6.5.3 Efficiency of Energy Transfers

• A very large proportion of the Sun's energy is not made available to photosynthetic plants, because
  • Light falls away from plants
  • Light passes through leaves or is reflected away
  • Light is a mixture of wavelengths, and only certain wavelengths stimulate photosynthesis
• During photosynthesis, primary producers such as plants and algae convert light energy to chemical energy in biological molecules
• The storing of this chemical energy as plant biomass makes a certain amount of energy available to the next trophic level, the primary consumers
• Only a small percentage of plant biomass becomes biomass in the primary consumer because:
  • Not all the plant's biomass is eaten by the primary consumer
  •  Not all the consumer's biomass intake is digested
    • Think about the energy content of cow dung, which can be dried and used as a heating/cooking fuel, as it contains a lot of undigested biomass e.g. cellulose
  • The primary consumer converts a lot of chemical energy to movement and heat, and only a small amount to new biomass in its own body
• The efficiency of biomass transfer from one trophic level to the next is low, typically around 10%

Calculating efficiency of biomass transfers between trophic levels

• Given the appropriate data, it is possible to calculate the efficiency of biomass transfer from one trophic level to the next, as a percentage

Efficiency of transfer = (biomass transferred  biomass intake)  100

• Where:
  • Biomass transferred = biomass that has passed to the higher trophic level
  • Biomass intake = biomass of the lower trophic level that has been consumed

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 formula

Efficiency of transfer = (biomass transferred  biomass intake)  100

(4.1  35) 100 % = 11.7 %

Calculating the efficiency of energy transfer between trophic levels

• A common way of working out the efficiency of energy transfer is calculating the net productivity of one trophic level as a percentage of the net productivity of the preceding trophic level

Net productivity of producers

• The net productivity of producers (also known as net primary productivity or NPP) can be calculated using the following equation:

NPP = GPP - R

• Where:
  • GPP = gross primary productivity
  • R = respiratory losses

Net productivity of consumers

• The net productivity of consumers can be calculated using the following equation:

N = I - (F + R)

• Where:
  • I = the chemical energy store in ingested food
  • F = the chemical energy lost to the environment in faeces and urine
  • R = the respiratory losses to the environment

Step 1: Calculate the net productivity of the toads

N = I - (F + R)

N = 10,000 - (2,000 + 7,000)

N = 10,000 - 9,000

N = 1,000 kJ m^-2 yr^-1

Step 2: Write out the equation for % efficiency and substitute in the known values

% Efficiency = (1,000 ÷ 10,000) × 100

Step 3: Calculate the efficiency

% Efficiency = (0.1) × 100

% Efficiency = 10%

How human activities can manipulate the transfer of biomass through ecosystems

• Human activity can adjust the efficiency of transfer of biomass between trophic levels, usually to maximise it in the context of maximising agricultural productivity
• For producers, arable farmers can adopt these methods
  • Providing artificial light in greenhouses on overcast days
  • Optimising planting distances between crops
  • Irrigation to maximise growth in dry weather
  • Use of fertilisers
  • Selective breeding for fast growth
  • Use of fungicides/pesticides
  • Fencing to exclude grazers
  • Ploughing and herbicides to kill weeds
  • Plant crops that store energy in edible form e.g. seeds, fruit, tubers

• Livestock farmers can adopt these methods for primary consumers (grazers)
  • Use of good quality feeds / food supplements
  • Use antibiotics and vaccines to reduce disease
  • Control predation with fencing or with indoor animal husbandry
  • Reduce competition for grazing e.g. rabbits, deer
  • Indoor husbandry to reduce energy loss from movement or from getting cold outside

Human activity can increase the efficiency of biomass transfer in an arable farm setting