Question

Eagle populations crashed because their eggshells were so thin that the fetuses were not able to survive, a result of DDT toxicity. Low levels of DDT were measured in nearby waterways, yet the fish living there appeared unaffected. Explain how the low levels of DDT in the water had no obvious effect on the fish populations but wreaked havoc on eagles, the top predators.

Answer 1

Answer-D.D.T. (dichloro-diphenyl-trichloroethane) is a highly persistant chemical called an organophosphate. It is readily absorbed by the soil and is hydrophobic. D.D.T. is problematic because it stays in the ecosystem for many, many years. It enters food chains and bioaccumulation occurs. At each trophic level of the food chain, the amount of D.D.T. accumulating in the organisms' tissues magnifies. Unfortunately for birds, many of their eggshells became weakened due to this pesticide and reproductive failure of the embryos occurred. The bald eagle almost went extinct but their population began to rebound once the ban on the use of D.D.T. went into effect. When runoff occurs on areas that were sprayed with D.D.T., the pesticide can be transported to lakes, streams, rivers, etc. This again caused problems to various fish species whose eggs develop in the water and many did not develop properly. D.D.T. is an agent known to be carcinogenic. It is also an endocrine disrupter and can cause reproductive and developmental problems in humans and other species.

But eagles are not particularly susceptible to DDT poisoning. What happened was much more insidious. When DDT entered water reserves, commonly by spraying for insects, the DDT did not immediately disappear. Some small amount would be consumed by a small fish. And some small amount would be consumed by a larger fish. Some small amount would be consumed by all of these fish, and they DDT deposits would stay within the fish, typically in the fishes’ body fat, the process is called bioaccumulation. Formally, it is the result when an organism absorbs more of a substance than it can process and remove as waste.

But it’s a food chain. The larger fish eat the smaller ones. The larger ones end up not just the with the DDT they bioaccumulated, but also whatever the smaller fish had since it ate that fish. The fish at the top of the food chain are the biggest and build up the most DDT or another toxin. It’s like a reverse pyramid scheme and it is called biomagnification. Those fish at the top of the water-based chain, like salmon, happen to be the favorite fish of the eagles.

So by the time the eagle gets around to dinner, its food is highly contaminated and it absorbs a much higher amount of DDT than it would get from the environment alone. Because of this, the eagles contamination was much higher than expected and it led to severe effects for the eagles including a drastic reduction in population.