Productivity at HL: Gross and net productivity are defined the same for SL and HL. They feed directly into the more detailed HL work on nutrient cycles and energy flow.
No HL-only content here — just get the productivity definitions and formulas clear.
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Productivity in ecosystems
Productivity describes how fast plants and other producers turn energy (mainly sunlight) into living material such as leaves, stems, and roots. In simple terms, it shows how much new life an ecosystem can support.
Most productivity comes from producers (like plants and algae) through photosynthesis, which forms the energy base for all food chains.
- Gross productivity (GP) — all the energy captured before anything is used.
- Net productivity (NP) — the energy left for growth, reproduction, and feeding consumers.
NP = GP − respiration
Only net productivity is available to be passed on to the next trophic level.
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IB-style question — One strength + one limitation of a productivity pyramid
State one strength and one limitation of using a pyramid of productivity to represent energy flow through an ecosystem. [2]
How to answer it, step by step
- Strength
• Shows the rate of energy production over time (not just a snapshot), so it is never inverted — energy flow always decreases up trophic levels. - Limitation
• Omnivores feed at more than one trophic level and are difficult to place correctly.
• OR: data collection requires killing organisms or complex calorimetry — expensive and time-consuming.
Final answer
One mark each. The examiner wants one strength AND one limitation — not two strengths. Most reliable strength: 'never inverted'. Most reliable limitation: 'hard to place omnivores' or 'destructive data collection'.
IB-style question — Calculate energy transfer efficiency
A meadow food chain has the following annual productivity values: grasses 40 000 kJ m⁻², voles 2 000 kJ m⁻², barn owls 80 kJ m⁻². Calculate the percentage efficiency of energy transfer from voles to barn owls. [1]
How to answer it, step by step
- Formula → substitution → answer
• Efficiency (%) = (higher level ÷ lower level) × 100
• = (80 ÷ 2 000) × 100 = 4%
Final answer
Show the working — examiners award the mark even if arithmetic is wrong as long as the method is right. Typical efficiency is 5–20%; if your answer is far outside this range, recheck which values you divided.
IB-style question — Compare shapes of number / biomass / productivity pyramids
Explain why a pyramid of productivity is always upright, but pyramids of numbers and biomass can sometimes be inverted. [4]
How to answer it, step by step
- Productivity always upright
• Energy is lost as heat (respiration) at every trophic transfer → less energy available at each higher level → bars always narrow going up. - Numbers and biomass can invert
• Numbers: one large tree supports thousands of aphids → producer bar narrower than consumer bar.
• Biomass: fast-reproducing phytoplankton can have lower standing stock than the zooplankton eating them (biomass = snapshot, not flow).
Final answer
Link every inversion to why productivity cannot invert. Never claim productivity pyramids can be inverted — examiners will deduct marks.