Estimating Biomass & Energy of Trophic Levels (DP IB Environmental Systems & Societies (ESS))

Estimating Biomass & Energy of Trophic Levels

• Estimating the biomass and energy of trophic levels in a community is an important step in understanding the structure and function of an ecosystem

• There are several methods for measuring biomass and energy, including:

  • Measurement of dry mass

  • Controlled combustion

  • Extrapolation from samples

Measurement of Dry Mass

• One common method for estimating biomass is to measure the dry mass of organisms

• This involves collecting samples of organisms and drying them in an oven to remove all water within the tissues

• The dry weight of the sample is then measured, and this can be used to estimate the biomass of the population

• For example:

  • If the dry mass of one daffodil plant is found to be 0.1 kg, then the dry mass (i.e. the biomass) of 200 daffodils would be 20 kg (0.1 x 200 = 20)

  • If the dry mass of the grass from 1 m² of a field is found to be 0.2 kg, we can say that the grass has a dry mass (i.e. biomass) of 0.2 kg m⁻² (this means 0.2 kg per square metre). If the grass field is 200 m² in size, then the biomass of the whole field must be 40 kg (0.2 x 200 = 40)

    

It is possible to estimate the biomass of organisms in a larger area if you know the dry mass of the organisms in a given (smaller) area

Controlled Combustion

• Another method for estimating biomass is controlled combustion

• This involves burning a known quantity of biomass and measuring the heat produced

• By knowing the heat value of the biomass, it is possible to estimate the total biomass of a population based on the amount of heat produced

• A piece of equipment known as a calorimeter is required for this process

  • The burning sample heats a known volume of water

  • The change in temperature of the water provides an estimate of the chemical energy the sample contains

    

A simple, inexpensive version of a calorimeter that can be set up using classroom equipment

An example of a more precise (and much more expensive) version of a calorimeter known as a bomb calorimeter - this type is used in professional scientific laboratories

Extrapolation from Samples

• A third method for estimating biomass is to take small samples of organisms and extrapolate to estimate the total biomass of a population

• This method can be particularly useful when dealing with large or difficult-to-sample populations

  

It is possible to estimate the biomass of a group of organisms if you know the dry mass of a single organism

• Data obtained from these methods can be used to construct ecological pyramids

  • Ecological pyramids (such as pyramids of biomass) are very useful in visually illustrating the relationships between different trophic levels in an ecosystem and how energy and biomass are transferred through the system

Limitations of Calorimetry

• It can take a long time to fully dehydrate (dry out) a plant sample to find its dry mass

  • This is partly because the sample has to be heated at a relatively low temperature to ensure it doesn’t burn

  • Depending on the size of the sample, the drying process could take several days

• Precise equipment is needed, which may not be available

  • A very precise digital balance should be used to measure the mass of the plant sample as it is drying (to detect even extremely small changes in mass)

  • It is preferable to use a very precise digital thermometer when measuring the temperature change of the water in the calorimeter (again, to detect even very small temperature changes)

• The more simple and basic the calorimeter, the less accurate the estimate will be for the chemical energy contained within the plant sample

  • This is due to heat energy from the burning sample being lost and not being transferred efficiently to the water

  • A bomb calorimeter ensures that almost all the heat energy from the burning sample is transferred to the water, giving a highly accurate estimate