Calculating Biomass Transfer Efficiency Revision Notes | GCSE AQA Higher Biology

1. Why Only 10% Makes It Through

Picture a field of grass feeding rabbits, which feed foxes. You might expect thousands of grass plants to support hundreds of rabbits and dozens of foxes. But the numbers drop off much more dramatically than that. Why?

The answer lies in energy transfer efficiency. When a rabbit eats grass, it doesn't magically transform 100% of that plant material into rabbit biomass. Most of it gets lost along the way.

Trap: Many students think 'efficiency' means how good an animal is at hunting. Actually, it's about how much of the food eaten becomes new body mass. Even the best predators only convert about 10% of their prey's biomass into their own.

2. Where Does the Other 90% Go?

When a rabbit munches grass, here's what happens to that plant material:

Lost as heat (biggest chunk): The rabbit's metabolism generates heat to keep it warm. This energy can't be passed to the next level.

Lost in waste: Not all plant material gets digested. Rabbit droppings contain undigested cellulose that the rabbit couldn't break down.

Used for movement and life processes: Running from predators, breathing, and maintaining body functions all use energy that doesn't become biomass.

3. The Calculation Toolkit

You can also express this as a fraction instead of a percentage. An efficiency of 10% is the same as 0.1 or 1/10.

Worked example: Calculating Transfer Efficiency

In a pond ecosystem, 5000g of algae supports 400g of water fleas, which support 30g of small fish. Calculate the efficiency of transfer from algae to water fleas. [3 marks]

1. $$\text{Efficiency} = \frac{\text{Biomass at higher level}}{\text{Biomass at lower level}} \times 100\%$$ — [M1] for correct formula
2. Higher level (water fleas) = 400g Lower level (algae) = 5000g — Water fleas eat algae, so they're the higher trophic level
3. $$\text{Efficiency} = \frac{400}{5000} \times 100\% = 0.08 \times 100\% = 8\%$$ — [M1] for substitution [A1] for correct answer

Answer: 8%

4. Why This Limits Food Chain Length

Here's where the maths gets dramatic. If only 10% transfers at each level, let's see what happens in a food chain:

Worked example: Biomass Through a Food Chain

Starting with 10,000g of grass, calculate the biomass at each trophic level assuming 10% efficiency.

1. Grass = 10,000g — This is our starting point - the primary producers
2. Rabbits = 10,000g × 0.1 = 1,000g — 10% of grass biomass becomes rabbit biomass
3. Foxes = 1,000g × 0.1 = 100g — 10% of rabbit biomass becomes fox biomass
4. Eagles = 100g × 0.1 = 10g — Only 10g left - not enough to support many large predators

Answer: Grass: 10,000g → Rabbits: 1,000g → Foxes: 100g → Eagles: 10g

Notice how quickly the biomass drops off. By level 4, there's barely enough energy to support a single large predator. This is why food chains rarely exceed 4-5 trophic levels.

5. Exam Technique - Getting Full Marks

Efficiency calculations are mark-heavy questions. Here's how to secure every available mark:

Exam depth extension: In calculating biomass transfer efficiency, connect why only 10% makes it through to where does the other 90% go? using clear cause-and-effect statements. When you explain biological processes, include what changes, why it changes, and the consequence for cells, organisms, or populations. If calculations appear, show each step, include units, and end with a sentence that interprets the result in biological context.

Common mistake: Students often calculate what's lost (90%) instead of what transfers (10%). Always check your answer makes sense - efficiency should be a small percentage, not a large one.

Understanding biomass transfer efficiency explains why there are millions of grass plants, thousands of herbivores, but only a few top predators in any ecosystem. The maths is unforgiving - energy losses at each level create a pyramid that gets narrower and narrower until there's not enough energy left to support more levels.