Secondary Productivity (AQA A Level Biology)

• During photosynthesis, primary producers (such as plants and algae) convert light energy into chemical energy in biological molecules
• The storing of this chemical energy in the biomass of primary producers is known as primary production
• This can be measured in units of mass per unit area or units of energy per unit area, for example, kg ha^–1 (kilograms per hectare) or kJ ha^–1 (kilojoules per hectare)
  • 1 hectare = 10,000 m² (i.e. 100m × 100m)

• The chemical energy stored as biomass during primary production is available to organisms in higher trophic levels in the ecosystem
  • These organisms are known as consumers
  • When consumers ingest producers, the chemical energy in the biomass of the producers is transferred to the consumers
  • Like producers, the consumers also store this chemical energy in their biomass
  • This process is known as secondary production or the production of consumers

Productivity

• When measuring the production of producers or consumers, it is often useful to measure the average production over a period of time. For example:
  • A farmer’s crops might grow very fast in the summer but very slow in the winter (the production of producers often varies during the year due to abiotic factors such as temperature and light availability). It may be useful to the farmer to know the average growth rate of his crops per month or year so that they can estimate how much crop biomass they can expect to harvest
  • A cattle farmer may want to know the average biomass they can expect their cows to produce per month or year. They need to know how much secondary production is occurring in a given period of time

• Productivity means the rate of production
  • The rate of primary production is known as primary productivity
  • The rate of secondary production is known as secondary productivity
  • In the examples above, the crop farmer needs to know the primary productivity of his crops and the cattle farmer needs to know the secondary productivity of his cows

• This means primary and secondary productivity is expressed in units of mass per unit area per unit time, for example:
  • g m^–2 day^–1 (grams per square metre per day)
  • kg ha^–1 yr^-1 (kilograms per hectare per year)
  • In this case, ‘area’ refers to the area of land that is being studied (this land contains the producers or consumers that are producing the biomass)

• Primary and secondary production can also be expressed in terms of how much chemical energy is stored in the biomass of producers or consumers
• This means primary and secondary productivity can also be expressed in units of energy per unit area per unit time, for example:
  • J m^–2 day^–1 (joules per square metre per day)
  • kJ ha^-1 yr^-1 (kilojoules per hectare per year)

Step 1: calculate the mean primary production per year

150 + 200 + 175 + 150 + 225 = 900

900 ÷ 5 = 180

Step 2: Give the appropriate units

 180 kJ ha^-1 yr^-1

Step 3: Draw a graph of the data provided, adding a line showing the primary productivity of the sunflower crop over the five year period

• Organisms that get energy by ingesting other organisms (including producers or other organisms that feed on producers) are known as consumers
  • When consumers ingest other organisms, the chemical energy in the biomass of these organisms is transferred to the consumers
  • The consumers then store this chemical energy in their own biomass
  • This process is known as secondary production or the production of consumers

• However, not all the chemical energy in the consumers’ food is transferred to the consumer
  • In fact, only around 10% of the energy is available to the consumer to store as new biomass
  • This means around 90% of the energy is lost

• Not all the energy is transferred to the consumer because:
  • Not all of the biomass of the food is eaten (e.g. the roots and woody parts of plants or the bones of animals), meaning that the chemical energy this biomass contains is lost to the environment
  • Consumers are not able to digest 100% of the food they ingest, so some is always egested as faeces. Again, the chemical energy in this undigested biomass is also lost to the environment
  • Lots of chemical energy is lost to the environment when consumers respire (this energy is mainly lost as heat) and during the excretion of waste products (e.g. urine)

• The energy that is left after these losses is available to the consumer to store as new biomass and is known as the net production of consumers (or simply as the net production)
• The net production of consumers (N) can be calculated using the following equation:

N = I - (F + R)

• Where:
  • I = the chemical energy store in ingested food
  • F = the chemical energy lost to the environment in faeces and urine
  • R = the respiratory losses to the environment

The chemical energy that consumers can store as new biomass after energy losses due to respiration, egestion (faeces) and excretion (urine) is known as the net production

Step 1: Write out the equation and substitute in the known values

N = I - (F + R)

N = 2,000 - (1,200 + 600)

Step 2: Calculate the net production (N) and give appropriate units

N = 2,000 - 1,800

N = 200 kJ m^⁻² yr⁻¹