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Question: How do toxins build up in the food chain? 
 
Answer: The build up of toxins in the food chain is an example of biomagnification.

The toxins that become magnified are those that do not readily break down and are usually retained in fatty tissue. An example of such a toxin is DDT.

If we think of insects that have been poisoned by DDT and die, and the bodies end up in, for example, a lake, several things will happen. The insects sink to the bottom of the lake, where the DDT is picked up by organisms at the bottom of the food chain. These might be algae, which will only pick up a minute quantity of DDT, say 1 nanogram. Next, plankton may eat a thousand of these algae, which means it now has accumulated 1 microgram of DDT. A small fish could now eat a thousand of the plankton, which means it has now accumulated 1 milligram. Larger fish will eat a number of the smaller fish and in turn will increase the amount of the DDT in their bodies. Eventually, a bird, such as an osprey, which is higher up the food chain, will eat a number of fish and increase the amount of toxin up to a considerable number of grams. The effects can now be quite devastating, with the possibility of the toxin weakening eggshells which can result in chicks being crushed in the nest, and of course, eventually no more ospreys.

So, the concentration at the end of the food chain can be millions what it was initially in the environment. What started off as relatively unharmful can become a killer due to the cumulative effect of the toxin.

 
 

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