SILENT SPRING: AN AMERICAN ICON AND DDT
America’s national symbol—the Bald Eagle—was once headed towards extinction. It is likely that several factors contributed to its decline including habitat loss and hunting, but pesticides were the major contributor. Many raptors (birds of prey) such as Bald Eagles, Ospreys, Peregrine Falcons, and others underwent dramatic population declines.
DDT used in agriculture and forestry was washed into rivers and lakes where it entered the food chains. The DDT or its breakdown product, DDE, accumualted in the fatty tissues of fish and birds. At each level of the foodchain, their concentratoins increased,a process called biomagnification. When Bald Eagles ate fish with DDT, the pesticide was concentrated further in the eagles’ tissues.
These high concentrations of DDT and DDE caused eggshell thinning and sterility, leading to widespread nesting failures. Egg numbers reduced and eggs broke in the nest. Across North America, the Bald Eagle began to disappear. By the early 1906s, there were only 417 breeding pairs in the lower 48 states. This represented a 95% decline in the eagle population. The Bald Eagle was declared an Endangered Species in 1967.
DDT was banned in the United States in 1972. Since then, the number of nesting Bald Eagles has increased dramatically. There are now more than 10,000 Bald Eagle nests in the lower 48 states, a twenty-fold increase over their low in the 1960s.
In 2007, the Bald Eagle was removed from the Endangered Species List. Populations of Ospreys, Peregrine Falcons, and Brown Pelicans have also recovered.
Some argue that the decline in raptors was not due to DDT and DDE, but the majority of environmental scientists and ornithologists regard the pesticides as the primary cause.