Lesson 3 of 9 / Organisms and their environment
Pyramids of numbers and biomass
Can one tree support a wider level of consumers?
I can construct and interpret pyramids while distinguishing counts from mass.
I can interpret pyramids of numbers and biomass.Syllabus
Syllabus K325, Topic 11(d). Interpret pyramids of numbers and biomass.
About 5 min
Make a guess. You are not marked.
One oak tree feeds 5,000 caterpillars. What shape is the pyramid of numbers?
Show the answer
Narrow at the bottom: one tree, many caterpillars.
A pyramid of numbers counts organisms. One large producer can support many small consumers.
The key idea
A numbers pyramid counts organisms; a biomass pyramid compares biological mass. They can have different shapes because individual organisms differ in size and turnover.
Each horizontal bar represents a trophic level, with producers at the bottom. Check what the width measures before interpreting the shape.
Explore the idea
One tree: count it or weigh it?
The tree is only one individual, so its bar is very narrow. The diagram counts organisms without accounting for size.
Original illustrative data for one defined area at one time. Widths are proportional within each diagram; the numerical scales differ between diagrams. This terrestrial example does not imply all biomass pyramids must be upright.
Explanation
A pyramid of numbers represents the number of organisms at each trophic level. A single tree can support many herbivorous insects. Its producer bar may therefore be narrower than the consumer bar even though the tree is much larger than each insect.
A pyramid of biomass compares the mass of biological material at each level, usually dry mass per unit area. Dry mass reduces the influence of differing water contents. In the tree example, the tree's large biomass can form a broad base despite being counted as only one organism.
Most familiar terrestrial biomass pyramids are upright, but some aquatic examples are inverted at a particular moment. Phytoplankton may have a small standing biomass because they grow and are consumed rapidly. Their production over time can support a larger consumer biomass measured at one instant.
Numbers and biomass are different from energy transfer per unit time. Do not conclude that an inverted numbers or snapshot biomass pyramid means consumers create energy. Read the units, sampling area and time before making a comparison.
To draw a pyramid, use one horizontal bar for each trophic level, with the producer at the bottom and each consumer level above it. Make each bar's width proportional to the number or the biomass, centre the bars on one vertical line, and label each bar with its trophic level or organism. To get biomass from counts, multiply the number of organisms by the mass of one organism. In an invented woodland example, 1 tree x 2000 kg = 2000 kg, 5000 caterpillars x 0.002 kg = 10 kg, 10 birds x 0.05 kg = 0.5 kg and 400 fleas x 0.000001 kg = 0.0004 kg.
Parasites change the shape of a pyramid of numbers. Many small parasites, such as fleas, can live on one bird, so the top bar of the numbers pyramid (400 fleas) is wider than the bar below it (10 birds). With the single tree as well, this pyramid is narrow at the base, wide in the middle, narrow, then wide again at the top. The pyramid of biomass for the same chain is still upright, because each flea has a tiny mass: 0.0004 kg of fleas against 0.5 kg of birds.
Step by step
- 1
Read the measure
Are the bars numbers of organisms or biomass, and what units are used?
- 2
Check the feeding order
Place the producer at the base and follow the specified chain upwards.
- 3
Explain the shape biologically
Use organism size for the tree example, or rapid producer turnover for an aquatic biomass snapshot.
Worked example
One tree, many insects
A tree supports 500 insects and 20 insect-eating birds. Explain why the numbers pyramid is not a simple upright triangle.
One way to explain it
The producer level contains only one individual, so its bar is narrow. The primary-consumer level contains 500 insects, so it is wider. Counting individuals ignores the much larger size of the tree; its biomass may still exceed the total insect biomass.
Why this answer works
- Use numbers rather than imagining equal-sized organisms.
- Explain the broad insect level.
- Do not infer biomass directly from counts.
Watch out for this
A student says: "Every ecological pyramid must get narrower at each level." What is wrong with this?
Show the answer
Pyramids of numbers can be inverted or irregular. Snapshot biomass can also be inverted when producers turn over rapidly. Identify what is measured.
Check your understanding
Why can a single tree have a narrow bar in a numbers pyramid but a broad bar in a biomass pyramid?
Show the answer
It is one organism but has a large mass compared with individual insects.
Counting individuals and measuring their biomass answer different questions.
Exam-style questions
Check your understanding
One oak tree supports 5 000 caterpillars, which feed 20 birds. Which describes the pyramid of numbers?
Show the answer
A narrow base, a wide middle and a narrow top
One tree is a small number, the caterpillars are many and the birds are few.
Quick recall
Can one tree support a wider level of consumers?