3.8.7Additional lessons
Primary and Secondary Production — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 97 of 142
Part 1 of 5Primary Production
Definition
- The accumulation of carbon compounds in the biomass of autotrophs as they photosynthesise, grow and reproduce.
Gross production
- The total carbon compounds made by photosynthesis, before any is used in respiration by the producers.
Net production
- Gross production minus the amount used in the producers' own respiration;
- this is the biomass available to consumers.
Units
- Measured as biomass per unit area per unit time, such as g m⁻² yr⁻¹, or g m⁻³ yr⁻¹ for aquatic volumes.
Biome differences
- Tropical forests have high production because of long hours of sunlight, warm temperatures and plentiful rainfall.
Part 2 of 5Secondary Production
Secondary Production
Definition
- The accumulation of carbon compounds in the biomass of heterotrophs, formed from the organic matter they have ingested.
Losses first
- Part of the food is egested and part is respired, so only the remainder is built into new tissue.
Calculation
- Secondary production equals the energy or biomass assimilated minus the amount lost through respiration.
Comparison
- Production at each consumer level is lower than the level below, so total secondary production is less than primary production.
Part 3 of 5Interpreting Rates
1
Rates are expressed per unit area or volume per year so that ecosystems of different sizes can be compared.
2
A desert has low primary production because water is scarce, even though light may be plentiful.
3
A tropical rainforest has high primary production because light, water and warmth all favour photosynthesis.
4
Consumers can only use the net production of producers, so their total production is capped by it.
5
Secondary production can then be compared between ecosystems or between different farm systems.
| Term | Organisms | Biomass gained from |
|---|---|---|
| Primary production | Autotrophs | Inorganic carbon dioxide |
| Secondary production | Heterotrophs | Ingested organic matter |
Part 4 of 5Worked Example
Question
- A meadow has a gross primary production of 5 500 g m⁻² yr⁻¹ and the plants lose 2 200 g m⁻² yr⁻¹ in respiration.
- Calculate net primary production and the percentage of gross production this represents.
- Net production = gross production − respiration.
- 5 500 − 2 200 = 3 300 g m⁻² yr⁻¹.
- Percentage = 3 300 ÷ 5 500 × 100 = 60%.
Answer Net primary production is 3 300 g m⁻² yr⁻¹, which is 60% of gross production.
Part 5 of 5Exam Checklist & Tips
3.8.7 – Key Points for the Exam
- Primary production The rate at which autotrophs accumulate biomass.
- Secondary production The rate at which heterotrophs accumulate biomass from ingested food.
- Net production Gross production minus the respiratory losses of the organism or group.
- Biome A large ecological area defined by its climate and characteristic communities.
Common Mistakes
- Defining primary production as biomass of consumers; it applies to autotrophs only.
- Leaving out the time unit when stating a rate of production.
- Believing secondary production could exceed primary production in a stable ecosystem.
- Mixing up gross and net production; respiration must be subtracted for net.
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