Energy and ecosystems - AL only (3.5.3)
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Biomass is the total mass of living material in an area at a given time.
Biomass is usually measured as dry mass because the water content of organisms varies and would make comparisons unreliable. Determining dry mass requires organisms to be killed and dried, so only small samples are usually taken, which may not be representative of the whole ecosystem.
Biomass is commonly measured in (grams per square metre) for terrestrial ecosystems.
The chemical energy stored in dry biomass can be estimated using calorimetry:
- The heat released when the biomass is burned warms a known volume of water.
- The temperature increase of the water is measured. The temperature change allows the energy content of the biomass to be calculated.
- This provides an estimate of the chemical energy stored in the organism.
Biomass (the total mass of living material in an area) decreases at each trophic level of a food chain. As a result, less biomass is available to support organisms at higher trophic levels. This is why most food chains only have four or five trophic levels.
Biomass is lost between trophic levels through:
- Respiration: energy is released and lost as heat.
- Excretion: waste products are not passed to the next trophic level.
- Heat loss: energy is lost to the surroundings during metabolic processes.
- Uneaten or indigestible material (bones, roots, etc.).
Gross Primary Production (GPP) represents the energy stored in plant biomass in a given area or volume and includes all energy fixed before any is lost through plant respiration.
Net Primary Production (NPP) is the chemical energy stored in plant biomass after subtracting energy lost in respiration.
It represents the energy available for growth, reproduction, and for consumers in the food chain. It is calculated using the formula:
Where:
- = total energy fixed in photosynthesis.
- = energy lost through respiration.
Net production (N) is the amount of chemical energy in consumers that is available for growth and reproduction. It is calculated using the formula:
Where:
- = chemical energy in ingested food.
- = energy lost in faeces and urine.
- = respiratory losses to the environment.
Net production reflects how efficiently consumers convert food energy into new biomass.
In an ecosystem, atmospheric carbon dioxide () and aquatic carbon dioxide is taken up by plants and algae during photosynthesis.
Most of the sugars produced by photosynthesis are used as respiratory substrates to release energy.
The remaining sugars are used to synthesise organic compounds (e.g., cellulose, starch, proteins, lipids), contributing to the plant’s biomass.
Productivity is the rate at which biomass is produced.
- Primary productivity is the rate at which producers (plants / algae) generate biomass.
- Secondary productivity is the rate at which consumers generate biomass.
Productivity is measured as biomass per unit area per unit time, e.g.,
Quantification of productivity allows comparison of energy flow and efficiency in different ecosystems.
Farming aims to maximise production by improving the efficiency of energy transfer in food chains. This means that more energy is channelled into growth and biomass, which increases the yield for human consumption.
Certain methods in farming reduce energy losses to non-human food chains:
- Use of pesticides to prevent crops from being eaten by pests.
- Herbicides reduce competition from weeds.
- Fencing or housing livestock to protect them from predators.
- Fertilisers boost plant growth, increasing biomass available to humans.
Farmers attempt to reduce respiratory losses in human food chains with processes such as:
- Intensive rearing of livestock (e.g., keeping animals indoors, restricting movement) to reduce energy lost via respiration.
- Temperature–controlled housing minimises heat loss.
- High-energy, easily digestible feed enhances growth efficiency.