5.3.7Energy Transfers In and Between Organisms (A Level only)

Primary and Secondary Productivity — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 8 parts Lesson 19 of 30 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision

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Lesson summary

5.3.7 Primary and Secondary Productivity — AQA A Level Biology 7402 notes: Distinguish primary and secondary productivity and use equations to calculate net production at consumer levels.Productivity is the rate at which energy is stored in biomass.Primary productivity applies to…

N = I − (F + R)
Food quality and digestibility
Body temperature regulation
Age and growth stage
Environmental conditions
Exam-style question 1

Section 1 of 8Production and productivity

Productivity is the rate at which energy is stored in biomass.

Secondary productivity

Primary productivity applies to producers; secondary productivity refers to the rate at which consumers convert the chemical energy of the food they absorb into their own biomass.

Both are measured as energy per unit area (or volume) per unit time, in kJ m^-2 year^-1, and are generally measured by the change in dry mass and calorimetry.

Section 2 of 8Net production of consumers

The same logic as NPP applies to consumers. A consumer takes in chemical energy as food, but only part is turned into body mass.

N = I − (F + R)

where N is net production (energy stored in new consumer biomass), I is the chemical energy in ingested food, F is the energy lost in faeces and urine, and R is the energy lost in respiration.

Net production of consumers

The term in brackets is energy not retained.

Energy in faeces was never absorbed, so some texts distinguish between ingested and assimilated energy: assimilated energy = I − F.

Net production is then assimilated energy minus respiration.

The net production of a consumer is the energy available to the next trophic level, and to decomposers when it dies.

Section 3 of 8Factors affecting secondary productivity

1Food quality and digestibility

Food quality and digestibility a diet rich in cellulose passes through quickly; carnivores digest more of their food.

2Body temperature regulation

Body temperature regulation endotherms respire a large proportion of intake and have low net production; ectotherms lose less and have higher net production.

3Activity

Activity animals that move more use more in respiration.

4Age and growth stage

Age and growth stage young animals convert more to biomass than adults, which mainly maintain body.

5Environmental conditions

Environmental conditions temperature, disease and the availability of food affect intake and growth.

Farmers exploit these in intensive rearing by restricting movement and keeping animals warm, which lowers respiratory loss and raises net production.

Section 4 of 8Data handling

When data on energy in a food chain are given, check carefully which quantity is which.

Producers: GPP and R give NPP.

For consumers: I, F and R give N.

To calculate how much energy is transferred between levels, compare N or I at a level with that of the level below.

For example, if the herbivore's ingestion is 1 200 kJ m^-2 year^-1 from a plant NPP of 12 000, the proportion eaten is 10%.

Symbol	Quantity
I	Energy in ingested food
F	Energy lost in faeces and urine
R	Energy lost in respiration
N	Net production = I − (F + R)

Section 5 of 8Worked example and extended application

A herbivore population in a field receives plant material containing 8 000 kJ m^-2 year^-1.

Worked example and extended application

Of this, 3 100 is lost in faeces and urine and 4 200 in respiration.

N = 8 000 − (3 100 + 4 200) = 700 kJ m^-2 year^-1, or 8.75% of intake.

If a predator eats all of this net production and has a net production of 85 kJ m^-2 year^-1, its efficiency is 85 / 700 × 100 = 12.1%.

Note how secondary productivity relates to farming: the aim is to maximise N.

A farmer can raise the food intake, lower F by feeding highly digestible food (grain rather than hay), and lower R by limiting activity and keeping animals warm.

The trade-off is welfare and cost.

Questions often give a table for several species; to compare them, calculate N as a percentage of I for each.

Typically a herbivorous insect or a fish has a higher percentage than a mammal or bird of similar size because it does not use energy to maintain a constant body temperature.

You should also be able to explain why detritivores and decomposers matter: they feed on the energy locked in faeces, urine and dead bodies, and in doing so release nutrients, so that mineral ions return to the soil for uptake by producers.

Section 6 of 8Exam-style questions

A cow ingests 40 000 kJ per day of grass. It loses 14 000 kJ in faeces and urine and 22 000 kJ in respiration. Calculate its net production per day and the percentage of ingested energy that this represents. [3]3 marks
Explain why net production in a farmed chicken kept in a warm barn is higher than in a free-range chicken. [3]3 marks

Section 7 of 8Key terms

Term	Meaning
Productivity	Rate of energy storage in biomass per unit area per unit time.
Secondary productivity	Rate at which consumers convert absorbed food energy into new biomass.
Ingestion	Taking food into the body.
Assimilation	Absorption of digested food and its use to build body tissue or release energy.
Net production	Energy retained in new biomass.

Section 8 of 8Exam technique

Using the formula N = I + F + R.

Confusing ingested and assimilated energy.

Forgetting urine in the energy lost.

Using productivity as if it were biomass; productivity is a rate.

Last updated: 18 October 2026