Biomass is how much living material is there. Productivity is how fast new material is made. Confuse the two and half this topic stops making sense — including why a pond full of almost invisible algae can feed far more fish than it looks capable of feeding.
📚 What you need to know
Gross productivity (GP) is the total gain in biomass in a given area and time — all the energy captured.
Net productivity (NP) is what remains after losses to cellular respiration: NP = GP − R.
NP is the energy available for growth and reproduction, and it is what the next trophic level can eat.
Respiratory losses are usually greater in consumers than in producers, because animals move, hunt and digest.
NP sets the maximum sustainable yield — how much can be harvested without depleting the population.
Biomass is measured as dry mass, in g or kg per unit area, because water is not organic matter.
Methods: measuring dry mass, controlled combustion in a calorimeter, and extrapolation from samples.
Gross and net productivity
The one equation to know
net productivity = gross productivity − respiratory losses
Think of it as a wage. Gross productivity is what an organism earns; respiration is the tax it cannot avoid; net productivity is what it takes home and can actually spend on growth.
Only the amber block can ever appear at the next trophic level. Everything in the red block has already left as heat.
Why consumers lose more: a plant stands still. A herbivore walks, chews, digests, escapes predators and, if it is a mammal or bird, holds its body temperature steady. All of that costs respiration, so a bigger share of GP disappears before anything is stored.
Net productivity and sustainable yield
Net productivity is also the answer to a very practical question: how much can we take? If a fish population adds 400 tonnes of new biomass in a year, harvesting 400 tonnes leaves the population exactly where it started. Harvest more and you are eating into the stock itself.
Sustainable harvesting
maximum sustainable yield = net productivity of the population
Go beyond it and the population shrinks, next year’s productivity is lower, and the safe harvest falls again. That downward spiral is exactly what happened to several major fisheries in the twentieth century, and it damages livelihoods as well as ecosystems.
Measuring biomass
You cannot weigh a living organism and call it biomass, because most of what you are weighing is water. Biomass means the organic material, so the water has to go first.
🧩 Measuring dry mass
Collect a sample of organisms from one trophic level.
Heat them in an oven at a low temperature, so the sample dries without burning.
Weigh, dry again, weigh again, and repeat until the mass stops changing. That is constant mass, and it means all the water has gone.
Record the dry mass per unit area, for example g m-2.
Scale up to the whole area to estimate total biomass.
Controlled combustion
The second method burns a dried sample and measures the heat it gives off, which tells you how much chemical energy the biomass contained. The equipment is a calorimeter: the burning sample heats a known volume of water, and the temperature rise is the measurement.
Every labelled part exists for one reason: to stop heat escaping anywhere except into the water being measured.
Extrapolation from samples
The third method is the one you will meet most in exam questions. Measure the dry mass of a small, manageable sample, then scale it up to the whole area or population. It is the only realistic option for a forest or a large field.
Why these methods have limits
Drying to constant mass takes a long time — potentially several days for a large sample.
Precise equipment is needed: a sensitive balance and an accurate thermometer, both of which cost money.
A simple calorimeter loses heat to the surroundings, so it underestimates the energy content.
Every method requires killing the organisms, which raises practical and ethical concerns.
If a question asks how to improve accuracy, two answers nearly always work: dry the sample to constant mass rather than for a fixed time, and reduce heat loss from the calorimeter with a lid, insulation and a draught excluder.
Worked examples
WORKED EXAMPLE
A crop captures 3 600 kJ m² per year and loses 1 450 kJ m² per year to respiration. Calculate its net productivity.
Step 1: write the equation
NP = GP − R
Step 2: substituteNP = 3 600 − 1 450NP = 2 150 kJ per m² per yearStep 3: say what it means
This is the energy stored as new plant biomass, and the maximum that could be harvested each year without reducing the crop.
Carry the units through to the answer. Productivity is always per area per time.
WORKED EXAMPLE
A 1 m² sample of meadow grass has a dry mass of 0.35 kg. Estimate the biomass of a 250 m² meadow.
Step 1: state the dry mass per unit area
0.35 kg per m²
Step 2: scale upbiomass = 0.35 × 250about 87.5 kgStep 3: note the assumption
This assumes the grass grows evenly across the meadow. Taking several samples and using the mean would make the estimate more reliable.
Naming the assumption is often worth the final mark on a scaling question.
WORKED EXAMPLE
A fishery has a net productivity of 400 tonnes per year. Explain what happens if 550 tonnes are caught each year.
Step 1: compare with the sustainable yield
The maximum sustainable yield equals the net productivity, so 400 tonnes is the safe limit.
Step 2: what taking 550 does550 − 400 = 150 tonnes taken from the existing stockStep 3: the consequence
The population falls, so next year fewer fish are left to reproduce, net productivity drops and the sustainable yield falls further.
Overexploitation, and a stock that declines year after yearSay that the safe limit itself falls. That feedback is what makes overfishing so hard to reverse.
💡 Exam tip
Learn NP = GP − R in symbols and in words. Both versions appear in questions.
Always give productivity units: energy or mass, per square metre, per year.
Say dry mass whenever you mention biomass measurement. The word “dry” carries the mark.
For “how would you improve this method”, mention drying to constant mass and reducing heat loss.
Link NP to sustainable yield whenever a question involves harvesting, fishing or forestry.
⚠ Common mix-up
Biomass and productivity confused. Biomass is an amount at one moment; productivity is a rate.
Weighing organisms wet. Water is not biomass, so the sample must be dried first.
Adding respiration instead of subtracting it. NP is always smaller than GP.
Forgetting that producers respire too. They have respiratory losses just like animals, only smaller.
Thinking a big standing biomass means high productivity. A forest holds huge biomass and grows slowly; algae hold little and grow fast.
Dropping the units. A productivity answer without units rarely gets full marks.
Up next: Drawing Ecological Pyramids — pyramids of numbers, biomass and energy, when they turn upside down, and how to draw one that scores full marks.
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