When herbivores consume plants containing pesticide residues, some may be retained by the body and be stored in fatty tissues. Over time, this builds up the levels within its body. When consumers eat herbivores, they accumulate these residues from the many that they have eaten. This bioaccumulation continues as you go up the food chain. This may result in dangerous or lethal levels in the top consumers, normally carnivores. The top consumer could be a human.
Heavy metals can also accumulate in this way.
The diagram above illustrates this process for a typical pesticide. Click on it to enlarge the diagram.
1. Case study of pesticide damage to birds of prey
There have been many well documented cases of pesticide damage to the environment. The most famous was the case of DDT and egg-shell thinning in birds of prey.
It was claimed that the widespread use of DDT was the main cause for the decline of birds of Prey in the 1960’s. This has led to a ban on the use of DDT
Read the following articles.
Article 1 – DDT, Eggshells, and Me - Ronald Bailey
Cracking open the facts on birds and banned pesticides
Article 2 – DDT - Wikipedia on-line Encyclopedia
Question: Do you think that the ban was justified?
Construct your argument ‘For’ or ‘Against’, using the evidence from each article.
Task: Locate a ‘case-study’ which clearly shows that ‘pesticides’ do damage biodiversity.
2. A complexity pollution problem.
Fishermen in a lake noticed that fish were dying – it was a hot and dry summer. The dead fish were sent off for analysis and found to contain high levels of a banned pesticide, mercury and cadmium. Scientists visited the lake and collected samples. They found high levels of cadmium (a toxic heavy metal). Later in the year (winter) the levels rose.
Further research showed that in the past waste from a local factory, which used cadmium and mercury, had been dumped in the lake. One year local farmers also used the banned pesticide to protect their crops against a pest of wheat by coating the seeds (legal if applied in this way) in the pesticide. This did coincide with the first recording of fish dieing in significant numbers.
Some questions for discussion:
- How did the pesticide get into the lake?
- Why did the level of cadmium rise in the winter?
- Why did the fish have high levels of mercury in their bodies.
- The levels of the pesticide, cadmium and mercury were all very low from the samples taken from the lake (sediments and plants) – why were they so high in the fish?
- No other contaminants were found – why did the fish die?
(Adapted from Applied Ecology by Geoff Hayward)