Food Chains and Food Webs: A Complete PSLE Guide

Every living thing needs energy to survive. That energy originally comes from the Sun, and it passes from one organism to another through feeding relationships. Understanding how this works — through food chains and food webs — is one of the most important and frequently examined topics in Singapore Primary Science from P4 through PSLE.

This guide covers everything you need: how to read and draw food chains, how food webs work, what producers and consumers are, how energy flows and is lost at each level, and — crucially — how to answer the classic PSLE question about what happens when one species is removed from a food web.

What is a Food Chain?

A food chain shows the transfer of energy from one organism to the next through feeding. It always starts with a producer — a plant or other organism that can make its own food through photosynthesis — and progresses through a series of consumers.

The arrows in a food chain represent the direction of energy flow. The arrow points FROM the organism being eaten TO the organism that eats it. A common mistake is to draw the arrow pointing the wrong way — remember, the arrow follows the energy, not the eating direction.

Example: Grass → Grasshopper → Frog → Snake → Eagle

In this chain: grass is the producer; the grasshopper is the primary consumer (herbivore); the frog is the secondary consumer; the snake is the tertiary consumer; and the eagle is the apex predator (top consumer). Each arrow represents energy being transferred when one organism eats another.

Producers, Consumers and Decomposers

Every organism in an ecosystem plays one of three roles in the flow of energy and nutrients:

• Producers — organisms that make their own food using sunlight (photosynthesis). All producers are plants, algae, or photosynthetic bacteria. They form the base of every food chain. Examples: grass, trees, algae, phytoplankton.
• Primary consumers (herbivores) — animals that eat producers directly. Examples: grasshoppers eating grass, rabbits eating carrots, caterpillars eating leaves, deer eating shrubs.
• Secondary consumers — animals that eat primary consumers. They may be carnivores (meat-eaters) or omnivores (eat both). Examples: frogs eating grasshoppers, foxes eating rabbits.
• Tertiary consumers — animals that eat secondary consumers. Examples: snakes eating frogs, eagles eating snakes.
• Decomposers — organisms (mainly bacteria and fungi) that break down dead organisms and waste products, returning nutrients to the soil. Decomposers are not usually shown in food chain diagrams but play a critical role in nutrient cycling.

An important distinction for exams: herbivores only eat plants; carnivores only eat animals; omnivores eat both plants and animals.

What is a Food Web?

In reality, most animals eat more than one type of food, and most animals are eaten by more than one predator. A food web is a more accurate representation of feeding relationships in an ecosystem — it shows multiple food chains interconnected together.

Food webs are more realistic than food chains because they show that energy flows along many different pathways. If one pathway is disrupted (for example, a species goes extinct), the energy can potentially flow along other pathways, making the ecosystem more resilient.

In a food web diagram, you will see organisms connected by many arrows going in different directions. When answering exam questions about food webs, always trace the arrows carefully to understand who eats whom.

Energy Flow and Why There Are Fewer Predators Than Prey

Energy is lost at each step of a food chain. When a grasshopper eats grass, it does not absorb 100% of the energy stored in the grass. Most energy is lost as heat during the grasshopper's life processes — movement, growth, keeping warm, and excretion. Only about 10% of the energy at one level is transferred to the next level.

This explains why food chains are rarely longer than 4–5 levels: by the time you reach the top predator, there is very little energy left to support a large population. It also explains why there are always far more prey animals than predators in any ecosystem — it takes many grasshoppers to support one frog, many frogs to support one snake, and many snakes to support one eagle.

This pattern — large numbers of producers, smaller numbers of primary consumers, even smaller numbers of secondary consumers — is called a pyramid of numbers. It reflects the energy loss at each trophic (feeding) level.

What Happens When One Species is Removed?

This is the most important question type in PSLE ecology. Examiners will give you a food web and ask what happens to the population of one organism if another is removed (or its population increases). You must trace the effects through every link in the web — not just one step.

The key rules to follow:

• If a prey organism is removed → its predators will have less food → predator population decreases
• If a predator is removed → its prey will have less predation → prey population increases
• If a prey population increases → its predators will have more food → predator population increases
• Always trace effects through every connected link, not just the immediate one

Example: In the chain Grass → Grasshopper → Frog → Snake → Eagle, if all frogs are removed:

1. Grasshoppers have fewer predators → grasshopper population increases
2. More grasshoppers eat more grass → grass population decreases
3. Snakes have less food (no frogs) → snake population decreases
4. Eagles have less food (fewer snakes) → eagle population decreases

Biodiversity and Why It Matters

Biodiversity refers to the variety of living organisms in an ecosystem. An ecosystem with high biodiversity — many different species — is generally more stable and resilient than one with few species. This is because if one species is lost, other species can fill similar roles, and energy can flow along alternative pathways.

In Singapore, biodiversity is a particularly relevant topic. The Central Catchment Nature Reserve and Bukit Timah Nature Reserve are home to hundreds of species in a tiny area — making them globally significant biodiversity hotspots. Conservation efforts in Singapore focus on protecting these areas precisely because losing species destabilises entire food webs.

Threats to biodiversity include habitat destruction (deforestation, urbanisation), pollution, invasive species, and climate change. When key species (especially producers at the base of food chains) are lost, the cascading effects can be devastating for entire ecosystems.

Decomposers — The Unsung Heroes

Decomposers (bacteria and fungi) are rarely shown in food chain diagrams, but they are essential to all ecosystems. When organisms die, decomposers break down their bodies into simple nutrients — nitrogen, phosphorus, potassium — that are returned to the soil. Plants then absorb these nutrients through their roots to support new growth.

Without decomposers, dead organic matter would accumulate indefinitely, nutrients would be locked away in dead bodies, and plants would eventually run out of the minerals they need to grow. The entire food chain would collapse from the bottom up.