Key Idea: Most marks in topic 2.2 come from productivity calculations and from pollutants building up in food chains. Learn those first, then chains, webs and pyramids.
1. Productivity (most marks)
- Gross productivity (GP) is the total gain in biomass; net productivity (NP) is what is left.
- NP = GP − R, where R is the energy lost in respiration.
- At HL also: NPP = GPP − R, GSP = food eaten − faecal loss, NSP = GSP − R.
- Only net productivity passes to the next trophic level.
2. Pollutants in food chains
- Bioaccumulation: a pollutant builds up in one organism over its life.
- Biomagnification: it becomes more concentrated at each trophic level.
- DDT, PCBs and mercury do not break down, so top predators get the highest dose.
- Microplastics carry these pollutants and pass them along food chains.
3. Chains, webs and pyramids
- Arrows point from the food to the feeder: the way energy flows.
- About 10% of the energy passes to the next level; the rest is lost, mostly as heat.
- So food chains are short, and pyramids of energy are never upside down.
- Ecological efficiency (%) = (energy in the higher level ÷ energy in the lower level) × 100.
How it is examined:
| Question | How to score |
|---|---|
| [1] State a trophic level or name a producer | Use the exact word: producer, primary consumer, secondary consumer. |
| [1] to [2] Calculate NP, GP or an efficiency | Formula first, then the numbers, then the answer with units. |
| [2] Explain why energy is lost along a food chain | Respiration releases it as heat; not all is eaten or absorbed. |
| [2] to [4] Explain biomagnification | Not broken down, stored in fat, eaten in large amounts at each level. |
| [7] Explain or describe how energy flows in an ecosystem | Inputs, transfers, losses as heat, with a named ecosystem. |
Key terms, one line each:
| Term | In one line |
|---|---|
| Producer (autotroph) | makes its own food from inorganic substances, usually by photosynthesis |
| Consumer (heterotroph) | gets energy by eating other organisms |
| Trophic level | the position of an organism in a food chain |
| Net productivity | the gain in biomass left after respiration losses |
| Biomagnification | a pollutant becoming more concentrated at each step up a food chain |
| Second law of thermodynamics | every energy transformation is inefficient: some energy is lost as heat |
Last-minute check: answer, then tap to see.
Herbivores eat 500 J and lose 200 J in faeces. What is GSP? GSP = food eaten − faecal loss = 300 J.
A level receives 14 000 kJ and passes on 1 600. Efficiency? (1 600 ÷ 14 000) × 100 = 11.4%.
What is a chemoautotroph? An organism that makes food using energy from chemical reactions, not light.
Why are sustainable yields higher at lower trophic levels? Less energy has been lost to respiration before harvest.
What happens to entropy as biomass passes along a food chain? It increases: ordered biomass becomes heat and simple molecules.
A farmer can feed people either wheat directly or pigs fed on the same wheat. Explain why the sustainable yield of food from wheat is higher than from pigs.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
Exam tips
- In a calculation, write the formula first, then the numbers, then the answer with units.
- Say where energy goes: lost as heat in respiration, not destroyed.
- Food-chain arrows point from the food to the feeder.
- For HL yields and efficiency, say which trophic level and how much energy was lost before it.