Primary & Secondary Production (DP IB Biology)
Revision Note
Primary Production
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
Secondary Production
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
Examiner Tips and Tricks
The terminology here can be confusing, so be sure that you are happy with the following main ideas:
Primary production is the accumulation of biomass in autotrophs while secondary production is the accumulation of biomass in heterotrophs
The rate of production, i.e. how quickly or slowly production is taking place, is usually measured in g m-2 yr-1
Secondary production is lower than primary production due to energy losses at each trophic level
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