Lesson 2 of 9 / Organisms and their environment
Why less energy reaches the next level
Why can a food chain not recycle its energy?
I can explain one-way energy flow and calculate the efficiency of transfer between trophic levels.
I can explain the non-cyclical flow of energy.Syllabus
Syllabus K325, Topic 11(a). Explain the non-cyclical flow of energy.
I can explain losses and calculate trophic energy-transfer efficiency.Syllabus
Syllabus K325, Topic 11(c). Explain losses and calculate trophic energy-transfer efficiency.
About 5 min
Make a guess. You are not marked.
A cow eats 100 units of energy in grass. About how much of it becomes new cow body tissue?
Show the answer
Only a small fraction, a few units.
Most is lost in respiration as heat, or egested in faeces. Little is left to build new biomass.
The key idea
Energy enters mainly as sunlight and leaves as dispersed heat. Only part of the chemical energy at one trophic level becomes biomass at the next.
Biomass is the mass of living biological material. Chemical energy in new biomass is the portion potentially available to the next consumer.
Explore the idea
Account for 1,000 kJ
- 120 kJ: New consumer biomass
- 480 kJ: Respiration: heat dispersed
- 250 kJ: Uneaten material
- 150 kJ: Egested material
120 / 1,000 x 100% = 12%. Uneaten and egested material can supply decomposers. Respiration ultimately disperses energy as heat; it is not returned to producers as usable sunlight.
Original alternative budgets for the same area and time. Changing this control compares examples, not a biological prediction. No universal transfer percentage is assumed.
Explanation
Producers capture a fraction of incoming light energy in photosynthesis and store energy in organic molecules. When consumers feed, some of this chemical energy passes along the chain. Producers, consumers and decomposers all respire, ultimately transferring energy to their surroundings as heat.
Not every part of an organism is eaten. Some eaten material cannot be digested and is egested; some absorbed material is used in respiration rather than becoming new biomass. These routes mean that less energy is available as new biomass at each successive trophic level.
Uneaten and egested material can transfer chemical energy into a decomposer food chain. It has not all vanished instantly as heat. Nevertheless, respiration throughout the ecosystem ultimately disperses energy as heat, which cannot be recycled by producers into the same biological pathway.
Calculate transfer efficiency from comparable energy values, using the same area and time period. Divide the energy transferred into new biomass at the next level by the energy available at the previous level, then multiply by 100%. A fixed ten-percent rule is not a universal law; use the supplied data.
Less energy is available for production at each successive trophic level. This limits the length of food chains. It does not mean that counts or standing biomass must decrease at every level: organism size and the rate at which biomass is replaced also affect those measurements.
Step by step
- 1
Identify the two levels
Use the energy in the lower level as the denominator and the next level as the numerator.
- 2
Calculate a percentage
Divide next-level energy by previous-level energy, then multiply by 100%.
- 3
Explain the missing transfer
Distinguish respiration and heat from uneaten, undigested or egested material that may enter decomposer pathways.
Worked example
An original grassland energy budget
Over the same area and year, producers form biomass containing 20,000 kJ and herbivores form biomass containing 2,400 kJ. Calculate transfer efficiency.
One way to explain it
Efficiency = 2,400 / 20,000 x 100% = 12%. The remaining energy does not all become herbivore biomass: some producer material is not eaten, some food is not assimilated, and energy is used in respiration and dispersed as heat.
Why this answer works
- Use comparable area and time units.
- Keep the direction of division correct.
- Explain several routes, rather than assuming all lost transfer is one process.
Watch out for this
A student says: "Decomposers recycle energy back to plants." What is wrong with this?
Show the answer
Decomposers help recycle matter such as carbon and mineral nutrients. They also respire and transfer energy as heat. Plants require a continuing light energy input.
Check your understanding
A lower level contains 8,000 kJ and the next level contains 600 kJ of new biomass energy over the same period. What is the transfer efficiency?
Show the answer
7.5%
Correct: 600 / 8,000 x 100 = 7.5%. Use the given values rather than assuming every transfer is 10%.
Exam-style questions
Check your understanding
Producers in a food chain capture 20 000 kJ. About 10% is passed on at each step. How much energy reaches the secondary consumers?
Show the answer
200 kJ
20 000 x 0.1 = 2 000 kJ for primary consumers, and 2 000 x 0.1 = 200 kJ for secondary consumers.
Quick recall
Why can a food chain not recycle its energy?