Presentation on theme: "Poisons in the food chain"— Presentation transcript:
1Poisons in the food chain Pesticides:Poisons in the food chain
2PesticidesAs much as 30% of the annual crop in Canada is lost to pests.The pests include weeds, rusts and moulds, birds, small mammals, and insects.For each of these pests, we have created a pesticide.Pesticides are chemicals designed to reduce the populations of unwanted organisms, both plant and animal.
3Tires and Mosquitoes What do we do with worn out old tires? Can’t bury them in landfills – they just keep popping up.Can’t burn them – thick choking smoke – bad for the environmentLet them sit???
5Still WaterThe still water – warmed by the sun hitting black rubber, makes an agreeable ecosystem for hatching mosquitoes.Why is it a problem to create a favourable ecosystem for mosquitoes??
6Mosquitoes and Disease Some mosquitoes carry microbes that cause disease. Malaria is one such disease.Thousands of people in tropical areas of the world die of this disease every year.Used tires are often stored in one area and then moved to another area for disposal. What problem might this cause?
7Using PesticidesScientists developed a pesticide spray that could eliminate mosquitoes. When it was tried on an island, other effects were soon noted.Insects other than mosquitoes began to disappear, and then the number of lizards began to fall.What do you think caused some other insects to begin to disappear?Why did the lizards begin to disappear?
8The Problems SpreadsMost people on the island were not too worried about the disappearance of a few insects and some lizards.However, they took notice once the local wildcats, which had fed on lizards, began to get sick and die.Without cats the rat population soon increased.Fearing an outbreak of diseases linked with rats, the local people imported domestic cats.What problem could be caused by bringing in domestic cats?
9The InvasionChanges to the food web became even more obvious when caterpillar populations began to increaseThe pesticide affected wasps and other predators of the caterpillar, but had little effect on the caterpillar.One the predators were gone, the caterpillar population increased greatly.Eventually hungry caterpillars moved into the fields and devastated food crops
10Biological Amplification Pesticides tend to stay in the bodies of animals that come in contact with them.The result is that the concentration of harmful pesticides increases at each level in a food chain.Predators always have more toxic chemicals in their bodies than their prey.
11Biological Amplification If the body of a prey contains harmful pesticides, the pesticides will be taken in by the predator.Predators eat many prey over their lifetimes. Each time prey is eaten, the amount of pesticide in the predator increases.This process is called biological amplification.Predict how scavengers, such as beetles, might be affected by biological amplification.