The Effects of DDT on Wild Birds

The eagle population declined during the use of DDT.

Silent Spring, by Rachel Carson, illuminated the unforeseen effects of widespread use of a pesticide, dichlorodiphenyltrichloroethane, or DDT. In the late 1930s and early 40s, this pesticide was heavily used in the United States as well as in other countries. It appeared to be the perfect pesticide; it was effective against many common insect pests, had no deleterious side effects, and was persistent in the environment. It was the environmental persistence, in both biotic and abiotic sources, that caused unanticipated problems. It gathered in the tissues of animals on land and in the waterways, and these animals had detectable amounts of the chemical. Birds, especially predatory birds, suffered severe population decreases that were linked to the use of DDT.

Toxicity Effects

The reason that DDT was particularly damaging to birds of prey was due to processes called bioaccumulation and biomagnification. DDT enters the food web through bioaccumulation; the zooplankton, for instance, consume it when they are feeding. When a fish eats zooplankton, it also ingests the DDT the plankton has in its tissues. Fish eat thousands of these smaller creatures, each of them contributing small amounts of DDT to the fish's diet. The pesticide accumulates in the fish's fatty tissue, so that when a bird of prey, such as a pelican, eats that fish, it ingests all the chemicals present in that fish as well as the others that it eats. This process is called biomagnification. The higher the animal in the food chain, the more pesticide it can accumulate and the more vulnerable it is to the effects.

Reproduction Effects

Birds of prey were more susceptible to DDT because they reside at the top of the food web. Though DDT is not very toxic to birds, it has a cumulative effect, which was present in what the birds of prey were consuming. The chemical interfered with the calcium metabolism of the birds, resulting in thinner eggshells that collapsed under the weight of the adult bird. Population numbers of the predatory birds--such as osprey, eagles, and pelicans--nosedived as these birds crushed their entire clutch while incubating.

Fatality Effects

The effects of the chemical are persistent in the environment as well. According to Stanford University, researchers noticed a high amount of robin deaths following treatment of an area with DDT. The birds were dying while displaying tremors, a characteristic symptom of DDT poisoning. Stranger still, the birds were still dying prior to further spraying the following spring. Studies revealed that earthworms were eating soil and leaves that contained the pesticide. The robins were then eating the worms, and this was proving toxic to the birds.

References

• Extension Toxicology Network, Pesticide Information Profile: DDT (dichlorodiphenyltrichloroethane)
• National Academic Press: Ecological Knowledge and Environmental Problem-Solving: Concepts and Case Studies: Ch. 24. Environmental Effects of DDT (358-374)
• Stanford University: DDT and Birds
• Marietta College: Bioaccumulation and Biomagnification

Resources

• "Silent Spring"; Rachel Carson; 1962.