# Feeding relationships

> Biology · Edexcel IGCSE (4BI1)
> Source: https://www.owlsprep.com/study/edexcel-igcse-biology-s4-feeding-relationships/

This guide covers all Edexcel IGCSE Biology feeding relationships content (spec points 4.6–4.9), including trophic levels, food webs, ecological pyramids, and energy transfer rules with exam-focused examples.

**Prerequisites:** [Basic understanding of ecosystems, populations and communities](https://www.owlsprep.com/study/edexcel-igcse-biology-s4-ecosystem-basics/)

## Learning objectives

- Name and define all core trophic levels including producers, consumers and decomposers
- Interpret and compare food chains, food webs, and the three types of ecological pyramids
- Explain how both energy and substances are transferred along feeding relationships
- State the ~10% energy transfer rule and describe standard reasons for energy loss between trophic levels

## Trophic Levels in Feeding Relationships

**Trophic Level** — The position an organism occupies in a feeding sequence, determined by its source of nutrition.

1. **Producer**: Autotrophic (usually green plant) that makes food via photosynthesis, forming the base of all feeding sequences
2. **Primary consumer**: Herbivore that eats producers
3. **Secondary consumer**: Organism that eats primary consumers
4. **Tertiary consumer**: Apex predator that eats secondary consumers
5. **Decomposer**: Bacteria/fungi that break down dead organisms and waste to recycle nutrients

**Worked example:** Name the trophic level of each organism in the sequence: Grass → Caterpillar → Sparrow → Hawk

1. 1. Grass produces its own food via photosynthesis = Producer
2. 2. Caterpillar eats grass (herbivore) = Primary consumer
3. 3. Sparrow eats caterpillar = Secondary consumer
4. 4. Hawk eats sparrow = Tertiary consumer
5. 5. Decomposers break down all dead organisms and waste from every level in the sequence

> **Exam tip:** Decomposers are not usually drawn on standard food chains, but you should reference them explicitly if asked to name all trophic levels in an ecosystem.

*Calculator:* allowed

## Food Chains and Food Webs

**Food Chain** — A linear sequence showing the transfer of energy, biomass and nutrients between organisms, with arrows pointing from the organism being eaten to the organism that eats it.

A food web is a network of interconnected food chains that shows all the feeding relationships in an ecosystem. Most organisms have multiple food sources, so food webs give a more accurate representation of real ecosystems than simple linear chains.

> **warning**
>
> Arrows in food chains *always* point in the direction of energy flow, from prey to predator. Drawing arrows backwards is the most common mark-losing mistake on this topic.

**Worked example:** A student draws the sequence: Fox → Rabbit → Grass. Explain the mistake and correct the sequence.

1. 1. The arrows point backwards, implying energy flows from fox to rabbit to grass, which is incorrect.
2. 2. Grass is eaten by rabbits, which are eaten by foxes, so arrows should point from the eaten organism to the eater.
3. 3. Correct sequence: Grass → Rabbit → Fox

*Calculator:* allowed

## Ecological Pyramids

Ecological pyramids show the relative amount of a given parameter at each trophic level in a feeding sequence. There are three core types you need to distinguish for your exam:

| Pyramid Type | What it measures | Can it be inverted? |
| --- | --- | --- |
| Pyramid of number | Count of individual organisms at each level | Yes (e.g. 1 large tree supporting 1000+ insects) |
| Pyramid of biomass | Total dry mass of organisms per unit area at each level | Rarely, almost always upright |
| Pyramid of energy | Total energy available at each level per unit time | No, always upright |

**Worked example:** Explain why a pyramid of energy can never be inverted, even if a pyramid of number for the same ecosystem is inverted.

1. 1. Energy is lost at every trophic level: as heat from respiration, in waste, or used for movement.
2. 2. The total energy available at a higher trophic level is always less than the total energy at the level below it.
3. 3. A pyramid of number counts individuals, so one very large producer can support hundreds of smaller consumers, making it inverted, but this does not apply to total energy available.

> **Exam tip:** If asked to compare pyramids, always reference energy loss as the core reason for the always-upright shape of energy pyramids.

*Calculator:* allowed

## Energy Transfer and the 10% Rule

**10% Energy Transfer Rule** — Approximately 10% of the energy stored in biomass at one trophic level is passed on to the next level, with the remaining 90% lost from the food chain.

- Key reasons for energy loss between levels: energy used in respiration (released as heat to surroundings), energy used for movement, energy lost in undigested faeces (egestion), energy lost in urine (excretion), not all parts of the organism are eaten by consumers
- Only energy used to build new biomass is passed on to the next trophic level

**Worked example:** If 50,000 kJ of energy is stored in the producer level of a food chain, calculate how much energy is available to the secondary consumer level.

1. 1. 10% of producer energy is passed to primary consumers: $0.1 \times 50,000 = 5,000$ kJ
2. 2. 10% of primary consumer energy is passed to secondary consumers: $0.1 \times 5,000 = 500$ kJ
3. 3. Final answer: 500 kJ

> **Exam tip:** Exam questions asking for reasons for energy loss usually require 2–3 valid points for full marks, so learn at least 3 of the standard reasons listed above.

*Calculator:* allowed

## Common pitfalls

- **Wrong:** Drawing food chain arrows from predator to prey
  - Why it fails: Arrows represent energy flow from the eaten organism to the eater, not the direction of consumption action.
  - Correct: Always point arrows from the organism being consumed to the consumer (prey → predator).
- **Wrong:** Stating pyramids of biomass are always upright
  - Why it fails: Rare inverted cases exist, e.g. aquatic ecosystems where phytoplankton biomass fluctuates seasonally below zooplankton biomass.
  - Correct: State that pyramids of biomass are *usually* upright; only pyramids of energy are always upright.
- **Wrong:** Only referencing energy flow in feeding relationships, not substances
  - Why it fails: The spec explicitly requires you to understand transfer of both energy and substances (nutrients/carbon) along food chains.
  - Correct: Mention both energy flow and nutrient/substance transfer when describing feeding relationships.
- **Wrong:** Listing only 1 reason for energy loss between trophic levels
  - Why it fails: Exam questions usually require 2–3 valid reasons for full marks.
  - Correct: Memorize 3+ standard reasons: respiratory heat loss, egestion of undigested food, not all parts of the organism are eaten.
- **Wrong:** Including decomposers as a single level on energy pyramids
  - Why it fails: Decomposers break down waste from every trophic level, they do not occupy a single linear position in standard pyramids.
  - Correct: Only reference decomposers as separate to the linear producer-tertiary consumer sequence unless explicitly asked.

## Cheatsheet

| Concept | Key Exam Fact | Example |
| --- | --- | --- |
| Trophic level order | Producer → Primary consumer → Secondary consumer → Tertiary consumer | Grass → Grasshopper → Frog → Hawk |
| Food chain arrows | Point in direction of energy flow (eaten → eater) | Grass → Rabbit (energy flows from grass to rabbit) |
| Pyramid of number | Can be inverted | 1 oak tree → 1000 caterpillars |
| 10% energy rule | ~10% of energy passed between trophic levels | 10,000 kJ producers → 1000 kJ primary consumers → 100 kJ secondary consumers |
| Energy loss reasons | Respiratory heat, egestion, excretion, uneaten parts | Warm-blooded animals lose large amounts of energy as heat via respiration |

## What's next

Now that you have mastered feeding relationships, you are ready to move on to nutrient cycling, which covers how decomposers recycle carbon and nitrogen through ecosystems — a closely linked topic that is often tested alongside feeding relationships in extended response questions. You should also practice applying the 10% energy transfer rule to exam-style calculation questions, and learn to interpret complex food webs to identify trophic levels for different organisms. Understanding these concepts will also prepare you for later topics on human impacts on ecosystems, including overfishing and deforestation, which draw heavily on energy transfer and feeding relationship principles.

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