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Primary Science · Year 4Chapter 4 · Habitats, change and food chains
Year 4 Science · Habitats, change and food chains · Lesson 4.2

Year 4 Science: Dangers to living things

Part of the Year 4 Science habitats, change and food chains chapter. Chapter overview · whole chapter on one page

4.2

Dangers to living things

Year 4
LH3: environments can change, and this can sometimes pose dangers to living things
Story. After a long dry summer, Mr Patel counts the animals in the school pond. “We counted lots of them last year,” says Aisha. “Some have gone,” says Leo. Mr Patel says, “Let us look at the numbers, and find out what changed.”

Some changes in the environment are dangerous to living things. A change can take away their water, their food, or their place to hide and nest. The table gives five changes, and the danger each one can pose.

Change in the environmentDanger to living things
A pond dries up in a long hot summerfrogs, newts and water beetles have no water to live in
Litter, such as plastic bags, is left in a parkanimals can eat it, or get tangled in it
A hedge is cut downbirds and small animals lose a place to nest and hide
Chemicals spill into a riverfish and water plants can die
A field floods after heavy rainsoil animals and plants can be drowned, and die

The pond survey counted the same living things before and after the dry summer. The fall in number is the number before, minus the number after.

Living thingBefore the dry summerAfter the dry summerFall in number
Frogs1239
Newts808
Snails25205
Water beetles15411
Total602733
04812fall in number9Frogs8Newts5Snails11Water beetles
The fall in number for each group of living things in the pond. Water beetles had the biggest fall.

Key ideas

  • A change in the environment can pose a danger to living things.
  • A dry summer can take away the water that frogs and newts need.
  • The fall in number is the number before the change, minus the number after.
  • Add the numbers in a column to find the total.
Worked example 4.2.1
In the pond survey, how many frogs were counted before the dry summer? Write the number.

Method. Read the table.

  1. Find the frogs row. The frogs row shows 12 before the dry summer.
    12
  2. Read the number. So 12 frogs were counted before the dry summer.
    12
  3. Write the answer.
    12

Check. After the dry summer there were 3 frogs. ✓

Worked example 4.2.2
In the pond survey, how many newts were counted after the dry summer? Write the number.

Method. Read the newts row.

  1. Find the newts row. The newts row shows 8 before, and 0 after.
    8 and 0
  2. Read the after number. So no newts were counted after the dry summer.
    0
  3. Write the answer.
    0

Check. Newts disappeared from the pond. ✓

Worked example 4.2.3
How many snails fell in number during the dry summer? Write the number.

Method. Subtract the after number from the before number.

  1. Read the snails row. Snails were 25 before, and 20 after.
    25 and 20
  2. Subtract. 25 minus 20 is 5, so 5 snails fell in number.
    25 - 20 = 5
  3. Write the answer.
    5

Check. Check: 20 plus 5 makes 25. ✓

Watch out. Subtract the number after the change from the number before.

Watch out

Watch out: a fall in number is the number before, minus the number after. If the number after is bigger, the number went up, not down.

Try it at home or school (with a grown-up). Count the number of different leaves, or small stones, in a pot with a grown-up. Then take away a few, and count again. Record the numbers in a table, and find how many fell. Do not touch any animals in a pond.

Exam tip

Grown-up tip. Ask: “How many were there before? How many are there now? What is the difference?” Let your child subtract the smaller number from the bigger number.

Exercise 4B  Dangers to living things

1
FLook at the table in section 4.2. Which change can trap animals in litter? Write the change.
2
FLook at the table in section 4.2. Which living things are in danger when chemicals spill into a river? Write one name.
3
FLook at the pond survey in section 4.2. How many frogs were counted before the dry summer? Write the number.
4
FLook at the pond survey in section 4.2. How many frogs were counted after the dry summer? Write the number.
5
FLook at the pond survey in section 4.2. How many fewer frogs were there after the dry summer? Write the number.
6
FLook at the pond survey in section 4.2. How many newts were counted after the dry summer? Write the number.
7
FLook at the pond survey in section 4.2. Which group had the biggest fall in number? Write the name.
8
FLook at the pond survey in section 4.2. Add the numbers before the dry summer. What is the total? Write the number.
9
FLook at the pond survey in section 4.2. Which group fell by 5? Write the name.
10
PSLook at the pond survey in section 4.2. How many living things fell in number altogether? Write the number.
11
PSWorking scientifically. Write one question you could ask to find out why the snail numbers fell.
12
RAli says the pond survey shows the snails were not harmed by the dry summer. Is Ali right? Write no, and give one reason.

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