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.
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.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
Why does a field of one crop hold fewer kinds of living thing than a meadow?
See the answer
Few kinds of plant mean few kinds of food, so fewer kinds of animal can live there.
A meadow has many plants, and each feeds different animals. A field of one crop offers one food, so fewer animals can live there.
If you chose A: A field has plenty of light. The difference is the number of different plants, and so of different foods. Go to S3.2.8.
If you chose B: A crop is a living plant. It is one kind of plant growing over a large area. Go to S3.2.8.
All the trees in the forest web are cut down. What happens?
See the answer
The web loses a producer, so the insect loses a food.
The tree is a producer. The insect ate it, so it loses a food. The web is smaller and weaker without it.
If you chose A: The insect still has the bush, but it lost one of its two foods. The animals that lived in the trees lose their homes, so something does change. Go to S3.2.8.
If you chose C: Taking away the producers does not help animals. They lose food and shelter, so fewer can live there. Go to S3.2.8.
The graph shows the trees left out of 100 in a forest. How many trees were lost in the first 20 years?
See the answer
35 trees
There were 100 trees at the start and 65 left after 20 years. 100 − 65 = 35.
If you chose B: That is how many trees were left after 20 years. The question asks how many were lost, so take that away from the start. Go to S3.2.8.
If you chose C: You have added. To find how many were lost, subtract what is left from what there was. Go to S3.2.8.
Most mayfly larvae need clean water. The chart shows larvae at four places along a river. Place B is at a town. What does it suggest?
See the answer
Something in the town may be polluting the water.
There were many larvae above the town and few at it. Larvae need clean water, so something at the town may be harming it.
If you chose B: It is the other way round. Most mayfly larvae need clean water, so few of them suggests the water is not clean. Go to S3.2.8.
If you chose C: Depth is not what the counts tell us. The larvae were plentiful above the town and far below it, so the river there suits them. Go to S3.2.8.
What is a good first step in planning an action to protect a place near you?
See the answer
Look at the place and choose one small problem you can solve.
A good plan starts by looking. A small, near problem is one you can finish, and you can then check whether it worked.
If you chose A: A plan that is too big never gets done. Start with a small, near problem that you can really solve. Go to S3.2.8.
If you chose B: Look first. Digging can harm nests and roots. A plan needs a problem, an action, helpers and a way to check. Go to S3.2.8.
From an earlier chapter
A student counts a mean of 4 plants in a frame. The field has 25 squares the size of the frame. Estimate the plants in the field.
See the answer
100 plants
Each square holds about 4 plants and there are 25 squares. 4 × 25 = 100.
If you chose A: You have added the two numbers. Each of the squares holds about the mean, so multiply the mean by the number of squares. Go to S3.2.7.
If you chose C: You have taken one number from the other. To estimate the whole field, multiply the mean by the number of squares. Go to S3.2.7.
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.
