Name: Date:
Unit 4: Food chains and food webs
Exercise 4.2: Harm to food chains and food webs
Focus
1. Find and circle the word in the word search grid that has
each of the following meanings. Circle each word you
find and write the word next to its meaning:
a. harmful or poisonous
b. to build up or increase
c. chemicals used by farmers to get rid of insects and other
living things that eat their crops
d. the air, land and water around us
e. a harmful metal in food chains
2. Decide if each of the sentences is true or false. Tick a box
( ) to show your answer
True False
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a. Harmful substances can damage living things in food
chains.
b. Harmful substances move through food chains instead
of energy.
c. Harmful substances in food chains cannot affect
humans
d. Some harmful substances in a food chain break down in
the bodies of the living things.
3. A factory has dumped a harmful chemical into a river.
The drawing shows a food chain in the river and the
number of units of the harmful chemical X in each living
thing.
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a. Suggest a way that chemical X entered the food chain
b. Which living thing contained the most units of chemical
X?
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c. How many times more time units are there in the fish
than in a pond weed leaf?
d.
i. What do you notice about the amount of chemical
X in a living thing and the position of the living
thing in the food chain?
ii. Explain why this is so.
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Challenge
Read the web page below
DDT was one of the first and most powerful pesticides
developed to kill insects. It was widely used to control the
spread of malaria. Mosquitoes spread the malaria parasite
when they bite people. DDT was also used a lot in the 1960s
to spray crops, mostly in Europe and North America.
Much later, scientists discovered that DDT can move
through food chains because animals’ bodies cannot get rid
of it. DDT is stored mainly in body fat. It also remains in the
environment for a long time before it breaks down.
Scientists also found that birds of prey, such as eagles and
hawks, which are affected by DDT, lay eggs with very thin
shells. Humans who eat plants or animals that contain DDT
are more likely to develop cancer and other serious diseases.
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DDT was banned in the 1980s and is no longer used as a
pesticide.
4.
a. Why was DDT used a lot in the past?
b.
i. Explain how DDT can enter a food chain
ii. Why can DDT move through food chains?
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c.
i. Suggest two reasons why arctic animals such as
seals and polar bears often have high levels of DDT
in their bodies, even though DDT is no longer used.
ii. Some people living in arctic regions eat seals. Why
can this be dangerous for them?
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d. In a food chain affected by DDT, explain why a frog
that eats three locusts does not die, but an owl that eats
three frogs dies. What is the word that describes this?
e. Why do you think the number of birds of prey in
Europe and North America decreased a lot in the
1960s?
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f. Do some research to find out the full name of DDT.
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