The geckos were full of poison
The Geckos ate the flies that were sprayed with DDT. The cats ate the geckos that were dead and dying from eating the poisoned flies. Then cats started dying which allowed the rats to proliferate which allowed the spread of Bubonic Plague.
Geckos are not at the top of the food chain. They are at the bottom.
geckos
the food chain is the way things are eaten - grass eaten by rabbit eaten by fox for example
food chain
food chain
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Cats
they have to get eaten so other aniamals in the food chain can have energy
a food chain tends to be linear eg he eats him who is then eaten by him only one link between two animals where as a food web is a lot of food chains interlinked so he is eaten by three people who are then eaten by four
it is eaten by leopard!