GCSE · Biology · AQA · Spec 8461

Pyramids of biomass and biomass transfer (biology only)

A rabbit eats grass all day, so why isn't the field full of foxes? Follow its dinner and watch almost all of it leave the food chain.

Follow the grass

Where does a rabbit's dinner go?

A rabbit grazes a patch of grass. Pick each piece of the grass's biomass and put it where it ends up. Only one box builds more rabbit.

Still to sort

Never eaten (0)

Left behind, so it never reaches the rabbit

Where the line is: Uneaten biomass never gets inside the rabbit at all. Droppings are different: that grass was eaten, it just wasn't absorbed.

Egested as faeces (0)

Eaten, but never absorbed

Where the line is: Egested material only passed through the gut. It was never absorbed, so it never became part of the rabbit.

Lost in respiration (0)

Absorbed, then broken down

Where the line is: This was absorbed, but respiration breaks it down and it leaves as carbon dioxide and water.

Lost in urine (0)

Absorbed, then removed as waste

Where the line is: This was absorbed too, then removed as water and urea. That's what makes it excretion, not egestion.

Becomes rabbit (0)

New biomass the next level can eat

Where the line is: The only route that passes biomass up. A fox eating the rabbit can only get what ended up here.

9 of 9 still to sort.

The cards aren't weighed, so counting them isn't measuring mass. The pattern is the point: out of five destinations, only one leads up the food chain.

Watch out: Faeces are egested, not excreted. Egested material was never absorbed. Urea and water in urine were absorbed first, then lost.

Put a number on it

How much actually makes it up?

Imagine one trophic level holds 100 kg of biomass. Roughly how many kilograms of it end up as biomass in the level above?

Your estimate

50kg

Imagine one trophic level holds 100 kg of biomass. Roughly how many kilograms of it end up as biomass in the level above? Estimate between 0kg and 100kg.

0kg100kg

What the picture means

What does each bar show?

A student draws the pyramid of biomass for grass → rabbits → foxes. It has three stacked bars, one per trophic level.

Which is closest to what you think the width of each bar shows?

How many organisms there are at that level.The top bar should be widest, because foxes are bigger than rabbits.The bars just get narrower going up. Their exact widths don't matter.The biomass at each trophic level, drawn to scale, with trophic level 1 at the bottom.

How sure are you?

I'm sureI think soI'm guessing

How many organisms there are at that level.

This is a common misconception

That's the most common mix-up. This pyramid doesn't count organisms, it weighs them. Each bar shows the biomass, the total mass of living material, at that trophic level. One big organism and thousands of tiny ones could have the same biomass.

The top bar should be widest, because foxes are bigger than rabbits.

This is a common misconception

The size of one fox isn't what's drawn. The bar is the total biomass of the whole level. Only about 10 % of each level's biomass is passed up, so each level holds less in total than the one below, and the bars shrink as you go up.

The bars just get narrower going up. Their exact widths don't matter.

This is a common misconception

They matter a lot. A pyramid of biomass is a scaled diagram, so every bar is drawn to the same scale and its width stands for a real amount. If the widths were guessed, you couldn't compare the levels at all.

The biomass at each trophic level, drawn to scale, with trophic level 1 at the bottom.

This matches the evidence

Exactly. Trophic level 1, the producers, goes at the bottom. Each bar above is drawn to the same scale, so you can read off how much biomass each level holds and compare them directly.

To describe a pyramid of biomass, say three things: it shows the biomass at each trophic level, the bars are drawn to scale, and trophic level 1 is at the bottom.

Your turn to draw

Build a pyramid from real numbers

Invented figures for a field: grass 5,000 kg, rabbits 450 kg, foxes 40 kg. Construct an accurate pyramid of biomass.

Order the levels: grass is the producer, so it is trophic level 1 and goes at the bottom. Rabbits go next, then foxes on top.The order comes from who eats whom, not from the order the table lists them in.
missing step

Which line is step 2?

Choose one scale for every bar: 1 cm = 500 kg.Give each bar its own scale, so even the foxes' bar is easy to see.Make each bar a fixed step narrower than the one below.

Check the working

Spot the slip in this efficiency calculation

In a pond, the algae hold 2,500 g of biomass and the water fleas that eat them hold 200 g. Calculate the efficiency of biomass transfer from the algae to the water fleas.

A student's answer — which line goes wrong?

Lower level (algae) = 2,500 g. Higher level (water fleas) = 200 g.
Both are in grams, so no unit conversion is needed.
Efficiency = 2,500 ÷ 200 × 100
= 12.5 × 100
= 1,250 %

From loss to numbers

Reason it through

Why can higher trophic levels usually support fewer organisms?

Link 1 of 4

First link · your turn

Start at any level. How much of its biomass reaches the level above?

Locked — reveal the link above first

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Follow a meal up a food chain and watch almost all of it escape

What you need to know

A pyramid of biomass shows how much biomass is at each trophic level, drawn to scale, with trophic level 1 at the bottom.
Only about 10 % of the biomass in one trophic level is passed to the level above. Producers transfer about 1 % of the light energy that reaches them for photosynthesis.
Biomass is lost between levels through uneaten parts, egestion (faeces), respiration (carbon dioxide and water) and excretion (water and urea in urine).
Efficiency of biomass transfer = biomass in higher level ÷ biomass in lower level × 100, with both masses in the same unit.
Because so little biomass passes up, higher trophic levels can usually support fewer organisms.

The big picture

Only about 10 % of the biomass in each trophic level reaches the level above; the rest is lost through uneaten parts, egestion as faeces, respiration and urine. A pyramid of biomass shows this as scaled bars, with trophic level 1, the producers, at the bottom. Transfer efficiency = biomass in higher level ÷ biomass in lower level × 100. With so little biomass passed up, higher levels can usually support fewer organisms.

Key points

1Pyramids of biomass are scaled diagrams that show the relative amount of biomass at each trophic level of a food chain. Trophic level 1 is at the bottom.

2All food chains start with a producer, which makes the biomass that is transferred to consumers at higher trophic levels. Producers are mostly plants and algae.

3Producers transfer about 1 % of the incident energy from light for photosynthesis.

4Only approximately 10 % of the biomass from each trophic level is transferred to the level above it.

5Biomass is lost because not all of it is eaten, not all ingested material is absorbed (some is egested as faeces), and some absorbed material is lost as waste: carbon dioxide and water from respiration, and water and urea in urine. Large amounts of glucose are used in respiration.

6To construct a pyramid from data, put trophic level 1 at the bottom and draw every bar to the same scale.

7% efficiency of biomass transfer = (biomass in higher trophic level ÷ biomass in lower trophic level) × 100. It can also be given as a fraction of the mass.

8Less biomass at each higher level means those levels can usually support fewer organisms.

.we-_R_18slubsnpfiv5ubrb_ [data-we-step]{display:flex !important}.we-_R_18slubsnpfiv5ubrb_ [data-we-solution]{display:block !important}.we-_R_18slubsnpfiv5ubrb_ [data-we-reveal]{display:none !important}

Worked example

Problem

In a meadow, the grass holds 3.2 kg of biomass and the grasshoppers feeding on it hold 280 g. Calculate the efficiency of biomass transfer from the grass to the grasshoppers, as a percentage and as a fraction. (Invented figures.)

⚠ Watch out

Mixing up egestion and excretion. Faeces are egested: that material was eaten but never absorbed. Urea and water in urine are excreted: that material was absorbed first and then lost as waste. If you write "faeces are excreted", you've swapped them.

🧠

Memory hook

One way up, four ways out. The way up is becoming the next animal's body. The ways out are left uneaten, egested as faeces, breathed out from respiration, and lost in urine. Only about 10 kg in every 100 kg takes the way up.

✓

Check yourself

Cover the page. Name the four ways biomass is lost between trophic levels, and say which were absorbed first. Why is the top bar usually narrowest?

Flashcards

(15)

What does a pyramid of biomass show?

The relative amount of biomass at each trophic level of a food chain, drawn to scale.

Which trophic level goes at the bottom of a pyramid of biomass?

Trophic level 1, the producers.

What does every food chain start with?

A producer, which makes the biomass that is passed on to the consumers above it.

What are most producers?

Plants and algae.

Roughly what percentage of incident light energy do producers transfer for photosynthesis?

About 1 %.

Roughly what percentage of a trophic level's biomass passes to the level above?

About 10 %.

What is egestion?

Eaten material that was never absorbed leaving the body as faeces.

Which waste products of respiration carry biomass out of the food chain?

Carbon dioxide and water. Large amounts of glucose are used in respiration.

What is lost in urine?

Water and urea, from material that was absorbed first.

Why do uneaten parts count as a loss?

Biomass that is never eaten can never reach the next trophic level.

Give the equation for % efficiency of biomass transfer.

(Biomass in higher trophic level ÷ biomass in lower trophic level) × 100

What must you check about the two masses before calculating efficiency?

That they are in the same unit. Convert one if they aren't.

You work out an efficiency of over 100 %. What went wrong?

You divided the lower level by the higher level. Swap them: higher ÷ lower.

When drawing a pyramid of biomass, how do you choose the scale?

Use one scale for every bar, so each width stands for a real amount of biomass.

Why can higher trophic levels usually support fewer organisms?

Only about 10 % of the biomass passes up, so there is much less biomass available to support organisms at each higher level.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

Learning with Lightbulb is opening soon

You can use this lesson now. Join the waitlist and we'll let you know when the full Lightbulb experience is ready.

Keep me posted

Where does a rabbit's dinner go?

How much actually makes it up?

What does each bar show?

Build a pyramid from real numbers

Spot the slip in this efficiency calculation

More AQA GCSE Biology topics