Polluting the food chain


A chemical which contaminates the environment is called a pollutant. Examples of these are:

  • pesticides - used by farmers to prevent pests eating their crops
  • heavy metals - used in industry

These types of pollutants affect organisms by a process called bioaccumulation.

Here is one way pollutants can enter a food chain.

  1. The pollutant is washed into rivers and lakes.
  2. It enters the food chain by adhering to or being absorbed by producers.
  3. When the producer is eaten, the pollutant it contains is passed onto the next trophic level as it is persistent. It remains in the body tissues of the organism because it is not easily broken down.
  4. The pollutant continues to accumulate, increasing in concentration, as it moves up the food chain.
  5. By the time it reaches the top carnivore, the pollutant will have accumulated to such an extent that it becomes toxic, causing reduced fertility or even death.


DDT is a chemical pesticide which was used by farmers to control insect pests eating their crops. It can be used to demonstrate bioaccumulation as shown in this diagram.

A pyramid diagram showing how the concentration of pesticide increases ten million times from water to algae to mayfly nymph to sunfish to the great blue heronThe concentration of DDT is shown in the diagram in parts per million (ppm)

The high concentrations of DDT in birds of prey weakened their egg shells, their young died and populations decreased.


To the nearest whole number, how many times greater is the concentration of DDT in the birds of prey than in the water?

25 ÷ 0.000003 = 8,333,333


Write the number given as the answer to the question above in words.

Eight million, three hundred and thirty three thousand, three hundred and thirty three.