IB Biology: Food Chains, Webs & Trophic Levels FAQ

It's examined across Paper 1 (data-based multiple choice on energy pyramids or pyramid of numbers), Paper 2 (short-answer and extended-response questions asking you to construct food webs or explain energy loss between levels), and occasionally in Paper 3 option data. According to the IB Biology guide (first exams 2025), this sits under Topic 4.2, Energy Flow.

A food chain is a single, linear sequence showing who eats whom — producer to primary consumer to secondary consumer. A food web shows the same community but with multiple interconnected chains, reflecting that most organisms eat more than one thing. Real ecosystems are always webs; chains are simplified teaching models.

A trophic level is the position an organism occupies in a food chain, based on the number of energy transfers from the original producer. Producers occupy the first level, primary consumers the second, secondary consumers the third, and so on. Trophic level is about feeding position, not species identity.

Roughly 90% of energy at each trophic level is lost as heat through respiration, or used for movement, growth and reproduction that isn't passed on when the organism is eaten. Only the energy stored in biomass gets transferred upward, which is why the '10% rule' is a useful approximation, not a fixed law.

Because energy loss compounds at every transfer, so little usable energy remains by the fifth or sixth level to support a viable population size. Beyond four or five levels, there typically isn't enough biomass or energy left to sustain a stable population of top predators long-term.

A pyramid of numbers counts individual organisms at each level and can be inverted (one tree hosting thousands of insects). A pyramid of biomass measures dry mass at each level and is rarely inverted. A pyramid of energy shows energy flow per unit area per unit time and is never inverted, since energy always decreases up the chain.

Read the feeding relationships given in the source material first, then draw one arrow per relationship pointing from the organism being eaten to the organism eating it — arrows always show energy flow direction, never 'who eats who' as an English sentence would suggest.

Assuming an organism always sits at one fixed trophic level, when in reality omnivores and organisms with varied diets can occupy different levels depending on which food source you're tracing. A fox eating berries sits at level 2; the same fox eating a rabbit sits at level 3.

Yes — you need to calculate percentage energy transfer between trophic levels using the formula (energy at level 2 ÷ energy at level 1) × 100, and be ready to explain the biological reasons behind the value you calculate. This appears regularly in Paper 1 data questions and Paper 2 calculations.

The core content on food chains, webs, trophic levels and energy flow is identical for SL and HL students — it sits in Topic 4.2 of the current guide. HL students face additional ecology depth elsewhere in Topic 4 (like nutrient cycling detail), but the trophic-level content itself isn't harder at HL.

Energy flow links directly to photosynthesis (Topic 2.9) and cellular respiration (Topic 2.8), since producers fix the energy that later flows through the food web. It also connects to nutrient cycling and climate change topics later in Topic 4, so examiners love cross-topic questions here.

Most students don't struggle with the ecology itself — they struggle with the maths (percentage energy transfer calculations) and with mixing up biomass versus energy in pyramid diagrams. If your child can define trophic level but can't calculate transfer efficiency from real data, that's the gap to target first.

Topical practice with real past-paper-style data questions works better than re-reading notes for this topic, since most marks are lost on calculation and diagram-construction skills rather than definitions. On RevisionPrep, students can work through topical worksheets and mock paper questions specifically on Ecology to build this exam technique.