Primary & Secondary Production (HL IB Biology)

• During photosynthesis, autotrophs can convert light energy into chemical energy stored in biological molecules
  • Organisms that do this are known as producers 
• The accumulation of carbon compounds in the biomass of autotrophs is known as primary production
  • Biomass accumulates when organisms grow and reproduce
• Primary production occurs more quickly in some biomes than in others
  • Biomes that have more hours of sunlight, optimum temperatures and higher levels of rainfall will allow photosynthesis to occur at a higher rate, leading to faster primary production
• E.g. tropical forest biomes have high levels of sunlight, rainfall, and ideal temperatures, so primary production occurs very quickly

CC BY-SA 4.0, via Wikimedia Commons

Tropical forest biomes have very high rates of primary production due to the high rate at which photosynthesis can occur

Measuring the rate of primary production

• The rate at which producers convert light energy into chemical energy is expressed in units of biomass, per unit area (or volume), per unit time, e.g.
  • Using area: g m^–2 yr^-1 (biomass, per square metre, per year)
  • Using volume: g m^–3 yr^-1 (biomass, per cubic metre, per year)
    • Volume would be used when calculating rate of production in aquatic habitats

• Organisms that gain carbon compounds by ingesting other organisms are known as heterotrophs
• When heterotrophs ingest other organisms, the chemical energy in the biomass of these organisms is transferred to the heterotrophs and stored in their own biomass
• This process by which biomass is stored in the tissues of heterotrophs is known as secondary production
• However, not all the energy stored in the carbon compounds consumed by heterotrophs is transferred to new biomass
  • Carbon is lost in the form of carbon dioxide as a waste product of respiration
  • Energy is lost during the excretion of other metabolic waste, such as water and urea
• Due to the subtraction of respiratory losses, the rate of secondary production is always lower than the rate of primary production in an ecosystem
  • The rate of secondary production is therefore calculated by subtracting respiratory losses from the stored energy ingested by a heterotroph