POISONS IN THE FOOD CHAIN - THE STORY OF DDT

One of the consequences of the web-like links between the animals and plants in an ecosystem is that chemicals that effect one of the levels in a food web can be passed on through the food web and become concentrated in the top carnivores, the larger predators. The experience of DDT and chemicals like it has become a very serious lesson for scientists and farmers.

In the 1950’s, DDT and several similar chemicals were produced in large quantities and were welcomed as a major step forward for farming as they were very efficient insecticides – they were very good at killing insects. Not only were crops sprayed with them and sheep dipped in them to kill parasites, but seeds were coated with them before being sown, to kill insect pests. What was not realised for a very long time was that DDT does not break down in the environment. Instead it is stored in fatty tissues in the bodies of insects and then in the bodies of the predators that feed on the insects. So animals further up the food chains concentrated DDT in their fat. Sometimes there was enough to kill them, but in the case of birds of prey, one of the first effects was that they started to lay very thin shelled eggs and as a result produced very few young. The numbers of many species of birds of prey like Peregrine Falcons*, Ospreys* and Sparrowhawks* crashed all over the western world. Shocking photos were published in the papers of heaps of dead farmland birds. The American author Rachel Carson drew attention to the crisis in her famous book ‘Silent Spring’ and eventually DDT and the other similar chemicals were banned. (although some of them are still being used in the Third World for mosquito control in malaria areas).

DDT was also widely used to kill a serious pest of orchards, the Codling Moth*, but another side effect of the poison emerged. The spray was also killing the carnivorous insects that preyed on the moth caterpillars and the scale insects that plague apple growers. So the other pests were increasing as a result of the DDT!

THE OSPREY STORY

In the estuaries near Long Island Sound off the east coast of the U.S. in the 1950's and 1960's, carnivorous birds of prey such as ospreys and eagles that fed on fish in the estuaries had high concentrations of the pesticide DDT in their bodies. But when the water in the estuary was tested, it had low concentrations of DDT. What accounted for such high levels of DDT in the birds? Poisons such as DDT that dissolve in fat can become more concentrated as they move up an aquatic food chain in a process called bioaccumulation. When the pesticide enters the water, small aquatic organisms such as algae and bacteria take in the poison. When fish eat the algae and bacteria, the poison is stored in the fat of the fish rather than diffusing back into the water. Each time a bird feeds on a fish, the bird accumulates more DDT in its fatty tissues. At Long Island Sound, DDT concentrations in fatty tissue were magnified almost 10 million times from the bottom to the top of the food chain in some estuaries. This bioaccumulation of toxins damages the carnivore at the top of the food chain. It may kill the carnivore, weaken its immune system, or impair its ability to reproduce successfully. High concentrations of DDT weaken the shells of bird eggs which causes the eggs to break and the chick embryos to die. This causes a tremendous drop in the population of the bird species.

The United States government recognized bioaccumulation as a major side effect of the use of DDT and in 1972 banned its sale except for emergency use. The aquatic food chains immediately started to recover. Unfortunately, the food chains are still not totally free of DDT. The pesticide breaks down very slowly in the environment. Also, DDT is still legal in some countries where it is used in large quantities. As a result, migratory birds may be exposed to DDT while wintering in Latin America or other locations outside the United States.