A PYRAMID OF BIOMASS is a diagram that shows the relative TOTAL MASS OF LIVING MATERIAL (biomass) at each trophic level in a food chain.
HOW TO CONSTRUCT:
Each trophic level is represented by a horizontal BAR.
The WIDTH of each bar is proportional to the total biomass at that level.
Trophic level 1 (producers) is at the BOTTOM.
Higher trophic levels are stacked above, getting narrower.
BIOMASS decreases at each higher trophic level because:
Energy is lost at each transfer (respiration, movement, heat).
Less energy → less mass can be supported.
RESULT:
Pyramid of biomass is almost always a TRUE PYRAMID shape — wide at bottom, narrow at top.
UNITS: biomass is measured in g/m² or kg/m² — mass per unit area of ground.
Pyramids of Biomass vs Pyramids of Numbers
PYRAMID OF NUMBERS shows the NUMBER of organisms at each level.
Can be an unusual shape — not always a true pyramid.
EXAMPLE WHERE NUMBERS PYRAMID IS INVERTED:
One oak tree → hundreds of caterpillars → dozens of birds.
The oak tree is one organism (trophic level 1) but the caterpillars (level 2) are hundreds.
Numbers pyramid: narrow at bottom (1 tree) → wide in middle (many caterpillars).
BUT the BIOMASS pyramid for this food chain IS a proper pyramid:
The one oak tree has far more biomass than all the caterpillars combined.
So biomass pyramid: wide bottom (large oak biomass) → narrower second level.
BIOMASS PYRAMIDS are more reliable than number pyramids because they account for SIZE — one large organism may have more biomass than thousands of small ones.
BOTH pyramids are always drawn with producers at the BOTTOM.
Constructing a Pyramid of Biomass
DATA REQUIRED:
Total biomass of all organisms at each trophic level in a defined area.
Example: in 1 m² of grassland:
Level 1 (grass): 500 g/m²
Level 2 (grasshoppers): 50 g/m²
Level 3 (frogs): 5 g/m²
Level 4 (herons): 0.5 g/m²
DRAWING THE PYRAMID:
1. Choose a scale — e.g. 1 cm = 50 g/m².
2. Draw level 1 at the bottom as the widest bar (10 cm wide for 500 g).
3. Draw level 2 above it, proportionally narrower (1 cm wide for 50 g).
4. Continue upward, each bar centred on the bar below.
5. Label each level with the organism name and trophic level.
NOTE: It is the RELATIVE WIDTH that matters — bars should be drawn proportionally to the data.
Always show the scale used.
⚠️ Common Mistake
Pyramids of BIOMASS are almost always true pyramid shapes — the ONE exception can occur with parasites (many small parasites on fewer large hosts). Pyramids of NUMBERS can be inverted (e.g. one tree → many insects). Don't confuse the two. Trophic level 1 (producers) is ALWAYS at the bottom.
📌 Key Note
Pyramid of biomass: bar width ∝ total biomass at each level. Producers at bottom. Almost always true pyramid shape. Units: g/m² or kg/m². More reliable than number pyramids — accounts for organism size. Biomass decreases at higher levels due to energy lost in respiration.
🎯 Matching Activity — Pyramid of Biomass
Match each statement to the correct description. — drag the symbols on the right to match the component names on the left.
Width of bar
Drop here
Trophic level 1 (producers)
Drop here
Why biomass decreases upward
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Units of biomass
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g/m² or kg/m² — mass per unit area
Always at the BOTTOM of the pyramid — widest bar
Proportional to total biomass at that trophic level
Energy lost at each level through respiration — less energy = less biomass at higher levels
🔬 Triple Science Only
Pyramids of biomass (4.7.4.2) is biology-only. Students must be able to construct accurate pyramids of biomass from data and compare them with pyramids of numbers.
🎯 Test Yourself
Question 1 of 2
1. Why is a pyramid of biomass usually a more accurate representation of an ecosystem than a pyramid of numbers?
2. In 1 m² of grassland, grass biomass is 500 g, grasshoppers 50 g, frogs 5 g. Using a scale of 1 cm = 50 g/m², how wide should the grasshopper bar be?
⭐ How Well Do You Understand This Topic?
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