Ecosystems and the Environment: Complete PSLE Science Guide

An ecosystem is a community of living organisms interacting with each other and with their non-living environment. Understanding ecosystems is one of the most important topics in Singapore Primary Science because it connects multiple concepts — food chains, adaptation, interdependence, and environmental responsibility. Questions about ecosystems appear consistently in PSLE and require both factual knowledge and the ability to reason about cause and effect.

What Is a Habitat?

A habitat is the natural environment where an organism lives. It provides everything the organism needs to survive: food, water, shelter, and space to reproduce. Different organisms are adapted to live in different habitats — a fish is adapted for an aquatic habitat, while a cactus is adapted for a desert habitat.

In Singapore, common habitats include rainforests, mangroves, coral reefs, freshwater ponds, and urban green spaces. Each habitat supports a unique community of organisms.

Producers, Consumers, and Decomposers

Every organism in an ecosystem plays a role in how energy and nutrients flow through the system.

Food Chains and Food Webs

A food chain shows the feeding relationships between organisms in a sequence, with arrows indicating the direction of energy flow. For example: Grass → Grasshopper → Frog → Snake → Eagle.

Arrows in a food chain mean "is eaten by" or "energy flows to." Always draw arrows pointing from the food to the feeder.

A food web is a more realistic picture — it shows how multiple food chains overlap and connect. Most organisms eat more than one type of food and are eaten by more than one predator, creating a complex network of feeding relationships.

Population Changes in Ecosystems

One of the most frequently tested PSLE skills is predicting what happens to populations when one organism in a food web changes. The key principle is: organisms in a food web are interdependent — a change in one population affects others.

How to Answer Population Change Questions

1. Identify which organism has changed (increased or decreased)
2. Trace the effect upward to predators and downward to prey
3. If a prey decreases → predator has less food → predator population decreases
4. If a prey increases → predator has more food → predator population increases
5. If a predator decreases → prey has fewer predators → prey population increases

Example: In a food web with Grass → Rabbit → Fox → Eagle, if the rabbit population decreases due to disease:

• Foxes have less food → fox population decreases
• Eagles have less food (since they eat foxes) → eagle population decreases
• Grass is eaten by fewer rabbits → grass population increases

Human Impact on the Environment

PSLE Science regularly asks students to evaluate how human activities affect ecosystems. Key topics include:

Deforestation

Clearing forests for farming, housing, or industry destroys habitats, reduces biodiversity, increases soil erosion, and contributes to climate change by releasing stored carbon dioxide. Animals lose their homes and food sources, leading to population decline or extinction.

Pollution

Water pollution from factories, farms, and sewage can kill aquatic organisms and disrupt food webs. Air pollution affects plant growth and human health. Oil spills devastate marine ecosystems by coating animals and plants in toxic oil.

Overhunting and Overfishing

Removing too many organisms from an ecosystem disrupts the balance of food webs. Overfishing reduces fish populations, which then affects species that depend on fish for food.

Conservation Efforts

Humans can take positive steps to protect ecosystems: establishing nature reserves, reducing pollution, recycling, practising sustainable farming and fishing, and replanting trees (reforestation). In Singapore, efforts include the Central Catchment Nature Reserve and Sungei Buloh Wetland Reserve.

Model Exam Answers

Question: In a pond ecosystem with algae → small fish → large fish → heron, explain what would happen if the large fish population decreased significantly.

Model Answer: If the large fish population decreases, herons would have less food and the heron population would decrease. At the same time, the small fish would have fewer predators eating them, so the small fish population would increase. With more small fish eating algae, the algae population would decrease.

Question: Explain why deforestation is harmful to ecosystems.

Model Answer: Deforestation destroys the habitat of many animals and plants, reducing biodiversity. Animals lose their food sources and shelter, causing their populations to decline or become extinct. The removal of trees also reduces the amount of photosynthesis occurring, which increases carbon dioxide levels in the atmosphere. With fewer plant roots to hold soil in place, soil erosion increases, which can pollute nearby rivers and streams.