Lesson 3 of 9 / Organisms and their environment
Pyramids of numbers and biomass
Can one tree support a wider level of consumers?
In this lesson: Construct and interpret pyramids while distinguishing counts from mass.
About 5 min
The key ideaA 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.
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.
Is this true? "Every ecological pyramid must get narrower at each level."
Pyramids of numbers can be inverted or irregular. Snapshot biomass can also be inverted when producers turn over rapidly. Identify what is measured.