Food Chains, Food Webs & Ecosystems

Producers, consumers, decomposers. Food chains and food webs. Factors affecting survival. Adaptation. Habitats. Impact of environmental change. Conservation.

food chains & websadaptationproducers & consumers
Ecosystems — key vocabulary
Organism, population, community
Organism — one living thing
Population — group of same kind in one place
Community — many populations in one habitat
Roles in a food chain
Producer — makes own food (plants)
Consumer — eats others for food
Decomposer — breaks down dead matter
Predator / Prey — hunter / hunted
Survival factors
Physical: temperature, light, water
Biotic: food availability, other organisms
Unfavourable → adapt, move or die
Food chain rules
Arrow shows direction of energy flow
Always starts with a producer
A → B means A is eaten by B (energy passes from A to B)
Food web: multiple overlapping food chains in the same habitat. Removing one organism affects all the chains it belongs to.
Part A — Multiple choice (Booklet A style · 2 marks each)
Question 1 — MCQ (2 marks) Booklet A Food chain · energy flow

The food chain below shows organisms in a pond.

Algae → Water flea → Small fish → Large fish

Which organism is the producer in this food chain?

AAlgae
BWater flea
CSmall fish
DLarge fish

The producer is always at the start of the food chain — it makes its own food through photosynthesis. Algae are aquatic plants and are the producer here. All animals in the chain are consumers. The arrow shows direction of energy flow, not "who eats whom" — though in a simple chain they happen to mean the same thing.

Question 2 — MCQ (2 marks) Booklet A Population change in food web

In a grassland food web: Grass → Grasshoppers → Frogs → Snakes. If the frog population suddenly decreases greatly, which prediction about the grasshopper population is most likely correct?

AGrasshopper population decreases because there are fewer frogs to eat.
BGrasshopper population increases because fewer frogs are eating them.
CGrasshopper population stays the same because frogs do not affect grasshoppers.
DGrasshopper population decreases because the grass will also decrease.

Frogs eat grasshoppers (frogs are predators; grasshoppers are prey). Fewer frogs = less predation of grasshoppers = grasshopper population increases. This "domino effect" through a food chain is the core concept of food web questions.

Part B — Structured questions (Booklet B style)
Question 3 — Structured (4 marks) Booklet B Food web · population changes

The diagram shows a food web in a forest ecosystem.

Trees / plants Caterpillars Squirrels Beetles Birds Foxes

(a) Name one food chain from this food web that contains exactly three organisms. (1 mark)

(b) A disease kills most of the squirrels. Predict what would happen to the fox population and the tree/plant population. Explain your answers. (3 marks)

(a)Accept any valid 3-organism chain, e.g.: Trees/plants → Caterpillars → Birds, or Trees/plants → Squirrels → Foxes, or Trees/plants → Beetles → Foxes.
(b) FoxesFoxes eat squirrels. With fewer squirrels, foxes have less food, so the fox population will decrease. (However, foxes also eat birds, so the decrease may not be as large if they eat more birds instead.)
Trees/plantsSquirrels eat trees/plants. With fewer squirrels eating them, trees and plants experience less predation, so the tree/plant population will increase.
Fewer squirrels → fewer foxes · Fewer squirrels → more trees/plants
For (b), always explain the link: state what eats what, then state the direction of change, then explain why. "Foxes decrease" alone earns 0. "Foxes decrease because squirrels are their food source, and with fewer squirrels there is less food for foxes" earns full marks.
Question 4 — Structured (4 marks) Booklet B Adaptation · survival factors Distinction level

The table describes two animals and their environments. Study the features and answer the questions.

Animal Habitat features Adaptations Polar bear (Arctic) Very cold, snow-covered, few plants, icy water Thick white fur, large body, large padded paws Cactus wren (Desert bird) Very hot, dry, little water, sparse vegetation Nests in cacti, gets water from food, active at dawn

(a) Explain how the polar bear's white fur helps it survive in its habitat. (2 marks)

(b) The cactus wren is active mainly at dawn rather than midday. Explain how this behaviour helps it survive in the desert. (2 marks)

(a)The white fur provides camouflage — it blends in with the snow and ice, making the polar bear harder for prey to detect as it approaches. This helps it hunt successfully and obtain food. (Also accept: white fur camouflages it from predators, though polar bears have very few predators.)
(b)At midday, the desert is extremely hot. Being active at dawn, when temperatures are cooler, allows the cactus wren to avoid excessive heat and reduce water loss from its body. This helps it survive in a habitat where water is very scarce.
Feature → function → survival benefit

The mark-earning structure for all adaptation questions: name the feature, explain what it does (function), then link it to survival in that specific habitat. "White fur helps it hide" earns 1 mark. "White fur provides camouflage in the snow, making it harder for prey to spot the polar bear, helping it hunt successfully" earns 2 marks.

Question 5 — Structured (5 marks) Booklet B Human impact · conservation · evaluate Distinction level

A coastal mangrove forest is home to a community of organisms including mangrove trees, mudskippers, herons, crabs and decomposers. A property developer proposes to clear the mangroves to build a resort hotel.

(a) Explain what would happen to the mudskipper population if the mangrove trees were removed. (2 marks)

(b) Apart from loss of habitat, state two other reasons why conservation of the mangrove forest is important. (2 marks)

(c) The developer argues: "The resort will bring economic benefits to the community, so it is worth clearing the mangroves." A conservationist disagrees. Suggest one argument the conservationist could make against the development. (1 mark)

(a)Mangrove trees provide food, shelter and breeding grounds for mudskippers. Without the trees, mudskippers would lose their habitat and food source. They would either move to other areas to survive or, if no suitable habitat is available, their population would decrease and they could die out locally.
(b)Accept any two: (1) Mangroves protect coastlines from erosion and storm damage by absorbing wave energy. (2) Mangroves maintain biodiversity — the loss of one species affects the whole food web and may cause other species to decline. (3) Decomposers recycle nutrients — removing the ecosystem disrupts nutrient cycling. (4) Mangroves absorb carbon dioxide, helping reduce the effects of climate change.
(c)Accept any valid argument, e.g.: Once destroyed, mangrove ecosystems are very difficult or impossible to restore — the loss is permanent. / The ecosystem provides services (coastal protection, fishing grounds) that benefit the community economically in the long run, more than a single resort would. / Loss of biodiversity may trigger a collapse of the food web, affecting local fishing communities who depend on the ecosystem for their livelihood.
Habitat loss → population falls · Conservation = biodiversity + ecosystem services
Part (c) is a STSE (Science-Technology-Society-Environment) question — the PSLE syllabus explicitly includes evaluating the ethical and environmental implications of scientific decisions. There is no single right answer, but the argument must be logically connected to scientific knowledge (e.g. food web disruption, ecosystem services) rather than just "it is wrong to destroy nature."