Food webs at HL: Food webs are the same for SL and HL. At HL you reuse them when analysing the more detailed energy and nutrient flows through an ecosystem.
No HL-only content here — just learn how food webs are built and read.
Free preview
This is the free notes preview
You're reading the free notes. Aimnova Pro unlocks the full study experience — and you can try it with your first topic free to keep:
- FlashcardsLock in vocabulary and key terms with spaced repetition.
- Practice questionsAnswer exam-style questions and get instant AI marking.
- Mock exams & past-paper vaultSit full mocks and see exactly how examiners award marks.
- Personalised study planA daily plan built around your exam date and weak areas.
Food webs: real ecosystems
Real ecosystems are more complex than simple food chains.
A food web shows how organisms are linked through multiple feeding relationships.
- Most organisms eat more than one species
- Energy can move through several pathways
- Food webs increase ecosystem stability
More connections = greater resilience if one species disappears.
Feeling unprepared for exams?
Get a clear study plan, practice with real questions, and know exactly where you stand before exam day. No more guessing.
Why food chains are short
Food chains rarely have more than four or five trophic levels.
This is because energy decreases at each step up the food chain.
- Less energy available at higher levels
- Top predators need large territories
- Ecosystems cannot support many high-level consumers
Short food chains are a direct result of energy loss at each transfer.
IB-style question — Predict a cascade effect in a food web
A simplified grassland food web contains: grasses → grasshoppers → frogs → herons; grasses → grasshoppers → shrikes. If heron numbers decline sharply, outline the likely effects on frog and grasshopper populations. [2]
How to answer it, step by step
- Start with the species that lost its predator
• Fewer herons → less predation on frogs → frog population increases. - Follow one level down
• More frogs eat more grasshoppers → grasshopper population decreases.
Final answer
One mark per valid change + cause. Always state BOTH direction of change AND the reason — 'frogs increase' alone earns nothing; 'frogs increase because heron predation is removed' earns the mark.