very few. Fewer kinds of plant mean fewer kinds of food, so fewer kinds of animal can live there.

An example. A meadow holds many more kinds than a field of one crop.

A pest is a living thing that harms a crop. With one crop and few predators, pests can spread fast. Farmers can help by leaving strips of wild plants, and by growing different crops in turn. Farming is needed to feed people, so how we farm matters.

The second change is cutting trees. Trees are the main producers of a forest. Cut them and the web loses its base. Look at the web after the tree is cut.

The insect lost one food. Animals that lived in the trees lost their homes too.

Animals that ate the trees lose food. Animals that lived in them lose their homes. Without roots, soil can wash away in the rain. Wood is useful, so people can cut some trees and plant new ones.

An example. The forest loses trees faster than they grow back.

The third change is pollution. Pollution is waste that harms living things. Litter, oil, smoke and chemicals are pollution. Too much fertiliser washed into a river is pollution too. It makes water plants grow too fast. When they die, bacteria use up the oxygen in the water, and fish die.

Some small animals need clean water. Most mayfly larvae need clean water with plenty of oxygen. If you find none, the water may be polluted. Scientists counted mayfly larvae at four places along a river. Place A is above a town. Place B is at the town. C is just below it, and D is far below.

An example. Few larvae at the town, so something there harms the water.

People can harm an ecosystem, and people can protect one. Sort some actions.

See it three ways

As a picture

In words

An action harms a place when it removes producers, poisons the water or air, or takes away homes. It helps when it does the opposite.

Try it

Plan a small action

You need: a pencil and paper, ten minutes to look at a place near you, such as a yard or a stream bank.

Be safe: Wear gloves to pick up litter. Do not touch broken glass or sharp things. Keep away from deep water. Leave nests and animals alone, and ask an adult before you dig.

Look at the place. Write down one problem you can see, such as litter or bare soil.
Choose one small action you can really do, such as picking up litter or planting a tree.
Write the plan: what you will do, who will help, when, and how you will check it worked.
Do it. Look again after a month and write down what has changed.

What you will see: A good plan is small, near and clear. You can finish it, and you can see whether it worked.

If you cannot try it, this is what happens.

A wrong idea many people have

One person cannot make a difference to an ecosystem.

Why it feels right: An ecosystem is huge, and one person is small.

The test that breaks it: Plant one tree. Insects come to eat it. Birds come to eat the insects. The soil under it stays in place. Ten neighbours who each plant one tree make a small wood.

What is true: Everything in an ecosystem is joined, so a small change spreads. Small actions, done by many people, add up.

Key words

pest: a living thing that harms a crop.
pollution: waste that harms living things, such as litter, oil, smoke and chemicals.

Back to your guess

The answer is C: Yes. Farming, cutting trees and pollution all change the living things in a place.

People change ecosystems all the time. Farming, cutting trees and pollution each change what lives in a place. The good news is that people can also help, and small actions add up.

Keep it

Copy this into your exercise book.

Farming, cutting trees and pollution change ecosystems. Fewer kinds of plant mean fewer kinds of animal. Plan a small action: a problem, an action, helpers, and a check.

Draw: Draw a plan as a table with four rows: what, who, when, check. Fill it in for one action near you.

The four groups of living things

Find and sort

A key for four small animals

Make a key for four animals

Join food chains into a web

Be the web

A pyramid of energy

Pass the beans

Dead things are recycled

Make a compost pot

What living things compete for

Crowded seedlings

Count plants with a frame

Count a patch

A forest food web

Plan a small action

Key words, A to Z