Energy transfer in ecosystemsIB MYP Biology: Revision notes
Section 1
Food chains and food webs
A food chain shows what eats what, with arrows pointing in the direction energy flows. For example: grass → zebra → lion.
- Producer: makes its own food by photosynthesis (green plants and algae). It is the start of every chain.
- Primary consumer: eats the producer (a herbivore).
- Secondary consumer: eats the primary consumer (a carnivore).
- Tertiary consumer: eats the secondary consumer.
- Decomposer: bacteria and fungi that break down dead organisms and waste, returning nutrients to the soil.
Most organisms eat more than one thing, so many food chains join to make a food web. Each step is a trophic level.
The arrow means 'is eaten by' and shows the direction of energy flow. It points from the food to the feeder.
Section 2
Pyramids of number and biomass
A pyramid of numbers shows the number of organisms at each trophic level. Each bar is as wide as the number of organisms. It can have an odd shape: one oak tree supports thousands of caterpillars, so the bottom bar is very narrow.
A pyramid of biomass shows the total mass of living material (dry mass) at each trophic level. It fixes the problem of size, so it almost always narrows towards the top. A pyramid of biomass is usually more reliable, because a single big tree and a single small caterpillar are not counted as equal.
Section 3
Where does the energy go?
Nearly all the energy in an ecosystem comes from the Sun. Producers capture a small amount in photosynthesis. When an animal eats, only part of that energy becomes new body tissue. The rest is lost as:
- heat from respiration (released to the surroundings)
- movement (muscles use energy)
- waste such as faeces and urine
- parts that are not eaten, such as bones or roots
Because energy is lost at every step, less energy and less biomass is available at each higher trophic level. This is why food chains rarely have more than four or five links.
Section 4
Efficiency of energy transfer
The efficiency of a transfer tells you how much of the energy supplied becomes useful growth.
efficiency (%) = (energy transferred / energy supplied) × 100
Worked example: a calf is supplied with 5000 kJ of energy in its feed and gains new body tissue containing 500 kJ.
Efficiency = 500 / 5000 × 100 = 10%
The other 90% is lost as heat, movement and waste. Farmers use this idea: keeping animals warm and limiting their movement can raise the efficiency of turning feed into meat.
Always put the energy that ends up in the animal on top, and the energy supplied on the bottom, then multiply by 100.
Must know
- Producers, primary, secondary and tertiary consumers and decomposers.
- Pyramids of number and biomass: biomass pyramids are more reliable.
- Energy is lost as heat, movement and waste, so each level has less.
- Efficiency = energy transferred / energy supplied × 100.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Energy transfer in ecosystems
- On the Serengeti plains in East Africa, grass is eaten by zebra, and zebra are hunted by lions. When animals die, bacteria and fungi in the soil break down their remains.State two ways in which a zebra loses energy so that it is not available to the lion that eats the zebra.2 marks
- In an English oak woodland, one large oak tree supports about 2000 caterpillars on its leaves. The caterpillars are eaten by about 20 blue tits, and the blue tits are hunted by a single sparrowhawk.Explain why the biomass at each trophic level is less than the biomass at the level below it.2 marks
- A farmer wants to know which of two feeds turns more of its energy into chicken growth. Each chick eats either Feed X or Feed Y for a week. Each chick on Feed X is supplied with 8000 kJ of energy and gains new body tissue containing 1200 kJ. Each chick on Feed Y is supplied with 9000 kJ of energy and gains new body tissue containing 1530 kJ. The farmer used one group of chicks for each feed.Calculate the efficiency of energy transfer for Feed X and for Feed Y, and state which feed is more efficient.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).