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.
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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.
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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.