Food Chains & Food Webs✓ Updated 2026
Master producers, consumers, decomposers, and energy flow. Learn how to read and draw food chains, analyse food webs, and nail every type of PSLE ecology question.
What Is a Food Chain?
Every living thing needs energy to survive — to move, grow, reproduce, and carry out life processes. That energy ultimately comes from the Sun. A food chain shows how this energy passes from one organism to the next through feeding.
Think of a food chain as a series of arrows showing who eats whom — and more importantly, which direction the energy flows. Each step in the chain is called a trophic level (trophic comes from the Greek word for "food" or "feeding").
In this chain:
- Grass traps energy from sunlight through photosynthesis
- Grasshopper gains energy by eating the grass
- Frog gains energy by eating the grasshopper
- Snake gains energy by eating the frog
- Eagle (apex predator) gains energy by eating the snake — nothing eats the eagle
Producers, Consumers & Decomposers
Every organism in an ecosystem plays one of three roles. Understanding these roles is the foundation of the entire topic.
Types of Consumers — Know Them All!
| Consumer Level | What They Eat | Diet Type | Example |
|---|---|---|---|
| Primary Consumer (1°) | Producers (plants) | Herbivore | Grasshopper eats grass |
| Secondary Consumer (2°) | Primary consumers | Carnivore or Omnivore | Frog eats grasshopper |
| Tertiary Consumer (3°) | Secondary consumers | Carnivore or Omnivore | Snake eats frog |
| Apex Predator | Other consumers; not eaten by anything | Top Carnivore | Eagle eats snake |
| Diet Name | Meaning | Examples |
|---|---|---|
| Herbivore | Eats only plants | Rabbit, cow, caterpillar, grasshopper |
| Carnivore | Eats only animals | Eagle, snake, lion, dragonfly |
| Omnivore | Eats both plants and animals | Human, rat, crow, monitor lizard |
| Scavenger | Eats dead animals (not killed by itself) | Vulture, hyena, some crabs |
Reading the Arrow — Don't Get It Backwards!
The arrow in a food chain has one specific meaning: it shows the direction of energy flow. The arrow points FROM the organism being eaten TO the organism that eats it.
A simple way to remember: "The arrow goes into the mouth of the eater." If the grasshopper eats the grass, energy flows from grass into the grasshopper — so the arrow points from grass to grasshopper.
Real Food Chains from Singapore
For PSLE, you should be comfortable with food chains from ecosystems that are common in Singapore: rainforests, mangroves, ponds, and gardens. Here are examples with full explanations.
🌳 Rainforest / Terrestrial Chain
💧 Pond / Freshwater Chain
🏞️ Singapore Pond Food Web — Full Diagram
This diagram shows a realistic Singapore pond food web — the kind of scenario that appears in PSLE Section B questions. Multiple organisms, multiple feeding relationships, multiple food chains within one web.
🌊 Mangrove / Coastal Chain
| Organism | Role in Chain | Why? |
|---|---|---|
| Algae / Grass / Leaves | Producer | Makes food using sunlight (photosynthesis) |
| Caterpillar / Grasshopper / Crab | Primary consumer (herbivore) | Eats plants directly |
| Sunbird / Frog / Small fish | Secondary consumer | Eats primary consumers |
| Hawk / Eagle / Monitor lizard | Apex predator | Eats other consumers; not eaten by anything in the chain |
Identify the Role of Each Organism in the Food Chain
A common PSLE exam question gives you a list of organisms and asks you to identify their role. Here is how to answer for the most common organisms:
| Organism | Role | Type | Why? |
|---|---|---|---|
| 🌿 Grass | Producer | Plant | Makes its own food through photosynthesis using sunlight |
| 🐸 Frog | Consumer (Secondary) | Carnivore | Eats insects like dragonflies or grasshoppers; does not eat plants |
| 🦅 Hawk | Consumer (Tertiary / Apex Predator) | Carnivore | Eats frogs or other animals; sits at the top of the food chain |
| 🐉 Dragonfly | Consumer (Primary) | Carnivore / Insectivore | Eats smaller insects; in a simple chain it may eat mosquito larvae (primary consumer) |
| 🍄 Mushroom | Decomposer | Fungi | Breaks down dead organisms into simple nutrients; NOT a producer or consumer |
Question: Identify the role of each organism in the food chain: Grass → Dragonfly → Frog → Hawk. What is the role of Mushroom in this ecosystem?
Answer:
• Grass — Producer. It makes its own food using sunlight through photosynthesis.
• Dragonfly — Primary Consumer. It obtains energy by feeding on the grass/smaller organisms (herbivore or omnivore depending on context).
• Frog — Secondary Consumer. It obtains energy by feeding on the dragonfly.
• Hawk — Tertiary Consumer / Apex Predator. It obtains energy by feeding on the frog and is not eaten by any other organism in this food chain.
• Mushroom — Decomposer. It breaks down the dead remains of organisms into simple substances that are returned to the soil.
2. Dragonfly is NOT always a primary consumer — adult dragonflies eat other insects, making them carnivores. Check what it eats in the given chain.
3. Hawk is the apex predator — nothing in the chain eats the hawk. Always check for this at the end of the chain.
What Is a Food Web?
In real ecosystems, animals rarely eat just one type of food, and most animals are eaten by more than one predator. A food web is a more realistic picture — it shows all the feeding relationships in an ecosystem, with many food chains interconnected together.
Notice how the mouse is eaten by both snakes AND owls. And the owl eats mice AND sparrows. This is why food webs are more accurate — animals have multiple food sources and multiple predators.
How to Read a Food Web in the Exam
- Find the producers first — they have no arrows pointing TO them (or are at the very base with no incoming arrows from other organisms).
- Identify the apex predators — they have arrows pointing TO them but no arrows pointing AWAY to another consumer.
- For any organism, the arrows pointing away from it tell you what eats it; the arrows pointing towards it tell you what it eats.
- Trace every food chain you can find — write them out if the question asks for it.
Why Are Predators Rarer Than Prey?
Energy is lost at every step of a food chain. When a grasshopper eats grass, the grasshopper does not absorb all of the energy stored in the grass. Most of the energy is used up or lost in several ways:
- Used for the grasshopper's movement and activity
- Lost as heat during the grasshopper's life processes (respiration, digestion)
- Released in waste products (excretion, faeces)
- Stored in parts the predator cannot digest (bones, shells)
This is why food chains are rarely longer than 4–5 steps. By the time you reach the 5th level, there is almost no energy left to support animals. It also explains why there are always many more prey animals than predators — it takes thousands of grasshoppers to support a small population of frogs, which in turn support an even smaller number of snakes.
The Pyramid of Numbers
Because energy is lost at each level, there must always be more organisms at the lower levels to support the organisms above them. When we show this visually, we get a pyramid of numbers — wide at the base (many producers), narrowing as we go up (fewer predators).
| Level | Organism | Approximate Number | Why? |
|---|---|---|---|
| Apex predator | Eagle | 1–2 | Needs enormous territory; very little energy available |
| 3° Consumer | Snake | 10–20 | Need many frogs to survive; scarce energy |
| 2° Consumer | Frog | 100–500 | Need large grasshopper population to feed on |
| 1° Consumer | Grasshopper | 5,000–10,000 | Need vast amount of grass; large population needed |
| Producer | Grass | Millions | Only 10% passes up; must have enormous base |
What Happens When a Species Is Removed?
This is the most important exam skill in this entire topic. Examiners love asking: "If species X is removed from the food web, what happens to the population of species Y?" You must trace the effect step by step through every connected link.
1. If a prey is removed → its predator has LESS food → predator population DECREASES
2. If a predator is removed → its prey has LESS predation → prey population INCREASES
Worked Example — Step by Step
Using the food web: Grass → Grasshopper → Frog → Snake → Eagle
Question: What happens if all the frogs are removed from this ecosystem? Explain the effect on each organism.
Step 2 — Grass: More grasshoppers eat more grass. Grass population decreases.
Step 3 — Snakes: Snakes eat frogs. With no frogs available, snakes have less food. Snake population decreases.
Step 4 — Eagles: Eagles eat snakes. Fewer snakes means less food for eagles. Eagle population decreases.
- Always start with the organism that was removed. Ask: who eats it? Who does it eat?
- Trace upwards (predators lose food → decrease) and downwards (prey lose predators → increase) at the same time.
- Continue tracing through every link until you reach organisms with no further connections.
- Use the exact words: "increases" or "decreases". Avoid vague words like "affected".
Second Worked Example — Predator Removed
Food web: Grass → Caterpillar → Sparrow → Hawk; Grass → Mouse → Hawk
Question: What happens if all hawks are removed?
Mice: Hawks eat mice. No hawks = less predation on mice. Mouse population increases.
Caterpillars: More sparrows eat more caterpillars. Caterpillar population decreases.
Grass: More mice eat more grass. Grass population decreases. (Also: fewer caterpillars would eat grass, but the mouse effect likely dominates.)
Note: In a food web with multiple pathways, different organisms may counteract each other's effects. Always state the most direct effect.
Biodiversity and Ecosystem Stability
Biodiversity refers to the variety of living organisms in an ecosystem — including all the different species of plants, animals, fungi, and bacteria.
A high-biodiversity ecosystem (many different species) is more stable than a low-biodiversity one. This is because if one species disappears, other species can fill similar roles and energy can flow along alternative pathways.
| Feature | High Biodiversity | Low Biodiversity |
|---|---|---|
| Number of species | Many different species | Few species |
| Food web complexity | Many interconnected pathways | Few pathways |
| Stability | High — losing one species has less impact | Low — losing one species can collapse the whole web |
| Example | Bukit Timah Nature Reserve, Singapore | A monoculture farm (only one crop) |
Threats to Biodiversity
- Habitat destruction — deforestation, land clearing for development
- Pollution — oil spills, chemical runoff into waterways
- Invasive species — non-native species outcompeting local ones
- Overhunting / overfishing — removing key species from the food web
- Climate change — changing conditions that species cannot adapt to quickly enough
Decomposers — The Unsung Heroes
Decomposers (mainly bacteria and fungi) are usually NOT shown in food chain diagrams — but they are absolutely essential to all ecosystems. Without them, life on Earth would grind to a halt.
What Would Happen Without Decomposers?
- Dead plants and animals would pile up everywhere and never disappear
- Nutrients (nitrogen, phosphorus, potassium) would be locked inside dead bodies
- Soil would become nutrient-poor; plants could not grow
- Without plants, the entire food chain would collapse
- Every ecosystem on Earth would eventually die
Common Mistakes Students Make Every Year
Everything You Need to Know — at a Glance
- All food chains begin with a producer — a plant or photosynthetic organism that makes its own food from sunlight
- The arrow in a food chain shows the direction of energy flow — from the food source to the eater
- Primary consumers eat producers (herbivores); secondary and higher consumers eat other animals (carnivores or omnivores)
- About 10% of energy transfers from one trophic level to the next — the rest is lost as heat
- This explains the pyramid of numbers: many producers, fewer primary consumers, even fewer secondary consumers, very few apex predators
- A food web is more realistic than a food chain — it shows all feeding relationships in an ecosystem
- If a prey species is removed → its predators decrease; if a predator is removed → its prey increases
- Always trace effects through every connected link, not just the immediate one
- Decomposers (bacteria and fungi) break down dead organisms and return nutrients to the soil — essential but not shown in food chain diagrams
- Biodiversity = variety of species; high biodiversity = more stable and resilient ecosystem
| Term | Definition | Example |
|---|---|---|
| Producer | Makes own food via photosynthesis | Grass, algae, phytoplankton |
| Consumer | Gets energy by eating other organisms | Grasshopper, frog, eagle |
| Herbivore | Eats only plants | Rabbit, caterpillar, cow |
| Carnivore | Eats only animals | Eagle, snake, dragonfly |
| Omnivore | Eats both plants and animals | Human, rat, crow |
| Decomposer | Breaks down dead matter into nutrients | Bacteria, fungi |
| Apex predator | Top of the food chain; not eaten by others | Eagle, tiger, shark |
| Food web | Multiple interconnected food chains in an ecosystem | All feeding relationships in a forest |
| Biodiversity | Variety of species in an ecosystem | Bukit Timah Nature Reserve |
| Trophic level | The feeding level of an organism in a food chain | Producers = level 1; primary consumers = level 2 |
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