IB Biology HLEnergy & Matter in EcosystemsPaper 1 & 2~9 min read
Primary & Secondary Production
Production is just biomass piling up. Primary production is biomass piling up in plants; secondary production is biomass piling up in animals. The terminology is the hard part, not the biology.
📚 What you need to know
Primary production is the accumulation of carbon compounds in the biomass of autotrophs.
Secondary production is the accumulation of biomass in heterotrophs.
Biomass accumulates when organisms grow and reproduce.
Primary production is faster in biomes with more sunlight, optimum temperatures and higher rainfall, because photosynthesis runs faster.
Rate of production is measured in biomass per unit area (or volume) per unit time: g m−2 yr−1, or g m−3 yr−1 for aquatic habitats.
Secondary production is always lower than primary production, because of respiratory losses.
Secondary production = energy ingested − respiratory losses.
Primary production
During photosynthesis, autotrophs convert light energy into chemical energy stored in biological molecules. Organisms that do this are the producers.
DefinitionPrimary production is the accumulation of carbon compounds in the biomass of autotrophs.
The word accumulation is doing the work here. Biomass accumulates when organisms grow and reproduce – not when they simply stay alive. A plant that is respiring away everything it photosynthesises is producing nothing.
Why some biomes produce far more than others
Primary production occurs more quickly in some biomes than in others. Biomes with more hours of sunlight, optimum temperatures and higher levels of rainfall allow photosynthesis to happen at a higher rate, which leads to faster primary production.
Tropical forest biomes have high light levels, high rainfall and ideal temperatures all year round, so primary production there is very fast. A hot desert has the light but not the water, and a tundra has neither the warmth nor the growing season.
Every one of these differences traces back to the rate of photosynthesis, which is set by light, temperature and water.
Measuring the rate
The rate at which producers convert light energy into chemical energy is expressed in units of biomass, per unit area (or volume), per unit time.
Unit
Meaning
When to use it
g m−2 yr−1
Biomass per square metre per year
Land habitats – the standard choice
g m−3 yr−1
Biomass per cubic metre per year
Aquatic habitats, where organisms are spread through a volume of water
Notice the difference between this page and the last one. Pyramids of energy use kJ m−2 yr−1 because they measure energy. Production uses g m−2 yr−1 because it measures biomass. Same shape of unit, different quantity – and swapping them is a classic dropped mark.
Secondary production
Organisms that gain carbon compounds by ingesting other organisms are heterotrophs. When they eat, the chemical energy in the biomass of their food is transferred to them and stored in their own biomass.
DefinitionSecondary production is the process by which biomass is stored in the tissues of heterotrophs.
But not all the energy in the carbon compounds consumed is transferred to new biomass:
Carbon is lost as carbon dioxide, a waste product of respiration.
Energy is lost during the excretion of other metabolic waste, such as water and urea.
Because respiratory losses are subtracted, the rate of secondary production is always lower than the rate of primary production in an ecosystem.
How it is calculated
secondary production = energy ingested − respiratory losses
Both boxes leak. The right-hand box leaks more, and it started with only what the left-hand box managed to keep.
Worked examples
WE 1
Calculate secondary production
A population of caterpillars ingests 4 800 kJ m−2 yr−1 of leaf tissue. Respiratory losses are 3 150 kJ m−2 yr−1 and 620 kJ m−2 yr−1 is egested. Calculate the secondary production. (3 marks)
Step 1: what is actually absorbed
4 800 − 620 = 4 180 kJ m−2 yr−1 assimilated (egested food was never absorbed).
Step 2: subtract respiration
4 180 − 3 150 = 1 030 kJ m−2 yr−1Step 3: state what it means
That is the energy stored as new biomass and available to the next trophic level.
1 030 kJ m−2 yr−1take egested material out first – it never entered the body, so it was never available to respire
WE 2
Compare two biomes
Explain why primary production in a tropical rainforest is much higher than in a tundra. (3 marks)
Point 1: light
A rainforest receives more hours of sunlight throughout the year.
Point 2: temperature and water
It also has optimum temperatures and higher rainfall, while a tundra is cold with a short growing season.
Point 3: the link
All three raise the rate of photosynthesis, so carbon compounds accumulate in autotroph biomass faster.
More light, warmth and water → faster photosynthesis → faster productiondo not stop at “it is warmer” – link every factor back to the rate of photosynthesis
WE 3
Explain the difference in rates
Explain why the rate of secondary production in an ecosystem is always lower than the rate of primary production. (3 marks)
Point 1: where it starts
Heterotrophs can only build biomass from carbon compounds that autotrophs have already made, so they start with less.
Point 2: respiration
Not all of the energy ingested becomes new biomass: carbon is lost as carbon dioxide in respiration.
Point 3: excretion
More is lost through excretion of metabolic waste such as water and urea, so respiratory losses are subtracted from what was ingested.
Secondary production = ingested − respiratory losses, so it must be smallerthe word “always” in the question is a hint that they want the reason it cannot go the other way
💡 Exam tips
Primary = autotrophs. Secondary = heterotrophs. Learn that pairing first.
Production units are g m−2 yr−1; energy pyramid units are kJ m−2 yr−1.
Use per cubic metre for aquatic habitats.
Say accumulation of biomass, and link it to growth and reproduction.
Quote the calculation: ingested − respiratory losses.
When comparing biomes, always route the explanation through the rate of photosynthesis.
⚠ Common mistakes
Saying primary production means “the amount of plants”. It is a rate of accumulation, not a standing quantity.
Using kJ for production. Production is measured in units of biomass.
Forgetting “per year”. Without a time unit it is not a rate.
Thinking secondary production could exceed primary. It is calculated by subtraction from what primary production supplied.
Ignoring plant respiration. Autotrophs lose energy by respiration as well.
Confusing secondary production with secondary consumers. Secondary production covers all heterotrophs, at every consumer level.
Up next: The Carbon Cycle – following the atoms rather than the energy, and learning to read a cycle diagram made of pools and fluxes.
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