Ecological Productivity (Edexcel IAL Biology)

Spec point — 5.9

Ecological Productivity

• During photosynthesis organisms such as plants convert light energy into chemical energy stored in biological molecules

  • Organisms that do this are known as **producers **

• The** rate** at which producers convert light energy into chemical energy is known as primary productivity

• Gross primary productivity, or GPP, can be defined as the rate at which chemical energy is converted into carbohydrates during photosynthesis

• Net primary productivity, or NPP, is the GPP minus plant respiratory losses

  • Of the total energy stored in glucose during photosynthesis, 90 % will be released from glucose to create ATP for the plant during respiration
  • 90 % of the energy originally converted by the plant will therefore not be stored as new plant biomass and will not be available to be passed on* to herbivores, also known as primary consumers

• The NPP can therefore be defined as the rate at which energy is stored in plant biomass

  • NPP is important because it represents the energy that is available to organisms at higher trophic levels* in the ecosystem, such as primary consumers and decomposers

• Net primary productivity can be calculated using the equation

NPP = GPP - R

Net primary productivity, or NPP, is the rate at which energy is stored in plant biomass and made available to primary consumers.

Calculation of Ecological Productivity

Spec point — 5.9

• Net primary productivity can be calculated using the equation

NPP = GPP - R

• Where

  • NPP = net primary productivity; the rate at which light energy is stored in plant biomass
  • GPP = gross primary productivity; the rate at which light energy is converted into carbohydrates during photosynthesis
  • R = respiratory losses; the carbohydrates used up by the plant that are not stored in plant biomass

• NPP is expressed in units of energy per unit area or volume per unit time e.g.

  • Using area: J m^–2 yr^-1 (joules per square metre per year)

  • Using volume: J m^–3 yr^-1 (joules per cubic metre per year)

    • Volume would be used when calculating NPP in aquatic habitats

Worked Example The grass in a meadow habitat converts light energy into carbohydrates at a rate of 17 500 kJ m^-2 yr^-1. The grass releases 14 000 kJ m^-2 yr^-1 of that energy during respiration. Calculate the net primary productivity of the grass in the meadow habitat.

Answer:

Step 1: Work out which numbers correspond to which parts of the equation

The meadow grass converts 17 500 kJ m^-2 yr^-1 into carbohydrates; this is its GPP

The meadow grass releases 14 000 kJ m^-2 yr^-1 of that energy in respiration; this is R

Step 2: Substitute numbers into the equation

NPP = GPP - R

NPP = 17 500 - 14 000

Step 3: Complete calculation

17 500 - 14 000 = 3 500

NPP = 3 500 kJ m^-2 yr^-1

Exam Hint The worked example above uses the equation in its basic form, but you may also be expected to rearrange the equation e.g. to calculate GPP or R

• If a question provides you with the NPP and R and asks you to calculate GPP, you will need to use the equation

GPP = NPP + R

• If a question provides you with the NPP and the GPP and asks you to calculate R, you will need to use the equation

R = GPP - NPP