ZIMSEC O Level · Biology · Form 3, Form 4
Ecosystems
Producers → primary consumers → secondary consumers → tertiary consumers. Decomposers (bacteria, fungi) break down dead matter and recycle nutrients.
- Pyramids of numbers and biomass show the amounts at each trophic level.
- Only about 10% of energy passes to the next level; the rest is lost in respiration, heat and waste.
Nutrient cycles
- Carbon cycle: photosynthesis removes CO₂; respiration, decomposition and combustion return it.
- Nitrogen cycle: nitrogen-fixing bacteria (in root nodules of legumes such as groundnuts and beans), nitrifying bacteria, denitrifying bacteria, lightning.
Populations
Population size depends on food, predators, disease and space. Growth curves show lag, exponential and stationary phases.
Human impact
- Deforestation, overgrazing and veld fires cause soil erosion and desertification.
- Pollution: fertiliser run-off causes eutrophication; mining releases heavy metals and silt into rivers; burning fossil fuels increases CO₂ and global warming.
- Conservation: afforestation, conservation farming, national parks, controlled hunting, recycling.
Exam tip: Eutrophication sequence: fertiliser run-off → algal bloom → algae die → bacteria decompose them, using oxygen → fish die.
Check yourself
- Why do farmers grow legumes in crop rotation?
- Name two groups of organisms that are decomposers.
- Explain why fish die when a river becomes eutrophic.
Show answers
- Root nodules contain nitrogen-fixing bacteria that add nitrates to the soil.
- Bacteria and fungi.
- Decomposing bacteria use up the dissolved oxygen, so fish suffocate.
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