Transfer of Biomass (A-level Biology)

Transfer of Biomass

Net Production of Consumers (N)

We have looked at GPP and NPP. Now it is time to put everything together to explain energy transfer between trophic levels of a food chain:

1. Plants only take up some available energy from the sun. Plants take up energy from the sun (gross primary production), but not all available energy is used. Some light energy is reflected by plants rather than being absorbed, and some light energy reaches the trunk and bark of the plant which cannot absorb light.
2. Plants use energy for respiration and biomass. Plants use some of the absorbed energy for respiration, and then use the rest for biomass (net primary production).
3. Primary consumers do not take up all the energy from plants. Primary consumers eat the plant, so energy is passed on to the next trophic level. However, not all of the plant is eaten, so some goes to waste.

Net Production

Net production is the total chemical energy that consumers retain and store in their biomass after ingesting material.

It takes into account the typical losses of energy through various means, including:

• Partially consumed food – not all food can be digested by the consumer, for example bones and plant roots.
• Faeces – material will be egested as faeces when not all the energy is used.
• Urination.
• Respiration.

Typically about 90% of the total energy that is available is lost as it passes through a tropic level.

There is a formula to calculate net production (all units kJ m^-2 yr^-1):

Where:

• N is the net production of the consumer
• I is the chemical energy stored in ingested foods
• F is the chemical energy lost to the environment (methods listed above)
• R is the respiratory losses

Net production is also referred to as secondary production.

Calculating Transfers Between Trophic Levels

1. Energy varies between two trophic levels. There is always a difference in energy between two trophic levels. We can compare each level’s net productivity to calculate the difference.
2. Energy can be calculated through biomass. As we have already seen, we can calculate the energy contained in an organism through it’s biomass. This can be performed by direct or indirect measurement.
3. Biomass of a population must be found. We have now found the biomass of a single organism. To find the biomass of a population, we must multiply the biomass of the single organism by the total number of organisms of that species in the population.
4. Energy is transferred between trophic levels. The difference in biomass between two trophic levels is the amount of energy that has been transferred between them.
5. This method is not completely accurate. Despite taking into account the biomass transferred between two trophic levels, this method is inaccurate. For example, a predator is in the trophic level above it’s prey. The predator may not completely consume the prey, leaving behind bones or fur. This means that the energy between these two trophic levels has also not been completely transferred.

Efficiency of Biomass Transfers

We can calculate the efficiency of biomass transfers using the following formula.

The efficiency of the transfer is expressed as a percentage.