Thanks to those of you who shared your eagle story with us!
DDT was once hailed as “a benefactor of all humanity,” especially in the fight against malaria spread by mosquitoes. However, DDT was also causing problems in eagle reproduction. In 1953 the National Audubon Society estimated there were about 1000 nesting pairs of Bald Eagles in the lower 48, but by 1963, there were less than 500 nesting pairs in the lower 48 states. From right outside the National Eagle Center in Wabasha Minnesota, all the way down the Mighty Mississippi River to Rock Island Illinois, between 1968 and 1972 there was only 1 nesting pair of Bald Eagles. What happened?
DDT accumulates in the fat cells of animals. It becomes concentrated as it moves up the food chain. Eagles were found to have high concentrations of it in their bodies that contributed to their population crash. Through the efforts of conservation organizations and individuals, DDT was banned in the US in the 1972. Almost immediately, eagle populations began to increase. However, DDT is still used in some parts of the world.
DDT is highly effective insecticide that kills disease-causing insects in humans, animals, and produce. It was banned in the US because links were made between DDT and declines in eagle and the other raptor populations.
DDT Didn’t Disappear
DDT doesn’t break down so it ends up in lakes and rivers. Each step in the food chain magnifies the amount of DDT in the bodies of the animals. In eagles, high concentrations of DDT affected their ability to process calcium, which is needed to make strong egg-shells and bones.
Here is how it affected eagles, raptors and other birds at the top of the food chain.
Plants and algae incorporate DDT into their tissues it is then at .000003 ppm (parts per million
Aquatic insects eat the plants and algae and then small fish eat the insects then the DDT is at .04 ppm
Large fish eat the small fish the DDT is then at 2ppm
When a predator—an eagle or a human catches and eats several fish, the DDT becomes even more concentrated at 25 ppm , this level was then affecting the females ability to produce calcium which then made thin or poorly formed egg shells so that when the female eagle went to incubate her eggs, she was crushing them with here own weight, thus producing no new eagles.
We must remember that those who created DDT were trying to help save lives.
DDT was banned in North America in 1972 and in 1973 the Endangered Species Act as we know it today was created and the Bald Eagle was added to it. Thankfully in 2007 the Bald Eagle was removed from the Endangered Species list and that we now estimate over 10,000 nesting pairs of Bald Eagles in the lower 48, including over 2,300 nesting pairs just in the state of Minnesota. That 260 mile stretch of Mississippi River that we call the Upper Mississippi River National Wildlife and Fish Refuge that only had 1 nesting pair, now has over 280 nesting pairs!
We can now celebrate! We must also be cognizant that there are still some threats out there ,and we will talk about them later. Now go back to watching Harmon, we only have a couple more weeks until he takes his first flight. Soon he will be branching; we will talk next time about what that means.