19.1.3Organisms and their Environment
Pyramids of Number and Biomass — Cambridge IGCSE Biology 0610 Revision Notes & Topical Questions
5 min read 6 parts Lesson 8 of 9 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 6Pyramids of Number
A pyramid of numbers shows how many organisms there are at each trophic level of a food chain.
What it shows
- What it shows The width of each bar represents the number of individual organisms counted at that trophic level.
Order
- Order Producers are at the base, then primary consumers, secondary consumers and tertiary consumers, in the same order as the food chain.
Unusual shapes
- Unusual shapes One large producer, such as an oak tree, can support thousands of small consumers, so the pyramid may be wider at the second level.
Size matters
- Size matters The pyramid ignores the size of the organisms, which is why numbers alone can give a misleading picture of the food chain.
Drawing rules
- Drawing rules Label each bar with its trophic level, keep the order fixed, and draw to scale if graph paper is provided.
Section 2 of 6Pyramids of Biomass
A pyramid of biomass shows the mass of living material at each trophic level.
Biomass
- Biomass This is the mass of living tissue, normally measured as dry mass because water content varies between organisms.
Shape
- Shape Biomass nearly always decreases at each level, so the pyramid is almost always a true pyramid shape.
Why it falls
- Why it falls Not all the biomass eaten is built into the tissues of the consumer, because some is used in respiration or lost as waste.
Advantage
- Advantage It takes account of the size of the organisms, so it gives a better picture of the living matter in an ecosystem.
Link to energy
- Link to energy Energy is stored in biomass, so biomass is an approximate guide to the energy available at each level.
| Feature | Pyramid of number | Pyramid of biomass |
|---|---|---|
| Measures | Count of organisms | Dry mass of living material |
| Shape | Can be any shape | Almost always pyramid-shaped |
| Organism size | Ignored | Taken into account |
| Data collection | Counting is quick but crude | Needs drying and weighing |
Section 3 of 6Drawing a Pyramid
Follow these steps whenever data are given for a food chain.
1
Write down the food chain and identify the trophic level of each organism, with the producer first.
2
Choose a scale so that the largest value fits on the page and the smallest bar is still visible.
3
Draw the bottom bar for the producers, centred on a vertical line, with its width in proportion to the data.
4
Draw the other bars above in the order of the food chain, each centred on the same line.
5
Label each bar with the name of the organism and its trophic level, and give the units on the scale.
Section 4 of 6Why Use Which?
Why Use Which?
Fair comparison
- Fair comparison Biomass allows comparison between organisms of very different sizes, such as a tree and an insect.
Limitations of biomass
- Limitations of biomass Harvesting and drying organisms kills them, so the method is destructive and usually uses only a small sample.
Limitations of numbers
- Limitations of numbers Counting is simple, but a single large organism and a single tiny organism are given the same width.
Section 5 of 6Worked Example
Question
- In a food chain, 1 oak tree supports 800 caterpillars, which are eaten by 20 birds.
- State the shape of the pyramid of numbers and explain it.
- Producers = 1, primary consumers = 800, secondary consumers = 20.
- The second level is wider than the first, and the third is narrower than the second.
- This is not a true pyramid because one very large tree provides food for many small caterpillars.
Answer The pyramid of numbers is narrow at the base, widest in the middle and narrow at the top, because the producer is a single large organism.
Section 6 of 6Exam Checklist & Tips
19.1.3 – Key Points for the Exam
- Trophic level The position of an organism in a food chain.
- Producer An organism that makes its own organic nutrients, usually by photosynthesis.
- Biomass The mass of living material, often measured as dry mass.
- Pyramid of numbers A diagram showing the number of organisms at each trophic level.
- Primary consumer An organism that feeds directly on producers.
Common Mistakes
- Changing the order of trophic levels to make the pyramid look like a pyramid.
- Using fresh mass instead of dry mass when measuring biomass.
- Drawing bars that are not in proportion to the data when told to use a scale.
- Assuming a pyramid of numbers always narrows towards the top.
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