3.8.5Additional lessons
Pyramids of Energy: Skills — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 95 of 142
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.
Part 1 of 5Features of a Pyramid of Energy
A pyramid of energy shows how much chemical energy is stored in the biomass at each trophic level over a year.
Bars by trophic level
- Bars by trophic level Each horizontal bar represents one trophic level, with producers at the base and the top consumer at the apex.
Bar length
- Bar length The length of each bar is proportional to the energy stored in the biomass of that level, so it should be drawn to scale.
Units
- Units Energy is given per unit area per year, written kJ m⁻² yr⁻¹, so that different ecosystems can be compared fairly.
Stepped shape
- Stepped shape Energy drops sharply at each level so the shape is a series of steps, and it is always widest at the base.
Labels
- Labels Each bar is labelled with its trophic level, such as producer or secondary consumer, and normally with the energy value.
Part 2 of 5Why the Pyramid Narrows
1
Only part of the producers' energy is eaten, because roots, bark and uneaten parts stay behind.
2
Part of what is eaten cannot be digested and is egested, so its energy does not enter the consumer's tissues.
3
Respiration transfers a large amount of energy to the environment as heat at every level.
4
Excretion of urea and other waste carries away some further chemical energy.
5
Roughly 10% of the energy at one level is passed to the next, giving a pyramid that always gets narrower.
Part 3 of 5Reading and Drawing Pyramids
Reading and Drawing Pyramids
Scale
- Scale Choose a scale so the largest bar fits the page, then draw every other bar using the same scale.
Symmetry
- Symmetry Centre each bar on the same vertical line so the pyramid looks balanced and the steps are easy to compare.
Comparison
- Comparison Dividing one level by the one below and multiplying by 100 gives the percentage transfer between those levels.
Contrast with biomass
- Contrast with biomass A pyramid of biomass shows mass of living material and can be inverted for some aquatic ecosystems,
- but energy pyramids never are.
Part 4 of 5Worked Example
Question
- A grassland produces 20 000 kJ m⁻² yr⁻¹ in its producers, 1 800 in primary consumers, 150 in secondary consumers and 12 in tertiary consumers.
- Calculate the percentage of energy transferred from primary to secondary consumers and state which transfer is least efficient.
- Transfer from primary to secondary = 150 ÷ 1 800 × 100.
- 150 ÷ 1 800 = 0.0833, so the percentage is 8.3%.
- Producers to primary1 800 ÷ 20 000 × 100 = 9.0%.
- Secondary to tertiary12 ÷ 150 × 100 = 8.0%.
Answer
- The primary to secondary transfer is 8.3% (to 2 s.f.);
- the secondary to tertiary transfer, at 8.0%, is the least efficient.
Part 5 of 5Exam Checklist & Tips
3.8.5 – Key Points for the Exam
- Pyramid of energy A diagram showing the energy stored in the biomass of each trophic level per unit area per year.
- kJ m⁻² yr⁻¹ The unit of energy flow per square metre of habitat each year.
- Energy transfer efficiency Energy at one level divided by energy at the level below, multiplied by 100.
- Biomass The total mass of organic matter in organisms, which stores chemical energy.
Common Mistakes
- Writing units as kJ alone; the area and time must be included.
- Drawing bars that are not to scale or are ordered with the producer at the top.
- Describing a 10% rule as exact; it is an approximate average that varies between ecosystems.
- Mixing up pyramids of energy with pyramids of biomass or numbers.
Last updated:
\n