When you first draw a food web in IB ESS, it feels less like science and more like trying to untangle headphones from the bottom of your bag. Arrows everywhere. Labels half-forgotten. And that nagging question: what exactly is “flowing” here--energy, matter, or both?
In the 2026 first assessment, energy flow and food webs show up because they are a perfect test of systems thinking. A small change at one trophic level can ripple through everything else, and exam questions love ripples.
Quick checklist for IB ESS energy flow
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Energy enters ecosystems mostly as sunlight, captured by producers.
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Energy moves through trophic levels via feeding relationships.
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Only about 10% transfers to the next trophic level (rule of thumb).
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The rest is lost as heat through respiration and other life processes.
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Food chains are simplified lines; food webs are realistic networks.
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Pyramids of energy are the most reliable way to show transfer losses.
If you want syllabus-aligned definitions and examples, anchor yourself with RevisionDojo’s ESS hub: IB ESS New Syllabus resources.
How energy flows through ecosystems in IB ESS
In IB ESS, energy flow is a one-way journey. It does not cycle. Ecosystems need constant input because eventually all usable energy leaves as heat.
Start with producers (autotrophs). Plants and algae convert light energy into chemical energy via photosynthesis. Then primary consumers eat producers, secondary/tertiary consumers eat other consumers, and decomposers break down dead material and waste.
The key exam idea is efficiency: most energy is used for metabolism, movement, and maintaining body temperature, and is lost as heat. That’s why food chains rarely stretch beyond 4–5 trophic levels.
To tighten your wording for Paper 2 explanations, use RevisionDojo’s topic notes on energy and biomass: 2.2 Energy and biomass in ecosystems (notes) and the matching practice: 2.2 Questionbank practice.
Food chains vs food webs (and why examiners prefer webs)
A food chain is a clean storyline: grass -> rabbit -> fox. Useful for learning trophic levels, but unrealistic.
A food web is closer to nature. One species often eats multiple foods, and many species share predators. That interconnectedness is exactly what Paper 1 data-response questions test: identifying trophic levels, predicting population change if a species is removed, or explaining stability.
If you need a focused refresher on feeding positions, RevisionDojo’s blog is a strong companion: What is a trophic level?.
Energy pyramids in IB ESS: what to remember
You’ll hear about pyramids of numbers, biomass, and energy, but IB ESS exam questions reward one message: pyramids of energy are always upright because energy is lost at each transfer.
Energy pyramids are typically shown as energy per unit area per unit time. They make the 10% idea visible and help you explain why top predators are fewer, need larger territories, and are more vulnerable to disruption.
For the broader systems link (flows, productivity, and energy laws), connect it to: 2.3 Flows of energy and matter (topic page) and 2.3 Flows of energy and matter (notes).
Human impacts that disrupt food webs (2026-ready examples)
In IB ESS, human impacts are rarely “extra.” They are the point of the system.
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Overfishing can remove top predators, triggering trophic cascades.
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Deforestation reduces producer biomass, shrinking the energy base of the web.
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Pollution can cause biomagnification, concentrating toxins in higher trophic levels.
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Climate change shifts species ranges and timing (phenology), breaking established feeding links.
These are ideal for evaluation questions because you can show cause, feedback, and consequences over time. For systems language and foundations, see: Foundations of ESS (notes).
Final takeaway: make IB ESS energy flow feel predictable
In IB ESS, energy flow and food webs aren’t about drawing prettier diagrams. They’re about predicting consequences. Once you internalize “energy enters, transfers inefficiently, and leaves as heat,” the rest becomes logic: shorter food chains, upright energy pyramids, and fragile top predators.
To revise efficiently, build a loop: learn with 2.2 Energy and biomass notes, drill with the 2.2 Questionbank, and use RevisionDojo’s Flashcards, Mock Exams, Predicted Papers, and Tutors when you want speed and confidence. IB ESS rewards students who can think in systems--RevisionDojo helps you practise that until it feels automatic.