By the end of this lesson
- Construct and interpret food chains and food webs.
- Draw and interpret pyramids of number, biomass and energy transfer.
- Explain why some pyramids are not pyramid-shaped.
BIG QUESTION
Why can a pyramid of numbers be upside down when a pyramid of energy never is?
One large tree can feed thousands of insects, but the energy passed on is always less than the energy received.
Chains and webs
A food chainA sequence showing how energy is transferred from one organism to the next by feeding, starting with a producer. shows one sequence of feeding, with arrows showing the direction of energy transfer from the food to the feeder.
A food webA network of linked food chains showing all the feeding relationships in an ecosystem. shows many interlinked food chains. It is more realistic because most consumers eat more than one type of food. A change in one population can then affect several others.
Pyramids of number and biomass
A pyramid of numbersA diagram whose bars show the number of organisms at each trophic level. shows the number of organisms at each trophic level. It can be inverted, for example one oak tree supporting many caterpillars, or have a narrow base when parasites are included.
A pyramid of biomassA diagram whose bars show the dry mass of living material at each trophic level. shows the dry mass of living material at each level, usually per unit area. It is usually pyramid-shaped, but may be inverted at one moment in some aquatic systems, where small producers reproduce rapidly and are eaten quickly.
Pyramids of energy transfer
A pyramid of energy transferA diagram whose bars show the energy transferred through each trophic level per unit area per year; it is always pyramid-shaped. shows the energy passed to each trophic level over a period of time, for example kJ m⁻² year⁻¹. It is always pyramid-shaped because energy is lost at every transfer.
Pyramids of energy are the most informative but the hardest to measure, because they need data collected over time.
Three ecological pyramids
Each pyramid measures something different.
| Pyramid | What is measured | Shape and limitation |
|---|---|---|
| Numbers | Number of organisms at each level. | Can be inverted; ignores organism size. |
| Biomass | Dry mass of organisms at each level (per area). | Usually upright; a snapshot that may be inverted in some aquatic systems. |
| Energy transfer | Energy transferred to each level over time. | Always upright; data are difficult to collect. |
Language that earns precision
- food chain
- A sequence showing which organism is eaten by which, starting with a producer.
- food web
- A network of interlinked food chains.
- biomass
- The mass of living material, usually measured as dry mass.
- pyramid of energy transfer
- A diagram showing the energy transferred to each trophic level over a set time.
Common misconceptions
Claim: Arrows in a food chain mean “eats”.
Correction: Arrows point from the food to the feeder and show the direction of energy transfer.
Claim: All ecological pyramids are pyramid-shaped.
Correction: Pyramids of number can be inverted; only pyramids of energy transfer are always upright.
Claim: Biomass is the number of organisms.
Correction: Biomass is the mass of living material, not the count.
Match the answer to the command word
Draw pyramid bars to scale where data are given, producer at the base, with each level labelled.
Refer to size of organisms (numbers) or energy loss at each transfer (energy).
Recall, apply, then justify
Explain why a pyramid of numbers for oak tree → caterpillars → blue tits may not be pyramid-shaped.
Hint
Size of the producer.
Suggested marking points
- There is only one (large) oak tree.
- It supports many (smaller) caterpillars, so the producer bar is narrower than the primary consumer bar.
Give one advantage and one disadvantage of a pyramid of energy transfer compared with a pyramid of numbers.
Hint
Shape reliability versus data collection.
Suggested marking points
- Advantage: always upright / accounts for size and energy actually transferred.
- Disadvantage: difficult or slow to measure because data are needed over time.
In a food web, a disease kills most rabbits. Explain the possible effects on foxes and on grass, using conditional language.
Planning hint
Follow the web in both directions and consider alternative food.
What a strong design includes
- Foxes have less food, so the fox population may decrease.
- Foxes may eat more of other prey, such as voles or birds.
- Those other prey populations may then decrease.
- With fewer rabbits grazing, grass may grow more or increase in biomass.
- Other herbivores may increase with more grass.
- The size of each effect depends on how many alternative food sources exist.