Pesticides and the food chain
Teaching notes
This activity is designed to demonstrate to students how toxins such as DDT, chlordane and
dieldrin can accumulate in the food chain, affecting organisms far removed from the original
source of the pesticide.
In the park the groundskeeper sprays herbicide around the base of trees, and around swings, slides and benches to kill unwanted plants.
Activity: 10 – 15 minutes
Students should be in pairs.
Each pair needs 20 small slips of paper.
How much herbicide will kill a plant?
On a board, visible to the class, list the amount of pesticide needed to kill different organisms,
as follows:
Plant – 1 dose
Caterpillar – 2 doses
Blackbird - 7 doses
Falcon – 10 doses
1 dose
When you are ready to carry out the activity reveal the first sentence of slide 2. Work your way
down through the information, revealing each sentence or section and eliciting responses from
the class as you go.
When the activity has been completed, students can use this form of presentation to design
and produce a large flow diagram demonstrating how toxins can build up in a food chain.
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© www.teachitscience.co.uk 2012
Each blackbird eats 5 caterpillars. How many blackbirds would your caterpillars feed?
How much herbicide will kill a caterpillar?
Put your slips of paper into groups of 5. Each group represents the amount of poison each blackbird consumed as it ate the caterpillars.
2 doses
How many doses will kill a blackbird?
The caterpillars have not absorbed enough poison to kill them, but they carry the chemical in their bodies.
To represent the caterpillars and the dose of poison they have received, label 20 slips of paper with the number 1.
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Blackbirds and thrushes live in the trees and bushes around the park.
They were not sprayed with the chemical, but they eat the caterpillars.
Caterpillars eating the weeds also get sprayed with poisonous herbicide.
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The blackbirds do not die, but they carry the poison in their bodies. 3
Blackbirds and thrushes are eaten by predators such as sparrowhawks and falcons.
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The poison accumulated in the bodies of different animals
as it passed along the food chain.
Each falcon eats 2 blackbirds.
Join together your paper slips to represent this.
Humans are at the top of many food chains. What are the implications for you and me?
How many doses of poison has the falcon received?
Is this enough to kill the falcon?
10 doses will kill a falcon.
So, even though the bird of prey was nowhere near the park when the Grounds Keeper sprayed herbicide around the benches and the falcon has not touched the herbicide, it has nevertheless been poisoned and has died because the toxic chemical was carried to it in its food. © www.teachitscience.co.uk 2012
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Keywords: Ecotoxicology – the study of the effects of pesticides and other contaminants on the environment.
Persistence – how readily substances can be broken down by environmental processes.
Bioaccumulation – the build up of pesticides and other toxins in the food chain.
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