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