The Energy Pyramid

The Energy Pyramid

Every meal is a transfer of **biomass**: the mass of living material. From one organism to the next. But the transfer is astonishingly wasteful: only about a **tenth** makes it up each step. This module follows the food from producers to top predators, and nails the calculation examiners test every year. (All of 4.7.4 is **Biology-only**.)

Trophic levels

A **trophic level** is a feeding stage in a food chain. They are numbered from the bottom of the chain upwards.

Match the trophic level

Spot the level

In the food chain **grass → rabbit → fox → eagle**, which statement is correct?

• The fox is a secondary consumer, because it eats a primary consumer
• The fox is a primary consumer, because it is the first animal to hunt
• The grass is a primary consumer, because it sits at level 1 of the chain
• The eagle is a secondary consumer, because it is the second predator shown

Pyramids of biomass

A **pyramid of biomass** shows the **relative amount of biomass** at each trophic level, drawn as a stack of bars **to scale**. The producers form a **wide base**, and each level above is **narrower**, because there is **less biomass** at each step up. That shrinking shape is the whole story of this topic.

Build the pyramid

A pond is surveyed. Order these four organisms from the WIDEST bar at the base of a pyramid of biomass to the NARROWEST bar at the top.

• Algae, 12000 kg
• Water shrimp, 1200 kg
• Small fish, 130 kg
• Heron, 9 kg

The 10% rule

Only about **10%** of the biomass at one trophic level is transferred to the level above. An answer that says where the other 90% went is the one that scores.

Efficiency calculation

A trophic level contains 500 kg of biomass, and 50 kg is transferred to the level above. Calculate the percentage efficiency of biomass transfer. (efficiency = biomass transferred ÷ biomass available × 100.)

The whole cascade

Work the biomass down this food chain. At every step only 10% of the biomass reaches the level above.

Working backwards

A tertiary consumer gains 8 kg of biomass. Using the 10% rule at every step, how much producer biomass (in kg) was needed at the bottom of that food chain?

Where did it go?

A student writes: "Only 10% is transferred because the predator does not eat all of its prey." Why is that answer incomplete?

• Even the biomass that IS eaten is mostly lost to respiration, movement and waste
• The true figure is nearer 50%, so the percentage quoted is simply wrong
• Biomass actually increases up the chain, so nothing is really lost
• The loss is explained entirely by the water content of the prey

Why so few links?

Producers in a meadow hold 100000 kg of biomass. Using the 10% rule, why does that chain rarely reach a sixth trophic level?

• By the fifth level only about 10 kg is left, far too little to support another level
• There is not enough physical space in the meadow for any more animals
• Predators at the top of a chain refuse to eat one another
• A food chain is defined as having a maximum of five trophic levels

In the exam

Pyramid climbed. Grade-9 habits for trophic levels: • Know the levels: **producer → primary → secondary → tertiary consumer**, plus **decomposers**. • The calc is a guaranteed mark: **% efficiency = (biomass transferred ÷ biomass available) × 100**: always **show your working**. • Explain the ~10%: losses to **respiration/heat**, **movement** and **waste**, which is also why chains rarely exceed **4–5 levels**.

Explain the losses

Explain why only about 10% of the biomass at one trophic level is transferred to the next, and why that limits the number of trophic levels a food chain can have.

• Start with the biomass that is never eaten at all.
• Then the biomass that is eaten but not absorbed, and where that ends up.
• Name respiration and the heat it releases, and name movement.
• Name both waste routes using the correct terms: egestion and excretion.
• Finish with the consequence: after several 10% steps too little biomass is left to support another level.