Ecology: The 10% Rule, Food Webs, and Carbon Cycling

By Sarah Mitchell, B.Sc. Biology·Updated April 18, 2026
A biological food web diagram showing arrows pointing from producers to various levels of consumers.

Why is energy transfer between trophic levels so inefficient?

When a rabbit eats grass, only about 10% of that energy is permanently stored as new rabbit tissue for a fox to eat later. The other 90% is lost. It is lost as body heat from cellular respiration, lost through movement, passed out completely undigested in faeces, or locked in indigestible parts like bones and roots that the predators don't eat.

Table of Contents

Ecology questions in Paper 2 often feel like "common sense," which lures students into writing vague, unscientific answers that score zero. CAIE examiners are looking for specific ecological vocabulary. This guide from our Ultimate O-Level Biology Guide provides the keywords needed to secure the marks.

1. Interpreting Food Webs (The Arrow Trap)

A food web shows how independent food chains interlock in an ecosystem. You must know the terminology for each level (Trophic Level).

  • Producer: Always at the start. Usually a plant or algae. They produce their own organic nutrients using energy from the sun.
  • Primary Consumer: Herbivores that eat the producers.
  • Secondary Consumer: Carnivores that eat the primary consumers.
  • Tertiary Consumer: Apex predators sitting at the top.

The Arrow Rule:

Arrows in a food web DO NOT mean "eats". The arrow means "direction of energy flow". The arrow points FROM the grass TO the rabbit, because the grass's energy flows into the rabbit's body. Drawing this backward is an instant fail.

2. The 10% Energy Transfer Phenomenon

examiners love asking: "Why are there rarely more than 5 trophic levels in an ecosystem?"

The answer is inefficiency. As energy moves up the chain, roughly 90% is lost at every single step. If the grass starts with 10,000 kJ of solar energy, the rabbit only gets 1,000 kJ, the snake gets 100 kJ, and the eagle gets 10 kJ. By level 5, there is simply not enough energy left to sustain another animal.

Where does the 90% go?

  • Lost as heat to the environment during respiration.
  • Lost in excretory products (urine) and egested waste (faeces).
  • Energy trapped in parts of the animal that aren't eaten (fur, bones, hooves).

3. Mastering the Carbon Cycle

Carbon atoms are recycled infinitely. It is vital you know exactly which biological processes move carbon from one box to another in the cycle diagram.

  • Out of the atmosphere: There is only one process! Photosynthesis pulls CO2 out of the air and locks it into organic molecules like glucose in plants.
  • Movement through animals: Animals get carbon through Feeding on plants or other animals.
  • Back to the atmosphere:
    • Respiration: Both plants and animals release CO2 as they burn glucose for energy.
    • Decomposition & Respiration: Microorganisms (bacteria/fungi) break down dead bodies and faeces, releasing CO2 via their own respiration.
    • Combustion: Burning fossil fuels releases millions of years' worth of trapped carbon back into the air instantly, causing the greenhouse effect.
Sarah Mitchell📋 From the Desk of Sarah Mitchell
When labeling a Carbon Cycle diagram in the exam, pay close attention to the arrows pointing from Dead Animals to the Atmosphere. Students often label this "Decomposition". That's only half right. The chemical process actually releasing the CO2 is the Respiration occurring INSIDE the decomposers. "Respiration of Decomposers" is the bulletproof answer.

Frequently Asked Questions

Why do food chains rarely have more than 5 trophic levels?▼
Because 90% of energy is lost between each level. By the 5th level, there isn't enough energy left in the chain to support another predator.
What do the arrows in a food web represent?▼
They show the direction of energy transfer. The arrow points from the prey to the predator.
How does carbon enter the biological food chain?▼
Exclusively through Photosynthesis, where plants convert atmospheric CO2 into glucose.
What role do decomposers play in the carbon cycle?▼
They break down dead matter and, through their own cellular respiration, release the trapped carbon back into the atmosphere as CO2.

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