Productivity & Biomass (DP IB Environmental Systems & Societies (ESS))

Productivity

• Gross productivity (GP) is the total gain in biomass by an organism or community in a given area or time period

  • It includes all the energy captured by organisms

  • E.g. by plants through photosynthesis or by consumers feeding on other organisms

    • For example, in a pond ecosystem, the total amount of energy captured by the aquatic plants and other species in the pond represents the gross productivity of that ecosystem

• Net productivity (NP) is the amount of energy or biomass remaining after losses due to cellular respiration

  • These energy losses are subtracted from the gross productivity

  • Net productivity reflects the energy available for growth and reproduction

    • For example, if a plant has captured 1 000 kJ of energy through photosynthesis (gross productivity) but has used 300 kJ for cellular respiration, its net productivity would be 700 kJ

• Losses due to cellular respiration are usually greater in consumers than in producers

  • This is due to the more energy-requiring activities of consumers

    • For example, herbivores need to spend energy on activities such as digestion and movement, resulting in higher respiratory losses compared to plants

Net productivity and sustainable yield

• The NP of any organism or trophic level represents the maximum sustainable yield that can be harvested without decreasing the availability of resources for the future

  • To maintain ecosystem stability and biodiversity, it is important to avoid harvesting beyond the sustainable yield of populations

    • For example, in fisheries management, the sustainable yield of fish populations is determined by considering the net productivity of the fishery

    • Harvesting beyond the sustainable yield can lead to overexploitation and depletion of fish stocks

    • This affects both the ecosystem itself and human livelihoods

Measuring Biomass

• 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 the biomass of a particular trophic, 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 from a particular trophic level and drying them in an oven to remove all water within the tissues

• The dry weight of the sample is then measured

• This can then be used to estimate the total biomass of the populations that have been sampled

  • Dry mass of samples is approximately equal to the mass of organic matter (biomass) since water represents the majority of inorganic matter in most organisms

• 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)

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 or trophic level, 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

    

Extrapolation from samples

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

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

  

• 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 biological 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 and can be very expensive

  • A very precise digital balance should be used to measure the mass of the sample as it is drying

    • This is 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

    • This is 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 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