IB Biology: Ecosystems & Energy Flow — FAQ

You need to explain how energy moves through trophic levels, calculate productivity (GPP, NPP, respiration), read energy-flow diagrams and pyramids, and connect nutrient cycling to ecosystem function. According to the IB Biology guide (first assessment 2025), this sits under Topic C4: Interaction and interdependence, alongside populations, communities and — at HL — climate change.

Trophic levels rank organisms by feeding position: producers (autotrophs), primary consumers (herbivores), secondary and tertiary consumers, plus decomposers recycling matter at every stage. In a food web, follow the arrows — each one points from the organism being eaten to the one eating it, showing the direction energy actually flows.

GPP is the total energy producers fix via photosynthesis; NPP is what remains after subtracting respiration, so NPP = GPP − R. Values are usually given in kJ m⁻² yr⁻¹, and exam questions typically supply two of the three figures and ask you to find the missing one, showing full working.

A food chain shows one linear energy pathway; a food web links several interconnected chains to show realistic community feeding relationships; an ecological pyramid (of numbers, biomass or energy) represents each trophic level as a horizontal bar that shrinks upward because energy is lost at every transfer.

The carbon cycle tracks matter, not energy — carbon atoms get recycled through photosynthesis, respiration, decomposition and combustion, while energy flows one way and is ultimately lost as heat. Examiners regularly test whether you can explain why matter cycles but energy doesn't, using the law of conservation of energy.

The three mistakes I see most in mock scripts: reversing arrow direction in food webs, forgetting only around 10% of energy transfers between trophic levels (the rest is lost as heat via respiration), and confusing GPP with NPP in calculations. Fix these three and you'll pick up marks most cohorts lose.

Expect "outline", "explain", "calculate" and "compare" most often. "Outline" wants a brief statement with no justification, "explain" needs a reasoned cause-and-effect chain, and "calculate" requires full working shown — even for a one-line formula like NPP = GPP − R.

It can appear anywhere. Paper 1 tests it through data-based multiple-choice items on food webs or productivity graphs, Paper 2 uses extended-response questions worth 6-9 marks on energy transfer and nutrient cycling, and Paper 3 occasionally links it to experimental data on decomposition or productivity measurement.

SL and HL share identical core content on trophic levels, productivity and nutrient cycling. The extra HL-only material sits in the climate change subtopic, where HL students must explain feedback mechanisms, evaluate evidence for anthropogenic climate change, and link disruption of the carbon cycle to ecosystem stability.

It's not conceptually difficult, but it's calculation-heavy and mark-scheme-strict — students who understand the theory fine still lose marks by skipping units or missing a step in a GPP/NPP question. Compared with something like enzyme kinetics, most students actually find ecology easier once they've drilled the standard question types.

Drill past-paper productivity calculations until the GPP/NPP/R formula is automatic, redraw energy-flow diagrams from memory instead of just reading them, and practise explaining — not just stating — why energy pyramids never invert. Timed, topic-specific past-paper practice is the fastest way to find which command term is actually costing you marks.

Ecosystems & energy flow is a full syllabus topic (Topic C4 in the current Biology guide, first assessed 2025) and regularly contributes questions across Paper 1 and Paper 2, so several weeks of teaching time is normal rather than a sign your child is behind. It's also one of the more calculation-heavy areas of the course.

Look for resources that pair concise notes on trophic levels and nutrient cycling with genuine past-paper-style calculation practice — reading alone won't build the numerical fluency examiners actually test. Structured revision notes combined with topical worksheets containing worked GPP/NPP examples and mark-scheme-style answers make self-checking far more effective than re-reading a textbook chapter.