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: • **Level 1 — Producers**: plants and algae that make biomass by **photosynthesis**. • **Level 2 — Primary consumers**: herbivores that eat producers. • **Level 3 — Secondary consumers**: carnivores that eat primary consumers. • **Level 4 — Tertiary consumers**: carnivores that eat secondary consumers; an **apex predator** has none of its own. • **Decomposers** break down dead organisms and waste, recycling nutrients.

Match the trophic level

Spot the level

In the food chain **grass → rabbit → fox → eagle**, what is the fox?

• A secondary consumer
• A primary consumer
• A producer
• A tertiary consumer

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.

Reading the pyramid

A pyramid of biomass has a very wide bar at the bottom and much narrower bars above it, getting smaller each level up. What does the wide bottom bar represent?

• The producers, which have the most biomass
• The apex predator
• The decomposers
• The level with the least biomass

The 10% rule 🔥

Only about **10%** of the biomass at one trophic level is transferred to the level above. Where does the rest go? • Not all of it is eaten, and not all that is eaten is absorbed. • Energy is **lost as heat** through **respiration**, and used for **movement** and life processes. • Biomass is lost in **waste** — **egestion** (undigested food, as faeces) and **excretion** (e.g. urea).

Efficiency calculation

An interactive activity.

How much gets through?

An interactive activity.

Where did it go?

Why is only about 10% of biomass transferred from one trophic level to the next?

• Most is lost as heat from respiration, used for movement, or lost in waste
• Because predators eat 100% of their prey
• Because biomass grows as it moves up the chain
• Because 90% of an organism is water

Why so few links?

Food chains rarely have more than 4 or 5 trophic levels. Why?

• So little biomass is left at each level that it cannot support another one
• There is not enough physical space
• Predators refuse to eat each other
• The animals run out of time to eat

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**.