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Human Influences on the Environment 1
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DDT was an insecticide used widely in the 1940's and 1950's. DDT is a very resistant chemical. This means that it does not break down into less harmful chemicals very easily.

All living things are linked in at least one food web. DDT passed through the food webs because it was washed into the soil by rain. It also reached lakes and sea water.

The following food chain exists in California where DDT had been sprayed to kill gnats.


Food Chain


Drawing of a grebe

Drawing of a Grebe

Although the amount of DDT was found to be very dilute in the water, the grebes, which are fish-eating birds, died in large numbers after a period of time.

(a) Explain carefully why the grebes died even though the amount of DDT in the lake water was very small.





(b) When the number of grebes decreased, how do you think that the rest of the food chain would be effected?





(c) Are there any less harmful ways of controlling pest populations?





(d) Explain why DDT has been found in the bodies of whales and penguins which live in the Antarctic even though DDT has never been used in the Antarctic.