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In teaching about the build-up of toxins in food chains and its effect on animals, I've found this role-play very effective.
Borrow two sets of hockey bibs from the PE department and make a set of 20 to 30 pieces of card marked to represent the "poisonous pest control substance".
The class goes to a suitable space, such as the tennis courts. Describe the food chain they are to investigate, for example insects - sparrows - sparrowhawk.
The students split into groups. With a class of 30, 20 children might become insects, seven sparrows (wearing bibs), three sparrowhawks (another set of bibs).
The teacher plays the farmer, spreading the "poison" over the field and asking questions: Why does the farmer do this? What are the possible benefits or hazards?
Then the insects roam the field, "eating the grass" by picking up the poison cards. Each insect ends up with one or two cards. Next discuss what effect this might have on them.
Let the sparrows loose on the field. They chase the insects and take the poison cards from them. Each sparrow gains a number of cards. How would this affect the sparrows?
Finally, let the sparrowhawks loose on the field. They catch the sparrows and take their cards. Sometimes you end up with one sparrowhawk having nearly all the cards.
This leads to a discussion of the role-play. The cards give the pupils a visual demonstration of the build-up of toxins in food chains.
After the discussion, it's best to repeat the role-play but with the parts changed.
After this active involvement children produce good write-ups when they are back in the classroom.
Mark Jeffery, former teacher at Windsor High School, Dudley, and lecturer at Halifax University, Birmingham