IB ESS HL Topic 2 — Ecology Paper 1 & 2 Practical skill ~9 min read

Biomass and Productivity

Biomass is how much living material is there. Productivity is how fast new living material is being made. Students lose marks constantly by treating those as the same thing — one is an amount, the other is a rate.

📘 What you need to know

Gross productivity (GP) is the total gain in biomass by an organism or community in a given area or time.
Net productivity (NP) is what remains after losses to cellular respiration.
NP is the energy available for growth and reproduction, and therefore for the next trophic level.
Respiratory losses are usually greater in consumers than in producers.
NP represents the maximum sustainable yield that can be harvested.
Biomass is measured by dry mass, controlled combustion or extrapolation from samples.

Gross and net productivity

Think of it like a wage. Gross productivity is what you earn; respiration is the money you have to spend just to live; net productivity is what is left over. Only the leftover can be saved — or, in ecological terms, eaten by something else.

The relationship Net productivity = Gross productivity − respiratory losses

The red block is not wasted from the plant’s point of view — it is the cost of living. It is simply unavailable to anything higher up the chain.

Worth noticing: respiratory losses are usually higher in consumers than in producers. Animals spend energy on digestion, movement and, if they are birds or mammals, on keeping warm. A plant just sits there.

If a question gives you two numbers and asks for productivity, work out which is which before you touch the calculator. The larger figure is nearly always gross; the amount described as “used in respiration” is R.

Net productivity and sustainable yield

The net productivity of an organism or trophic level is the maximum sustainable yield — the most that can be harvested without reducing the resource for the future.

Take a fishery. If you harvest exactly the net productivity, the population replaces what you took. Harvest more and stocks decline, which damages both the ecosystem and the livelihoods that depend on it.

The link worth remembering: NP is not just a number on a diagram. It sets the real-world limit on how much fish, timber or crop a system can give you year after year.

Measuring biomass

Estimating the biomass at each trophic level is how ecologists turn a food web into actual data. Three methods come up.

1. Measurement of dry mass

Collect samples of organisms from one trophic level and dry them in an oven until all the water has gone. Weigh what remains. Water is removed because water content varies enormously and is not organic matter — dry mass is a much better estimate of the biomass actually present.

Once you have the dry mass of a sample, you scale up. If one daffodil has a dry mass of 0.1 kg, then 200 daffodils hold roughly 20 kg.

2. Controlled combustion

Burn a known quantity of dried biomass and measure the heat produced. A calorimeter is used: the burning sample heats a known volume of water, and the temperature rise gives an estimate of the chemical energy the sample contained.

3. Extrapolation from samples

Take small samples of a population and scale up to estimate the total. Particularly useful for populations that are very large or difficult to sample directly — you cannot weigh every tree in a forest.

Limitation	Why it matters
Drying takes a long time	Samples must be heated gently so they do not burn, so it can take days
Precise equipment is needed	A very accurate balance and thermometer are required to detect small changes; both are expensive
Simple calorimeters lose heat	Heat escapes to the surroundings instead of reaching the water, so energy is underestimated
Organisms must be killed	Measuring dry biomass destroys the sample, which raises practical and ethical problems

A bomb calorimeter solves the heat-loss problem by ensuring almost all the heat from the burning sample reaches the water, giving a much more accurate estimate — at much higher cost.

WORKED EXAMPLE

A plant community captures 1 400 kJ m⁻² yr⁻¹ through photosynthesis and uses 520 kJ m⁻² yr⁻¹ in respiration. Calculate its net productivity and state what this figure represents.

Step 1: Identify the values 1 400 is gross productivity; 520 is R Step 2: Subtract 1 400 − 520 = 880 Step 3: Say what it means 880 kJ m−² yr−¹ This is the energy stored as new plant biomass and available to herbivores

WORKED EXAMPLE

The dry mass of grass from a 1 m² quadrat is 0.35 kg. The field measures 250 m². Estimate the total biomass of grass in the field and state one assumption you have made.

Step 1: State the biomass per unit area 0.35 kg m−² Step 2: Multiply by the total area 0.35 × 250 = 87.5 87.5 kg of grass Assumption: the grass is evenly distributed across the whole field

💡 Exam tip

Say rate when defining productivity, and give units per area per time.
Explain why samples are dried: water content varies and is not biomass.
When extrapolating, state your assumption — usually even distribution. It is often a separate mark.
Link net productivity to sustainable yield whenever harvesting is mentioned.
For calorimetry limitations, name heat loss to the surroundings as the main source of error.

⚠ Common mix-up

Biomass and productivity treated as the same thing. Biomass is an amount; productivity is a rate.
Using fresh mass. Water content varies too much, so dry mass is always used.
Forgetting producers respire. That is the entire reason NP is smaller than GP.
Assuming a sample is representative without saying so. State the assumption explicitly.
Ignoring units. A productivity answer without per area per time is incomplete.

Up next: Drawing Ecological Pyramids — turning these numbers into diagrams, and knowing which pyramid can be upside down.

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IB DEMYSTIFIED

Biomass and Productivity

📘 What you need to know

Gross and net productivity

Net productivity and sustainable yield

Measuring biomass

1. Measurement of dry mass

2. Controlled combustion

3. Extrapolation from samples

💡 Exam tip

⚠ Common mix-up

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