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7.4Chapter 7: The Health of the Environment

Pesticides and bioaccumulation — Alhussan Biology - Stage 8 Revision Notes & Topical Questions

5 min read 8 parts Lesson 4 of 8 1 quiz

📚 Alhussan Biology - Stage 8 Hub / Topical Revision
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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

Part 1 of 6Why pesticides are used

Pests
  • Pests insects, birds, molluscs, mammals, fungi and weeds feed on crops, and around 30% of the food grown is lost to pests.
Pesticides
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
  • Harm to other organisms pesticides can kill bees, which are needed for pollination, and fish in streams after chemicals wash off the fields.

Part 2 of 6Pesticides as toxins

Toxins
  • Toxins pesticides are poisons, so every use puts toxins into the environment.
Biodegradable pesticides
  • Biodegradable pesticides many break down quickly.
Persistent pesticides
  • 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.

Part 3 of 6Bioaccumulation

Bioaccumulation
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
  • 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
  • Evidence scientists found many dead predators, including sparrowhawks and herons, with high pesticide levels in their bodies.

Part 4 of 6Examples of food chains

Land chain
  • Land chain 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.
River chain
  • River chain 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.
Sea chain
  • Sea chain 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.

Part 5 of 6Effects on the ecosystem

Effects on the ecosystem
Effect on bird eggs
  • DDT caused thin eggshells in birds of prey, which broke, so their numbers fell sharply.
Interdependence
  • 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.

Part 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.

Key terms

TermMeaning
PesticideA substance that kills pests.
ToxinA poison.
Persistent pesticideA pesticide that does not break down and stays in the environment.
BioaccumulationThe build-up of a substance in an organism over time.
Biological controlUsing a natural predator or parasite to control a pest.
Top predatorThe consumer at the end of a food chain.

Avoid these mistakes

1
Saying predators take in the same amount of pesticide as the plankton — the concentration increases at each stage of the chain.
2
Thinking all pesticides persist — biodegradable ones break down.
3
Confusing bioaccumulation with biomagnification — one is build-up in a body, the other is increase up a chain.
4
Writing that pesticides only kill pests — they affect other organisms too.

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