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A plant is sprayed with a substance, perhaps an insecticide of some kind. Over the next few weeks, you notice that all the birds of prey of the area fall out of the sky. Why? Well, it's called bio-accumulation, and this is how it works:

When a plant is sprayed with a toxic substance, usually containing heavy metals, the effect on the plant is non-existent it seems. Let's say that hypothetically a field of wheat is sprayed with a substance. We'll call it substance "A" and decide that maybe 12 units of this substance becomes a lethal dose in a large bird. So this field is sprayed with substance A, and each plant contains perhaps 1 unit of it. A small mouse then may eat 3 of these plants, so it now has 3 units of substance A within his body (not a lethal dose for a mouse). The problem is that this substance (or any heavy metal for that matter) cannot be excreted, and so, any amount that was ever ingested by this animal remains within its body, stored in fat tissues, for the rest of its life. A hawk or an owl then approaches and eats maybe 5 of these small mice. That bird then absorbs 15 units of substance A - well over a lethal dose.

The problem does not stop here, however, as in all natural ecosystems the nutrients are reabsorbed into the food chain. A carrion bird - say a crow or raven - then eats the decaying matter.

The food chain given also has only 3 trophic levels - imagine the potential effect in a much larger food web.

The same thing happened with DDT, they sprayed it on children saying it was harmless, and it was, in very small doses, but toxins accumulate in food chains. What some spray to kill mosquitoes may jump straight into the fish on your dinner table.

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