العلوم (بالإنجليزية) الصف الخامس الابتدائي، الأسبوع 6: When the grass disappears

When food webs change

الأسبوع 6 · المحور الأول «الأنظمة»، الوحدة الأولى «العلاقات الغذائية بين الكائنات الحية»، المفهوم 1.3 «التغيرات في الشبكات الغذائية» (ب): تغير الأعداد وفقدان الموطن والتلوث · 3 دروس

When the grass disappears

Discovery Education Science P5: Term 1, Unit 1, pp. 82–84

GoalPredict what happens along a food chain when the grass disappears, and model how energy passes on.

New words

energy transfer · energy passing from one living thing to the nextpollution · harmful things added to air, water or landmodel · a way of showing how something works

Explanation

Suppose all the grass in a valley disappears after a long drought. The hare has nothing to eat, so hares starve or leave. Then the eagle that hunts hares loses a food, and it may go hungry or move away. A change at the bottom of a food chain passes upward. We can act this out with paper squares as energy. Grass starts with 12 squares. When the hare eats, it gets only 6, and the other 6 go into a “heat” box, because animals use energy to move and keep warm. When the fox eats the hare it gets 3, and 3 more go into the box. The numbers are only a model, but they show why there are fewer foxes than hares, and fewer hares than grass plants. If smoke or ash covers the grass, there is less energy for every animal.

grass: 12 squares6 lost as heat→hare: 6 squares3 lost as heat→fox: 3 squares

Worked examples

The grass started with 16 squares

Step	Squares passed on	Squares lost as heat
grass to hare	8	8
hare to fox	4	4

Marwan’s class counts the squares in the game. The squares lost as heat at each step are counted too.

	Clear day	Smoky day
grass	16	8
hare	8	4
fox	4	2

On a smoky day the smoke covers the grass, so the grass has only 8 squares instead of 16. The hare then gets 4 instead of 8, and the fox gets 2 instead of 4. Everyone gets less.

A drought dries up the grass in a valley. The hares have nothing to eat and leave, and the bird of prey that hunted them goes hungry.

Class activity

Energy game (a grown-up helps). You need about 24 small paper squares and a box labelled “heat”. One player is the grass and starts with 16 squares. The hare takes half of them and puts the other half in the heat box. The fox takes half of the hare’s squares and puts the rest in the box. Count the squares each player ends up with. Then play again for a smoky day, when the grass loses half of its squares first. Write both results in a table.

Practice

What happens to a hare if all the grass disappears?

What happens to the eagle that hunts hares, and why?

In a game the grass has 20 squares, the hare gets 10 and the fox gets 5. How many squares go into the heat box altogether?

grass: 20→hare: 10→fox: 5

Why are there usually fewer foxes than hares?