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.
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.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
Plants in a crowd compete. What do they compete for?
See the answer
Light, water and space
A plant needs light, water, space and nutrients. In a crowd there is less of each for every plant.
If you chose B: Plants make their own food from light. They do not eat. What they share is light, water and space. Go to S3.2.6.
If you chose C: Animals compete for mates and shelter. Plants compete for light, water, space and nutrients. Go to S3.2.6.
Two foxes hunt in the same wood. What are they competing for?
See the answer
The rabbits and mice, their food
Both foxes need the same food, and there is only so much of it. Animals compete for food, drinking water, shelter and mates.
If you chose A: Foxes do not grow in one place like plants. What they share is the food in the wood. Go to S3.2.6.
If you chose C: Foxes do not make food from light. Only green plants do. A fox has to find food to eat. Go to S3.2.6.
A waterhole holds 60 litres. 5 animals share the water equally. How many litres does each animal get?
See the answer
12 L
60 ÷ 5 = 12. Each animal gets 12 litres.
If you chose A: You may have used the number of animals as the answer. Divide the water by the number of animals. Go to S3.2.6.
If you chose B: You have multiplied. Sharing means dividing: more animals share the same water, so each gets less. Go to S3.2.6.
Why does a desert plant have a thick stem?
See the answer
To store water for dry times
A desert is very dry and rain is rare. A thick stem holds water so that the plant can live between rains.
If you chose B: That is why rainforest trees grow tall. In a desert the problem is water, not shade. Go to S3.2.6.
If you chose C: The stem is full of water, not warmth. Its job is to store water for the long dry spells. Go to S3.2.6.
A seal swims in a polar sea at about 0 °C. What helps it stay warm?
See the answer
A thick layer of blubber
Blubber is a thick layer of fat under the skin. It keeps the heat in, so the seal stays warm in cold water.
If you chose A: Big ears let animals of hot deserts lose heat. A seal in icy water needs to keep heat in. Go to S3.2.6.
If you chose C: White fur hides animals such as polar bears on the ice. A seal in cold water relies on blubber to keep warm. Go to S3.2.6.
Why do many rainforest trees grow very tall?
See the answer
To reach the light above the other trees
The forest floor is dark, because the trees above shade it. Tall trees compete for light, and the tallest get the most.
If you chose A: That is a desert adaptation. A rainforest is wet all year, so water is not the problem there. Go to S3.2.6.
If you chose B: A tropical rainforest is warm all night. The tall trunk is not for warmth. Go to S3.2.6.
From an earlier chapter
Which living things are the decomposers?
See the answer
Fungi and bacteria
Fungi and bacteria feed on dead things and break them down, so that nutrients return to the soil.
If you chose A: These are animals that eat plants or other animals. Decomposers break down dead things. Go to S3.2.5.
If you chose C: Grass and trees are producers, because they make food from light. Fungi and bacteria break down dead things. Go to S3.2.5.
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.
