IB Biology SLTopic 3 — Energy & MatterPaper 1 & 2Core idea~9 min read
Primary & Secondary Production
Production is just biomass being added. Autotrophs build biomass out of carbon dioxide, and heterotrophs build biomass out of whatever they ate. Two words, one clean distinction — and a calculation you should practise until it is automatic.
📘 What you need to know
Primary production is the accumulation of carbon compounds in the biomass of autotrophs.
Secondary production is the accumulation of carbon compounds in the biomass of heterotrophs.
Biomass accumulates when organisms grow and reproduce.
The rate of production is measured per unit area (or volume) per unit time, e.g. g m−2 yr−1.
Secondary production is calculated by subtracting respiratory and other losses from the energy ingested.
Secondary production is always lower than primary production in the same ecosystem.
Primary production
Autotrophs capture energy and use it to build carbon compounds. Some of those compounds are respired straight away to keep the organism alive; the rest are added to the plant’s own body as new leaf, stem, root or seed. That accumulated material is primary production.
Production is what is left after the losses have been paid for.
Why some places produce far more than others
Primary production depends on how fast photosynthesis can run, which depends on the conditions in that biome. Biomes with plenty of light, warm but not extreme temperatures and high rainfall produce enormous amounts of biomass. Cold, dark or dry biomes produce very little.
A tropical forest fixes more than twenty times as much biomass per square metre as a desert.
Do not confuse rate with total. The open ocean has a low rate per square metre, but it covers such an enormous area that its total contribution to global production is huge.
Measuring the rate
Production is always expressed as an amount of biomass, over an area or volume, over a period of time.
Units of production
g m−2 yr−1 on land g m−3 yr−1 in water, where depth matters
Energy units such as kJ m−2 yr−1 are also used, since biomass and stored energy are two ways of measuring the same thing.
Aquatic habitats often use volume rather than area because photosynthesising algae are spread through the water column, not sitting on a flat surface. If a question mentions a lake or the open sea, expect cubic metres.
Secondary production and how to calculate it
A heterotroph swallows a lot of energy, but most of it never becomes body tissue. Secondary production is what is left once the losses have been taken out.
Secondary production
energy ingested − (faeces + respiration + excretion)
WORKED EXAMPLE
A vole population ingests 4 200 kJ m−2 yr−1. It egests 1 850 kJ in faeces, loses 2 010 kJ in respiration and excretes 95 kJ. Calculate the secondary production and the percentage of ingested energy it represents. [3]
Step 1: total the losses1 850 + 2 010 + 95 = 3 955 kJStep 2: subtract from energy ingested4 200 − 3 955 = 245 kJ m−2 yr−1Step 3: percentage245 ÷ 4 200 × 100 = 5.83 %keep the units attached all the way through — marks are given for them
Why secondary is always lower than primary
Every heterotroph is downstream of an autotroph. The energy it takes in has already been reduced by the producer’s own respiration, and is reduced again by the consumer’s respiration, faeces and excretion. Two rounds of subtraction can only give a smaller number.
This is the same idea as the 10 % rule, dressed in different words. If a question asks you to explain the difference in rates, talk about respiratory losses at each step.
Primary production
Secondary production
Which organisms
Autotrophs (producers)
Heterotrophs (consumers, decomposers)
Source of carbon
Carbon dioxide from air or water
Carbon compounds in food ingested
Energy source
Light, or oxidation of inorganic chemicals
Chemical energy in the food
Typical rate
Higher
Lower, because losses have already occurred
💡 Exam tip
Define production as the accumulation of carbon compounds in biomass, not simply “growth”.
Say autotroph for primary and heterotroph for secondary. Those two words split the definitions cleanly.
Include per unit area and per unit time whenever you are asked about a rate.
In calculations, subtract every loss the question gives you, not just respiration.
If the habitat is aquatic, expect per cubic metre rather than per square metre.
Remember reproduction counts as biomass accumulation as well as growth.
⚠ Common mix-up
Primary production is not the same as photosynthesis. Photosynthesis is the process; production is the biomass left over after the plant has respired.
Secondary production is not “the second trophic level”. It applies to all heterotrophs.
Production is a rate, not a total. Missing the time unit is the most common error here.
Desert and open ocean are low per square metre, but that says nothing about their global totals.
Biomass is dry mass in most measurements, since water content varies wildly.
Decomposers do secondary production too, even though they are drawn to one side of the chain.
Up next: The Carbon Cycle — we have followed the energy to the end of the line. Now we follow the atoms, and they do something energy never does: they come back.
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