5.3.8 Calculating Productivity & Efficiency

Percentage efficiency =

$$\frac{\text{Energy transferred to next level}}{\text{Energy available at previous level}} \times 100$$

Key Concepts:

• Gross Primary Productivity (GPP): The total rate at which energy is incorporated into organic molecules by plants during photosynthesis, measured in a given area or volume over time.
• Net Primary Productivity (NPP): The chemical energy store in plant biomass after respiratory losses (R) have been accounted for. Formula:

$$\text{NPP} = \text{GPP} - \text{R}$$

• NPP is available for plant growth, reproduction, decomposers, and herbivores.

Efficiency in Consumers:

The net production of consumers (N) can be calculated using the formula:

$$N = I - (F + R)$$

Where:

• I = Chemical energy ingested from food
• F = Energy lost in faeces and urine
• R = Energy lost as heat during respiration

Using a Bomb Calorimeter:

• A bomb calorimeter is used to measure the chemical energy in biomass.
• A sample of known mass is burned in pure oxygen, and the heat released is measured by the temperature change in the surrounding water.
• This provides the energy content of the sample, helping calculate GPP and NPP.