DDT (dichlorodiphenyltrichloroethane) is an insecticide that was used extensively in the 1960's. It controlled the spread of insect pests on crops and stopped the spread of malaria by killing mosquitoes. Farmers used large amounts of DDT, thinking it would solve the problem of insects eating their crops and profits. However DDT has created huge problems, the most notable being its role in reducing the numbers of bald eagles because of its impact on the food chain.

The concentration of some pesticides such as DDT and heavy metals such as lead and mercury increase as you move along the food chain. Higher order consumers have a greater concentration of these substances than lower order consumers. This is due to the fact that they break down very slowly and therefore accumulate over an organism's lifetime. When an animal eats another animal that has a metal or pollutant in its body all of the contaminant is passed along the chain. As more animals are eaten more of the substance builds up in a process known as bio accumulation or biomagnification.

DDT can dissolve into fatty animal tissue and so is stored, and passed along the food chain. The compound enters the food chain with insects, or first level consumers. From there it is passed onto the second level consumers, birds and small reptiles. As the animal eats more insects more DDT is stored and when the birds or reptiles are consumed the stored DDT is passed on to the third order consumer. The high levels of DDT in animals affects the metabolism and reproductive systems of fish, reptiles, birds and mammals. It stops calcuim from being deposited in the shells of the animals eggs, making them thin and brittle.

In America this biomagnification of DDT has caused a large decline in the number of offspring produced by higher order consumers such as herring gulls, falcons, eagles and osprey. The concentrations are often as high as 250 times that found in the natural surroundings. These extreme levels of DDT cause the birds to produce egg shells which are thin and brittle, easily breaking before the chick is able to hatch. Many birds are unable to raise their chicks with all of their young dying.

Bibliography

Scott, M. Young Oxford Books - Ecology. (London: Oxford)

Time Understanding Life and Science, Ecology. (Nederlands: Time-Life, 1968)

Baker, et al. Pathways into Senior Geography. (Melbourne: Nelson, 1995)

Pain, Bliss & Smith. Pathways to Geography HSC Course. (Melbourne: Macmillan, 1995)

The Book of Popular Science Encyclopaedia. (New York: Grolier, 1961)

The Software Toolworks Multimedia Encyclopaedia, Release 6. (New York: Grolier, 1996)

Snyder, et al. Biology - The Spectrum of Life. (Melbourne: Oxford University Press, 1990)

Encarta 96 Encyclopaedia. (Redmond: Microsoft, 1996)