1. Herbicides
To know how herbicides and pesticides can affect food webs

2. Photosynthesis & Light
An experiment was set up, investigating the effect of red, green and blue light on the rate of photosynthesis in pondweed
Which gas is given off during photosynthesis?
Why collect oxygen as a measure of the rate of photosynthesis?
Oxygen
Oxygen and glucose are the products of photosynthesis – glucose is difficult to collect and measure whilst oxygen is given off and is easy to measure!

3. Photosynthesis & Light
What conclusions can we draw from the results?
What piece of equipment is missing from the experiment which would have been used to measure the rate of oxygen given off?
How was the same white light modified to producing the three different colours?
Different colours enable different rates of photosynthesis
Stop watch to measure a given time
Coloured filters / prism

4. Competition
Organisms that share a habitat compete with each other for limited living resources
A weed is any plant that is growing where people do not want it, and these are a problem for farmers as they compete with the crops for resources such as light, water, living space and mineral salts
Weeds compete with crops for living resources and this can reduce crop yield
One way farmers can control weeds is to use chemicals called herbicides (weed killers)

5. Herbicides
Complete the herbicides worksheet

6. Herbicides
Herbicides are sprayed to kill weeds, which would otherwise compete with the crops (which would not grow as well, making less money for the farmer)
Herbicides are quicker, less labour intensive (so cheaper) and often more effective at removing weeds
The plants are the producers
a) Paraquat would kill: dog rose, rye grass, hazel, nettle; b) 2,4 D would kill the broad leaved plants (dog rose, hazel, nettle)
Selective herbicides kill weeds, but not the crop plants (so can be sprayed over fields)
The herbicide will kill the nettles, so the butterfly numbers drop or they die out. If this happens there is less or no food for the blackbirds, so their numbers drop or they die out
2,4 D on a lawn would kill dandelions and thistles but grass will not be killed so its numbers will increase (as there is less competition)

7. Pests
Pests are animals that eat and damage crops, being a problem for farmers as they reduce crop yield and compete with humans for food
What pests can you think of?
Caterpillars are pests specific to a type of plant such as cabbages
Snails and slugs are pests that eat the leaves of many plants
Other common pests include insects, birds and mice

8. Pest Control What can farmers do to control pests?
Pesticides can be used as a method of pest control by killing the pests
Pesticides contain poisonous chemicals (toxins)

9. Pesticides There are some major problems with pesticides
Pesticides can kill useful animals as well as the pests that they were meant to kill
Pests are also part of the food web - the toxins in pesticides can affect other organisms in a food chain or food web if they eat an infected pest…

10. Food Chains & Pesticides
Introducing a pesticide at the bottom of a food chain can have huge effects on the organisms above
Bioaccumulation takes place when a toxin is passed on in a food chain, getting more concentrated at each step
This happened with the pesticide DDT…
DDT was sprayed on a lake to control mosquito larvae
DDT is a toxin that does not break down in the environment, staying in the body of an animal, even when it is eaten…

11. DDT plant plankton zooplankton small fish large eagle
DDT was sprayed on the lake to control mosquito larvae
Plant plankton at the bottom of the food chain absorbed some of the DDT from the water
Each zooplankton ate lots of plant plankton, getting several doses of DDT
Each small fish ate many zooplankton and so consumed even more DDT
Each large fish ate several small fish and so consumed even more DDT
Each eagle ate several large fish therefore getting more than one dose of DDT

12. DDT plant plankton zooplankton small fish large eagle 5 ppm 10 ppm
absorbed
Each eagle had 1600 ppm of DDT in its tissue, which could kill the bird
In most cases, this amount of toxin made the bird eggs have very thin shells. These eggs broke very easily and so not many chicks were born alive, devastating the eagle population
This example is based on real events that took place in the USA in the 1950s

13. Safety
DDT was considered as a safe pesticide when it was first used to kill insect pests
The problem was that DDT does not break down in the environment, and the levels of this toxin that built up in top carnivores proved to be a major hazard
DDT is now banned in many countries in order to protect the environment – alternative chemicals which break down quickly in the environment are now used as pesticides

14. Anagrams

15. Quiz