5.3.2

# Production & Productivity

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## Primary Production

As energy passes from one trophic level to the next, energy is lost. Gross primary production and net primary production are important metrics for studying the energy transfer across trophic levels.

### Gross primary production

• Gross primary production (GPP) is the chemical energy stored in plant biomass, in a given area or volume.
• During respiration almost half of GPP is lost as heat. This is respiratory loss (R).

### Net primary production

• Net primary production (NPP) is the chemical energy stored in plant biomass after respiratory losses (R) to the environment have been taken into account. This relationship can be described by the following equation:
• NPP = GPP − R

## Secondary Production

As energy passes from one trophic level to the next, energy is lost. Secondary production allows us to study how much energy consumers store in their biomass. It is also called consumer production.

### Consumer production

• Net production of consumers (e.g. herbivores, carnivores, decomposers) refers to the production of new biomass by consumer.
• Consumers cannot convert all of the energy available from the previous trophic level into new biomass. Energy is lost primarily by:
• Respiration.
• Excretion (faeces and urine).

### Calculating consumer production

• The net production of consumers can be calculated by:
• N = I – (F + R)
• N = net production of new biomass by consumers.
• I = chemical energy stored in ingested food.
• F = chemical energy lost to the environment in faeces and urine.
• R = chemical energy used in respiration.

### Example

• A population of deer consume 40 000 kJ m2 yr −1 of energy.
• 27 000 kJ m2 yr −1 of this energy is lost as excretion (urine and faeces).
• 5000 kJ m2 yr −1 energy is used in respiration.
• Net production of consumers is calculated as:
• N = 40 000 − (27 000 + 5000)
• N = 40 000 − 32 000
• N = 8000 kJ m2 yr −1

### Efficiency of energy transfer

• After calculating consumer production, the efficiency of energy transfer can also be calculated.
• Example -
• Total energy consumed by the deer is 40 000 m2 yr −1 and net productivity is 8000 m2 yr −1.
• Efficiency = (8000 ÷ 40 000) × 100
• Efficiency = 20%

## Productivity

Productivity is the rate of production of new biomass. There are two categories of productivity: primary and secondary.

### Primary productivity

• Primary productivity refers to the rate of primary production by green plants.

### Secondary productivity

• Secondary productivity refers to the rate of secondary production by consumers (e.g. herbivores and carnivores).
• Herbivores have a lower secondary productivity than carnivores.
• This is because herbivores eat plant material which is higher in cellulose content than meat. Cellulose is hard to digest so more energy is lost in excretion.

### Measuring productivity

• Productivity is measured as biomass in a given area in a given time.
• An example of a unit used for measuring productivity is kJ ha-1 year-1.