3.5Unit 3 — Life on Earth
Food Production — SQA National 5 Biology — SQA National 5 Biology Revision Notes & Topical Questions
5 min read 2 parts Lesson 5 of 6
Section 1 of 2Fertilisers and eutrophication
- Human population growth requires increased food yields. Farmers use fertilisers (providing nitrates for protein synthesis) and pesticides (killing crop pests).
- Sequence of freshwater algal blooms and eutrophication:
- 1. Leaching — Heavy rainfall washes excess synthetic fertilisers from agricultural fields into nearby freshwater streams, rivers, and lochs.
- 2. Algal Bloom — The high nitrate and phosphate concentrations cause rapid, uncontrolled proliferation of surface algae, forming a thick algal bloom.
- 3. Light Blockage — The dense blanket of algae blocks sunlight from penetrating the water, killing submerged aquatic plants as they can no longer photosynthesise.
- 4. Bacterial Decomposition — Dead plants and algae provide a vast food source for aerobic bacteria, whose populations explode.
- 5. Oxygen Depletion & Death — Multiplying bacteria consume vast quantities of dissolved oxygen for aerobic respiration. Dissolved oxygen levels plummet, causing fish and other aquatic invertebrates to suffocate and die.
Section 2 of 2Pesticides, bioaccumulation, and alternatives
1Pesticides and bioaccumulation4 points
:
- Pesticides sprayed onto crops can be washed into rivers and absorbed by microscopic aquatic organisms.
- Many pesticides are persistent (non-biodegradable): they cannot be broken down by animal metabolism.
- As energy transfers along the food chain, the concentration of the toxic pesticide increases at each successive trophic level (bioaccumulation / biomagnification).
- Apex predators at the top of the food chain accumulate lethal or sub-lethal toxic concentrations, causing eggshell thinning or mortality.
2Sustainable alternatives2 points
:
- Biological controlIntroducing a natural predator, herbivore, or pathogen to suppress pest populations (e.g. ladybirds introduced to feed on aphid pests, or parasitic wasps against whiteflies in greenhouses).
- Genetically Modified (GM) cropsEngineering crops with natural pest resistance (e.g. Bt corn expressing a bacterial toxin harmless to mammals but lethal to insect borers), eliminating the need for chemical pesticide sprays.
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