Four equations, and they are all the same idea repeated: take the total, subtract the losses, get the net. The marks here are lost on units and on picking the wrong equation, almost never on the arithmetic.
📘 What you need to know
Primary productivity is the rate at which producers make biomass using an external energy source.
GPP is the rate at which producers store chemical energy through photosynthesis.
NPP = GPP − R, where R is plant respiratory losses.
GSP = food eaten − faecal loss for consumers.
NSP = GSP − R.
Units are biomass or energy per unit area (or volume) per unit time.
Volume is used instead of area in aquatic habitats.
The four equations
The only real difference is that consumers lose material in faeces before respiration even gets a chance. Producers have no equivalent step.
Primary productivity
During photosynthesis, producers convert light energy to chemical energy stored within biological molecules — biomass. Primary productivity is defined as the rate at which biomass is produced using an external energy source such as sunlight, plus inorganic sources of carbon and other elements.
How it is measured
Laboratory techniques. Measure the rate of photosynthesis in samples such as plants or phytoplankton. Controlled experiments show how conditions like light intensity or carbon dioxide levels affect productivity.
Field techniques. Measure changes in biomass over time. In grassland, for example, researchers sample vegetation at different times to see how much biomass has been added.
Gross primary productivity
GPP is the rate at which plants store chemical energy or biomass through photosynthesis. It can be given in energy units or in mass units:
Energy per area per time: J m−2 yr−1, kJ km−2 yr−1
Mass per area per time: g m−2 yr−1, kg km−2 yr−1
In aquatic environments, per volume: kg m−3 yr−1, kJ m−3 yr−1
“Area” here means the area of land being studied, which contains the primary producers. No producers in that area means no gross primary production.
Net primary productivity
Net primary productivity
NPP = GPP − R
A large share of the energy stored in glucose during photosynthesis is released again during respiration, so it is not stored as new plant biomass and is not available to herbivores. NPP is therefore the rate at which energy is stored in plant biomass allowing for respiratory losses.
NPP is the basis of every food chain and food web, because it is the energy that supports everything above the producers. It can be thought of as the amount of plant growth available to be harvested by:
Primary consumers in natural ecosystems
Farmers in agricultural systems
Foresters in silvicultural systems
Rearranging. Questions often give you two of the three values and ask for the third. GPP = NPP + R. R = GPP − NPP. Practise both rearrangements — they appear as often as the original.
Secondary productivity
Consumers cannot photosynthesise, so their input is the food they eat. Two things go missing before anything can be stored: material that passes straight through as faeces, and energy released in respiration.
Secondary productivity
GSP = food eaten − faecal loss
NSP = GSP − R
Gross secondary productivity is the total energy or biomass assimilated by consumers — that is, actually absorbed into the body. Net secondary productivity is what is left after respiration, and is the amount stored as new consumer biomass.
A reliable order of operations: faeces first, then respiration. If you subtract them in the wrong order you will still reach the right final answer, but you will lose the method mark for GSP.
WORKED EXAMPLE
Grass in a meadow converts light energy into carbohydrates at 16 200 kJ m−2 yr−1 and releases 12 900 kJ m−2 yr−1 in respiration. Calculate the net primary productivity.
Step 1: Match the numbers to the equation16 200 is GPP; 12 900 is RStep 2: SubstituteNPP = 16 200 − 12 900Step 3: Calculate and give units= 3 300NPP = 3 300 kJ m−² yr−¹
WORKED EXAMPLE
Caterpillars in woodland ingest 2 400 kJ m−2 yr−1 from oak leaves. They lose 1 350 in faeces and a further 720 through respiration. Calculate the net secondary productivity.
Step 1: Calculate GSPGSP = 2 400 − 1 350 = 1 050 kJ m−² yr−¹Step 2: Calculate NSPNSP = 1 050 − 720 = 330NSP = 330 kJ m−² yr−¹Show the GSP line separately – it is usually a mark on its own
WORKED EXAMPLE
A patch of tundra covering 1 km2 produces 1 800 kg of biomass per year. Calculate the gross primary productivity in g m−2 yr−1.
Step 1: Convert the area1 km² = 1 000 000 m²Step 2: Find the mass per square metre1 800 ÷ 1 000 000 = 0.0018 kg m−² yr−¹Step 3: Convert kg to g0.0018 × 1 000 = 1.81.8 g m−² yr−¹Do the area conversion first – it is where most errors happen
💡 Exam tip
Write the equation out before substituting. It earns method marks even if the arithmetic slips.
Label the numbers first: which is GPP, which is R, which is faecal loss.
Always give units per area per time. An unlabelled number rarely gets full marks.
Practise the rearrangements: GPP = NPP + R and R = GPP − NPP.
Use volume units for aquatic habitats.
⚠ Common mix-up
Forgetting the faecal step for consumers. Food eaten is not the same as food assimilated.
Adding instead of subtracting. Net is always smaller than gross.
Mixing units in one calculation. Convert kg to g, or km2 to m2, before you start.
Treating GPP as available to herbivores. Only NPP is.
Using area units for a pond or ocean question. Aquatic systems use volume.
Up next: Maximum Sustainable Yield — using these numbers to decide how much can actually be harvested.
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