BIOLOGY · Stage 8 · Chapter 7: The Health of the Environment
Biology 7.4 Pesticides and bioaccumulation — Persistent Toxins in Food Chains — Alhussan International Academy - Biology (Stages 7-9) Revision Notes & Topical Questions
5 min read 6 parts Lesson 4 of 8
Lesson summary
7.4 Pesticides and bioaccumulation — Persistent Toxins in Food Chains — Alhussan International Academy — Biology (Stages 7–9) notes: Explain how persistent pesticides build up along food chains by bioaccumulation and harm top predators.Pests — insects, birds, molluscs, mammals, f…
- Harm to other organisms
- Biodegradable pesticides
- Along a food chain
- Land chain
- River chain
- Effect on bird eggs
Section 1 of 6Why pesticides are used
- Pests
- insects, birds, molluscs, mammals, fungi and weeds feed on crops, and around 30% of the food grown is lost to pests.
- Pesticides = substances that kill pests of all types, used by farmers and gardeners for many years.
- They can increase crop yield and so help produce enough food.
- Harm to other organisms
- pesticides can kill bees, which are needed for pollination, and fish in streams after chemicals wash off the fields.
Section 2 of 6Pesticides as toxins
- Toxins
- pesticides are poisons, so every use puts toxins into the environment.
- Biodegradable pesticides
- many break down quickly.
- Persistent pesticides
- some, such as the insecticide DDT, do not break down and stay in the environment.
- Many dissolve in fat rather than water, so animals cannot easily excrete them.
- They are stored in fatty tissue, and each time the animal feeds more is added.
Section 3 of 6Bioaccumulation
- Bioaccumulation = the increase in concentration of a substance in an organism over time.
- Persistent pesticides build up because organisms eat contaminated food.
- Along a food chain
- biomass passes along the chain, so toxins become more concentrated at each stage.
- Each predator eats many prey, so it takes in the toxin from all of them.
- Top predators therefore receive the highest doses, even if the concentration in the environment was very low.
- Evidence
- scientists found many dead predators, including sparrowhawks and herons, with high pesticide levels in their bodies.
Section 4 of 6Examples of food chains
1Land chain2 points
- insects and small mammals eat seeds and plants sprayed with pesticide.
- A top predator such as a sparrowhawk eats many of them and takes in enough pesticide to kill it.
2River chain3 points
- pesticides wash into rivers and small fish absorb tiny amounts and store them.
- Bigger fish absorb the toxins from the small fish.
- At the end of the chain a heron eats enough toxic fish and frogs to die.
3Sea chain2 points
- pesticides carried down rivers are absorbed by small fish, which are eaten by bigger fish and then by seals.
- One seal eats a lot of fish, so levels build up, and a polar bear that eats many seals can receive a lethal dose.
Section 5 of 6Effects on the ecosystem
- Effect on bird eggs
- DDT caused thin eggshells in birds of prey, which broke, so their numbers fell sharply.
- Interdependence
- the death of top predators affects other species.
- Prey numbers may build up and put stress on other food sources.
- Numbers of a different predator may increase.
Section 6 of 6Reducing the harm
- Ban or restrict persistent pesticides, as with DDT in many countries.
- Use biodegradable chemicals that break down quickly.
- Use biological control, where a natural predator or parasite of the pest is introduced.
- Use integrated pest management, with crop rotation and resistant crop varieties, so fewer chemicals are needed.
- Trade-off
- DDT also saved millions of lives by killing malaria mosquitoes, so benefits must be weighed against environmental costs.
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