5.1.7 Calculating the Efficiency of Biomass & Energy Transfers

Transfer of energy through a food chain

• During photosynthesis organisms such as plants convert light energy into chemical energy stored in biological molecules
  • Organisms that do this are known as producers 
• The chemical energy stored in plant biomass is passed to primary consumers when the plant is ingested
  • Primary consumers are animals that eat plant material; they can be herbivores or omnivores
• The primary consumer digests the plant tissues and uses the stored chemical energy either to fuel respiration or to build up biomass; this latter means that the stored chemical energy is transferred to the tissues of the primary consumer
• When the primary consumer is ingested by a secondary consumer the stored chemical energy passes to the secondary consumer, and so on up the food chain
  
  • When an organism dies, the chemical energy stored in its tissues passes to decomposers such as bacteria and fungi
• In a food chain the arrows represent the transfer of energy from one trophic level to the next by the process of feeding
  • The term trophic level refers to the stage in a food chain

The light energy converted into chemical energy by producers is passed up the food chain by the process of feeding

Energy losses in a food chain

• The transfer of energy in a food chain is not 100 % efficient; energy is lost to the environment at every trophic level
• A large proportion of the sun's energy is not available to producers for building biomass
  • Light passes through leaves or is reflected away
  • Light hits non-photosynthetic parts of the plant e.g. bark or flowers
  • Light is a mixture of wavelengths and only certain wavelengths are absorbed in photosynthesis
  • Plants release energy during respiration, some of which is lost to the environment in the form of heat 
• When a consumer ingests another organism, not all the chemical energy in the consumer's food is transferred to the consumer's biomass
  • Only around 10 % of the energy is available to the consumer to store in their tissues
  • This is because around 90 % of the energy is lost to the environment
• Around 90 % of the energy is lost to the environment because
  • Not every part of the food organism is eaten, e.g. the roots and woody parts of plants or the bones of animals, meaning that the stored energy in these uneaten tissues is lost to the environment
  • Consumers are not able to digest all of the food they ingest, e.g. cellulose in plants or the fur of animals, so some is egested as faeces; the chemical energy in this undigested food is also lost to the environment
  • Energy is lost to the environment in the form of heat when consumers respire
  • Energy is lost to the environment when organisms excrete the waste products of metabolism e.g. urea in urine
• The energy that is left after these losses is available to the consumer to fuel their life functions, including being stored in biomass during growth
• The rate at which energy is converted into biomass in the body of a consumer is known as net productivity
  • Note that this is slightly different to the rate at which energy is converted into biomass in producers, which is known as net primary productivity

Energy is lost to the environment at every trophic level of a food chain

Calculating the efficiency of energy transfer

• The efficiency of energy transfer in a food chain can be calculated using the following equation

Energy efficiency = (net productivity energy received)  100

• Note that net productivity can be calculated by subtracting energy losses from energy received
• Efficiency of energy transfer is given as a percentage 

Worked Example

Step 1: Calculate the net productivity of the toads

Toad energy received= 10 000 kJ m^-2 yr^-1 

Toad energy losses = 7 000 + 2 000 = 9 000 kJ m^-2 yr^-1 

Toad net productivity = 10 000 - 9 000 = 1 000

Toad net productivity = 1 000 kJ m^-2 yr^-1

Step 2: Substitute values into the equation

Energy efficiency = (net productivity  energy received) x 100

Energy efficiency = (1 000 10 000)  100

Step 3: Complete calculation

Energy efficiency = (1 000 10 000)  100 

Energy efficiency = 10 %

Calculating the efficiency of biomass transfer

• It is also possible to calculate the efficiency of biomass transfer from one trophic level to the next in a food chain
• In order to calculate this scientists need to know the biomass of the organisms concerned
  • Dry biomass is used, as the amount of water stored in tissues can vary
  • Dry biomass can be measured by drying a sample of the organism in an oven at a low heat and weighing the sample at regular intervals until the mass becomes constant; this will be the dry biomass
    • The organism needs to be dead for this process to be carried out, so an estimate is often used
  • Depending on the transfer being studied, it may then be necessary to multiply up the sample to take into account the size of the area or number of organisms being studied
• The efficiency of biomass transfer can be calculated using the following equation

Efficiency of biomass transfer = (biomass transferred biomass intake) 100

• Biomass transferred refers to biomass that has passed to the higher trophic level while biomass intake refers to biomass of the lower trophic level that has been consumed
• Efficiency of biomass transfer is given as a percentage 

Worked Example

Step 1: Ensure both units are the same

In this case, both are expressed in kg so the units do not need to be converted

Step 2: Substitute the values into the equation

Efficiency of biomass transfer = (biomass transferred biomass intake) 100

Efficiency of biomass transfer = (4.1 35)  100

Step 3: Complete calculation

Efficiency of biomass transfer = (4.1  35) 100

Efficiency of biomass transfer = 11.7 %

Exam Tip