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Ecological Efficiency (HL) (DP IB Environmental Systems & Societies (ESS)) : Revision Note

Alistair Marjot

Written by: Alistair Marjot

Reviewed by: Bridgette Barrett

Updated on

Ecological Efficiency

  • The transfer of energy in a food chain is not 100 % efficient

    • Energy is lost to the environment at every trophic level

  • 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

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  • So much 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

      • This means all 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

      • This means some is egested as faeces

      • The chemical energy in this undigested food is then 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

Diagram showing the energy losses by organisms at particular trophic level
Energy losses by organisms at particular trophic level
  • 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

  • These energy loss limit the number of trophic levels in ecosystems

    • Eventually, the amount of energy remaining becomes insufficient to support further trophic levels

    • This is why most terrestrial ecosystems are unable to support more than five trophic levels

  • Ecological efficiency is:

The percentage of energy received by one trophic level that is passed on to the next level

  • The exact percentage varies between ecosystems, trophic levels and species

    • The percentage of energy transferred from one trophic level to the next is very variable

    • The commonly used value of 10% is not a fixed amount or a true average

Calculating ecological efficiency

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

    • The equation for calculating ecological efficiency is shown below:

Ecological efficiency = (energy used for new biomass ÷ energy supplied) × 100

Worked Example

A butterfly lays an egg on a blackberry bush. In its first day, the caterpillar that hatches consumes blackberries containing a total of 35 J of energy. 4.1 J of this energy are used to form new caterpillar biomass. Calculate the ecological efficiency of this step of the food chain.

Answer

Step 1: Ensure both units are the same

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

Step 2: Substitute the values into the equation

Ecological efficiency = (energy used for new biomass ÷ energy supplied) × 100

Ecological efficiency = (4.1 ÷ 35) × 100

Ecological efficiency = 11.7 %

Worked Example

A wheat farmer decides to use biological control against insect pests that are eating her wheat crop. The farmer introduces a species of toad. By eating the insect pests, the toads ingest 11 000 kJ m-2 yr-1 of energy. The toads lose 7 500 kJ m-2 yr-1 of this energy as heat from respiration and 2 800 kJ m-2 yr-1 of energy in faeces and urine. Calculate the ecological efficiency of energy transfer from the insects to the toads.

Answer

Step 1: Calculate the energy used for toad growth (new biomass)

Toad energy received = 11 000 kJ m-2 yr-1 

Toad energy losses = 7 500 + 2 800 = 10 300 kJ m-2 yr-1 

Energy for growth = 11 000 - 10 300 = 700 kJ m-2 yr-1

Step 2: Substitute the values into the equation

Ecological efficiency = (energy used for new biomass ÷ energy supplied) × 100

Ecological efficiency = (700 ÷ 11 000) × 100

Ecological efficiency = 6.4 %

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Alistair Marjot

Author: Alistair Marjot

Expertise: Biology & Environmental Systems and Societies Content Creator

Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.

Bridgette Barrett

Reviewer: Bridgette Barrett

Expertise: Geography Lead

After graduating with a degree in Geography, Bridgette completed a PGCE over 25 years ago. She later gained an MA Learning, Technology and Education from the University of Nottingham focussing on online learning. At a time when the study of geography has never been more important, Bridgette is passionate about creating content which supports students in achieving their potential in geography and builds their confidence.

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