Productivity & Maximum Sustainable Yield (SL IB Environmental Systems & Societies (ESS))

• During photosynthesis, primary producers (such as plants and algae) convert light energy to chemical energy stored within biological molecules
• Gross primary production can be defined as the amount of chemical energy stored in the carbohydrates within plants (during photosynthesis)
  • Roughly only 1% of the light falling on a plant is used in photosynthesis to produce glucose
  • 99% of the light either passes through the leaf without hitting chloroplasts, is reflected off of the leaf, or is transferred to heat energy
  • After that 1% is successfully absorbed and used to form glucose, the quantity of energy now stored in glucose is the gross primary production

Gross Primary Productivity

• The rate at which plants are able to store chemical energy via photosynthesis is referred to as gross primary productivity (GPP)
• Gross primary productivity can be expressed in units of energy per unit area per unit time, for example:
  • J m^-2 yr^-1 (joules per square metre per year)
  • kJ km^-2 yr^-1 (kilojoules per square kilometre per year)
  • In this case, ‘area’ refers to the area of land that is being studied (this land contains the primary producers that are producing the biomass - if there are no primary producers present in this area of land, there will be no gross primary production)

• Gross primary productivity can also be expressed in units of mass per unit area per unit time, for example
  • g m^-2 yr^-1 (grams per square metre per year)
  • kg km^-2 yr^-1 (kilograms per square kilometre per year)

• In aquatic environments, it may be more suitable to measure gross primary production per unit volume
  • For example, for aquatic algae, gross primary productivity could be given in kg m^-3 yr^-1 (kilograms per cubic metre per year) or kJ m^-3 yr^-1 (kilojoules per cubic metre per year)

Net Primary Productivity

• Net primary productivity (NPP) is the GPP minus plant respiratory losses (R)
  • Of the total energy stored in glucose during photosynthesis, 90 % will be released from glucose 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 (primary consumers)

• NPP can therefore be defined as the rate at which energy is stored in plant biomass, allowing for respiratory losses
  • 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

• NPP is expressed in units of biomass or energy per unit area or volume per unit time e.g.
  • Using area: g m^-2 yr^-1 (grams per square metre per year) or J m^–2 yr^-1 (joules per square metre per year)
  • Using volume: kg m^-3 yr^-1 (kilograms per cubic metre per year) or kJ m^-3 yr^-1 (kilojoules per cubic metre per year)
  • As with GPP, volume would be used when calculating NPP in aquatic habitats

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

• Gross secondary productivity (GSP) is the total energy/biomass assimilated by consumers and is calculated by subtracting the mass of faecal loss from the mass of food eaten
• Gross secondary productivity can be calculated using the equation:

GSP = food eaten - faecal loss

• Net secondary productivity (NSP) is calculated by subtracting respiratory losses (R) from GSP
• Net secondary productivity can be calculated using the equation:

NSP = GSP - R

• As with gross primary productivity and net primary productivity, GSP and NSP are expressed in units of biomass or energy per unit area or volume per unit time e.g.
  • Using area: g m^-2 yr^-1 (grams per square metre per year) or J m^–2 yr^-1 (joules per square metre per year)
  • Using volume: kg m^-3 yr^-1 (kilograms per cubic metre per year) or kJ m^-3 yr^-1 (kilojoules per cubic metre per year)
  • Volume would be used when calculating GSP or NSP in aquatic habitats

• The annual yield for a natural resource (such as a forest) is the annual gain in biomass or energy, through growth
• The maximum sustainable yield is the maximum amount of a renewable natural resource that can be harvested annually without compromising the long-term productivity of the resource (i.e. without a reduction in natural capital)
• It is the level of harvest that can be maintained indefinitely
• The concept of maximum sustainable yield applies to various resources, such as crops, fish, timber, and game animals
  • For example, in fisheries, the concept of maximum sustainable yield is used to determine the maximum amount of fish that can be harvested sustainably from a given population
  • This is calculated based on the population size, growth rate, and reproduction rate
  • If the fishing rate exceeds the maximum sustainable yield, the population may decline, and the long-term productivity of the fishery may be compromised
  • Similarly, in forestry, the concept of maximum sustainable yield is used to determine the maximum amount of timber that can be harvested sustainably from a forest
  • This is calculated based on the growth rate and regeneration capacity of the trees
  • If the harvesting rate exceeds the maximum sustainable yield, the forest may become depleted, and the long-term productivity of the forest may be compromised
• In this way, maximum sustainable yield is equivalent to the net primary productivity (NPP) or net secondary productivity (NSP) of a system (as these values represent the amount of energy stored and new plant or animal biomass per year)