ZIMSEC O Level · Combined Science · Form 1, Form 2, Form 3, Form 4
An ecosystem is a community of living things interacting with each other and with their non-living environment.
Food chains and webs
Energy flows from the Sun to producers (green plants), then to primary consumers (herbivores), secondary consumers and so on.
Example: grass → impala → lion. Arrows show the direction in which energy flows.
Only about 10% of energy passes from one level to the next; the rest is lost as heat, movement and waste. This is why food chains are short.
Cycles
- Carbon cycle: plants take in carbon dioxide in photosynthesis; respiration, decay and burning return it to the air.
- Water cycle: evaporation, condensation, precipitation and run-off.
Human impact
- Deforestation (for firewood, farming and tobacco curing) causes soil erosion and loss of habitats.
- Veld fires destroy grazing and kill organisms.
- Pollution: litter, sewage, chemicals and mine waste damage rivers; burning fossil fuels adds carbon dioxide, contributing to climate change.
- Conservation: planting trees, contour ridges, protected areas such as Hwange and Mana Pools, and recycling.
Exam tip: In a food web question, follow the arrows – they point TO the organism that eats.
Check yourself
- In the food chain maize → mouse → snake → hawk, name the secondary consumer.
- Why are food chains rarely longer than five organisms?
- Give two effects of deforestation.
Show answers
- The snake.
- Energy is lost at each level, so too little is left to support more levels.
- Any two: soil erosion, loss of habitats, less rainfall/climate change, loss of firewood and medicines.
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