Syllabus Edition
First teaching 2015
Last exams 2025
Energy Transfer & Transformation (DP IB Environmental Systems & Societies (ESS))
Revision Note
Written by: Alistair Marjot
Reviewed by: Bridgette Barrett
Transfer & Transformation of Energy
Pathways of energy through an ecosystem include:
Conversion of light energy to chemical energy
Transfer of chemical energy from one trophic level to another with varying efficiencies
Overall conversion of ultraviolet and visible light to heat energy by an ecosystem
Re-radiation of heat energy to the atmosphere
A food chain experiences a loss of chemical energy from one trophic level to the next
Ecological efficiency is the percentage of energy transferred from one trophic level to the next, and varies from 5% to 20%, with an average of 10%
Energy losses occur due to various reasons, such as movement, inedible parts (e.g. bone, teeth, fur), waste products (e.g. faeces), and the inefficient energy conversions that occur during the process of respiration
Ultimately, energy is lost as heat due to the second law of thermodynamics
An ecosystem converts light energy into heat and chemical energy
Energy is converted from one form to another but cannot be created or destroyed due to the first law of thermodynamics
The inputs of the system as a whole, and of any individual trophic level, are equal to the outputs
Energy is lost to the environment at every trophic level of a food chain
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
This value is known as the ecological efficiency
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 10 000 kJ m-2 yr-1 of energy. The toads lose 7 000 kJ m-2 yr-1 of this energy as heat from respiration and 2 000 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 = 10 000 kJ m-2 yr-1
Toad energy losses = 7 000 + 2 000 = 9 000 kJ m-2 yr-1
Energy for growth = 10 000 - 9 000 = 1 000 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 = (1 000 ÷ 10 000) × 100
Ecological efficiency = 10 %
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