Ecology And Environment

Ddt and Bald Eagles



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Shortly after World War Two, the pesticide DDT was introduced to American agriculture.  This pesticidal wonder eradicated any insect it came in contact with and was used not only on crops, but in coastal marshlands to control salt marsh mosquitoes.  It wasn’t until the 1960’s, twenty years after its introduction and extensive use that a connection was made between its utilization and the reproductive decline of the American Bald Eagle and other birds of prey.

Dichlorodiphenyltrichloroethane (DDT) was first synthesized in 1874 by an Austrian chemist.  It wasn’t until 1939 that its use as an insecticide was discovered by Paul Muller.  Muller suggested the use of DDT during WWII to control malaria and typhus, both vector diseases. (A vector disease is one transmitted by insects such as mosquitos or lice).  DDT was highly effective in answering the problem of vector control and Muller received a Nobel Prize in Physiology and Medicine for his work. 

When the war was over, DDT came home to the American coasts and farmland. After its agricultural introduction, an increasing number of farmers began dusting their crops with the powder as its insecticidal effectiveness was proven.  In the 50s, crop pests began developing immunity to the insecticide so farmers began to double, and even triple, the amount of DDT.  Along the American coast, DDT was used to suppress and control swarms of mosquitoes which led to the eventual eradication of Malaria in North America.

Until the 1972 ban, nearly 600,000 tons of DDT were dispensed in the United States.  The DDT that was sprayed on crops eventually found its way to American water sources (via crop runoff). DDT is insoluble, which means that it does not dissolve in water.  Once in streams, rivers and lakes, the DDT was ingested by fish and other water organisms.  These were ingested by larger wildlife, which in turn were ingested by larger animals until DDT was found at the top of the food chain in birds of prey.  Because DDT is trapped in the fatty tissue of animals and has a long rate of decay, the amount of DDT compounds as it moves up the food chain in a process called bioaccumulation.  For example, a bald eagle ingests the toxins not only in the fish it consumes, but in everything that fish consumed, and so on. 

Charles Broley was among the first to suspect that there was a correlation between a decline in Bald Eagles and the extensive use of DDT and other pesticides.  A retired banker and self-titled naturalist, Broley banded and studied eagles along the Florida coast. In the early 50s Broley noted that the eaglet population was declining rapidly. In six years the eaglets he was able to find had dropped from 150 to 15.  This was attributed to a thinning of the eggshell which caused the egg to be crushed during incubation, and a rising number of infertile or deformed embryos.  DDT was used along the Florida coast to quell mosquito swarms and Broley believed this was the source of the eagle reproductive issues.

It wasn’t until nearly a decade after Broley’s suspicions that the potentially harmful effects of DDT on birds of prey, specifically the Bald Eagle, was given national attention. It took another decade before the ban in 1972 which made illegal the use of DDT in America. 

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