Herbicides

D. Crowley, 2008

HerbicidesTo know how herbicides and pesticides can affect food webs

Tuesday, April 18, 2023

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!

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?

Volume Oxygen Production Over Time

0

1

2

3

4

5

6

7

8

9

10

Red Green Blue

Colour Of Light

Vo

lum

e (c

m3)

Oxy

gen

Pro

du

ctio

n In

12

Ho

urs

Different colours enable different rates of photosynthesis

Stop watch to measure a given time

Coloured filters / prism

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)

Herbicides• Complete the herbicides

worksheet

Herbicides

1. 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)

2. Herbicides are quicker, less labour intensive (so cheaper) and often more effective at removing weeds

3. The plants are the producers

4. a) Paraquat would kill: dog rose, rye grass, hazel, nettle; b) 2,4 D would kill the broad leaved plants (dog rose, hazel, nettle)

5. Selective herbicides kill weeds, but not the crop plants (so can be sprayed over fields)

6. 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

7. 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)

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

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)

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…

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…

DDT

• 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

plantplankton zooplankton

smallfish

largefish eagle

DDT

• 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

DDT absorbe

d

5 ppm 10 ppm

250 ppm 1600 ppm

plantplankton zooplankton

smallfish

largefish eagle

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

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