DSE Biology | Energy Flow in Ecosystems — Teacher Prep Guide | 1.5% capture, 10% transfer and the pyramid

2026-11-05

Tutor lesson-prep guides · Secondary Biology

Students can recite the pyramid of energy, then answer why top predators are few with they get eaten. This lesson puts the missing ninety per cent into three rows of numbers.

Objectives — Students will be able to: (1) calculate gross and net primary production from incident light and capture rate; (2) apply the ten per cent rule to each trophic level and say where the other 90 per cent goes; (3) explain predator numbers with the pyramid of energy and separate one-way energy flow from material cycling.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Photosynthesis	Which two raw materials does it need	Carbon dioxide and water
Respiration	In what form is energy released	Chemical energy, some lost as heat
Food chains	Producer versus consumer	Makes its own food versus eats

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Guess the number	How many eagles can one hectare of grassland support	Make an estimate
8-20 min	Capture and NPP	1,200,000 kJ in, 1.5% captured, minus respiration	Calculate it
20-34 min	Ten per cent rule	12,000.0 then 1200, 120, 12, 1.2	Fill the table
34-46 min	Where energy goes	Respiration, heat, excretion, not eaten	Sort four destinations
46-56 min	Pyramid and carbon	Energy one way, matter cycles, decomposers	Draw two loops
56-60 min	Exit check	Collect and mark three questions	Work alone

03 Teaching script (read it out)

Scene 1: From incident light to net production

About 1,200,000 kJ per square metre per year reaches the ground and producers capture 1.5 per cent, so gross primary production is 18,000 kJ. Subtract their own respiration of 6,000 kJ and net primary production is 12,000 kJ. That net figure is the capital for the whole chain.
The low capture rate is not laziness: pigments absorb only part of the spectrum, leaves reflect and transmit light, and water or carbon dioxide may limit the rate.

Ask students: put incident, captured, respired and net into four boxes and draw arrows only left to right.

Scene 2: The ten per cent rule needs the other ninety per cent

From net production, herbivores eat part and absorb part, building about a tenth of it into their bodies: 1200 kJ, then 120, 12 and 1.2. At each step 90 per cent goes to respiration and heat, is lost in excreted material, or is simply never eaten.
So one kilogram of top predator needs about 1,000 kilograms of producer. That number explains why long chains have so little at the top.

Ask students: why the same field feeds far more people when they eat plants instead of beef.

Scene 3: Pyramids of energy and of numbers are different objects

A pyramid of energy is drawn in kJ and is always upright, because every level loses 90 per cent; a pyramid of numbers can invert, since one tree supports many insects, and a pyramid of biomass can invert briefly when phytoplankton are grazed as fast as they grow.
Energy flows one way and leaves as heat; carbon, nitrogen and water cycle through photosynthesis, respiration, decomposition and combustion.

Ask students: write one sentence each using one-way and cycle to compare energy with carbon.

04 Question types and worked steps

Type	Example	Steps	Answer
Energy calculation	GPP and NPP from incident light	Multiply by capture, then subtract respiration	State units and the time basis
Transfer efficiency	Available energy at each level	Multiply by 0.1 and name the losses	Numbers alone earn little
Graph interpretation	Which pyramid can invert	Ask first: energy, numbers or biomass	Energy never inverts
Carbon cycle	Four processes and their directions	Photosynthesis, respiration, decomposition, combustion	Name the carbon form

Sanity check (teach this): Sanity checks: 1,200,000 times 1.5 per cent is 18,000; minus 6,000 leaves 12,000; three transfers later only 12 remains, one thousandth of the start; a pyramid of energy is upright while a pyramid of numbers may not be.

05 Independent practice (3 questions)

Question	Answer	What to watch
Capture calculation	Incident 1,200,000, capture 2 per cent, find GPP	Multiply, then attach units
Four levels	Fill three rows from net production	Name the loss at each row
Pyramid choice	A tree with insects, plankton with whales	Say which pyramid each is
Carbon diagram	Add four arrows and labels	Photosynthesis and respiration point opposite ways

06 Common mistakes and what to say

Mistake	Student writes	Say this
Using GPP as the base	Starting the chain at 18,000	Producers respire too; subtract 6,000 before you start.
Describing loss as eaten	Ninety per cent is eaten by the next level	Eaten becomes the next level. The rest is respiration, excretion and uneaten material.
Drawing an inverted energy pyramid	Top level wider than the base	Energy falls at each step, so an inverted drawing claims energy is created.
Calling energy cyclical	Energy cycles within the ecosystem	Energy flows one way and leaves as heat; carbon, nitrogen and water are what cycle.

07 Homework (paper, section and question numbers)

Secondary Biology chapter index — Every chapter in the subject
All prep guides — Parallel chapters in other subjects
Redo this chapter — Recalculate the four-level table with a named loss at each row

08 Exit check (last 5 minutes)

Question	Answer	Decision
GPP versus NPP	Gross production versus gross minus respiration	Answering they are equal means restart act one
Energy left at level three	12 kJ, one thousandth of the start	Answering thirty per cent means redo act two
Can energy cycle	No, it flows one way and leaves as heat	Answering yes means redo act three

09 Tutor FAQ

Is ten per cent a law?

It is a working approximation, with real values from five to twenty per cent. Use the numbers given, and ten per cent only when none are.

Why are decomposers needed?

They return inorganic nutrients from dead matter to producers; the energy itself is lost as heat along the way.

Can a pyramid have more than five levels?

Most terrestrial chains stop at four or five because too little energy remains to support a breeding population.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.