Form 5 · Common mistakes

Ecosystem, common mistakes

The mistakes SPM students make on Ecosystem, why each one loses marks, and the correct version.

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The mistakes, why they lose marks, and the fix

Common mistake	Why it loses marks	What earns the mark
Drawing food chain arrows pointing towards the food source.	Arrows must show the direction of energy flow, from the organism eaten to the organism that eats it, not the direction food physically moves.	Draw each arrow from the prey or producer towards the consumer that eats it, showing energy flowing forward.
Saying decomposers are a type of producer.	Decomposers break down dead matter and waste; producers make their own food by photosynthesis, which is a completely different process.	State that decomposers, such as bacteria and fungi, recycle nutrients from dead organisms, while producers photosynthesise.
Saying energy is recycled in an ecosystem in the same way as nutrients.	Energy flows through an ecosystem in one direction only and is lost as heat at every level; only chemical nutrients such as carbon and nitrogen are recycled.	State that energy flows one way and must be constantly resupplied by the Sun, while nutrients are reused repeatedly.
Confusing a food chain with a food web.	A food chain is a single pathway; a food web is a network of interconnected food chains showing that most organisms eat more than one type of food.	Use 'food web' only when describing multiple interconnected food chains, not a single pathway.
Assuming every pyramid of numbers, biomass and energy must have the same shape.	A pyramid of numbers can appear inverted, for example when one large tree supports many small herbivores, even though the pyramid of energy for the same chain is always a true pyramid.	State that only the pyramid of energy is always a true pyramid shape; numbers and biomass can occasionally be unusual.
Confusing a habitat with a niche in an answer.	A habitat is the physical place an organism lives; a niche is its functional role, including diet and interactions with other species.	Give the habitat as a short physical location and the niche as a fuller description of diet and interactions.
Saying competition only happens between different species.	Competition can be intraspecific, between members of the same species, or interspecific, between different species, and intraspecific competition is often more intense.	State that competition can be either intraspecific or interspecific, and name which type an example shows.
Naming only one fixed example, such as bee and flower, for every symbiosis question.	Exam questions often describe an unfamiliar relationship and expect the student to classify it as mutualism, commensalism or parasitism from the description, not to recall a memorised pair.	Identify who benefits, who is harmed and who is unaffected in the given description before naming the type of relationship.
Assuming a population can never exceed its carrying capacity.	A population can temporarily overshoot its carrying capacity before limited resources cause the death rate to rise and the population to fall back.	Describe the overshoot-and-fall pattern as a peak followed by a dip on a population growth graph.
Saying every limiting factor becomes more severe as a population grows.	Only density-dependent factors, such as food supply and disease, become more severe as a population grows; density-independent factors affect a population by a similar proportion regardless of size.	State clearly which limiting factors are density-dependent and which are density-independent in an answer.
Describing the S-shaped growth curve as having only two phases.	The curve has three recognisable phases: an initial slow phase, a rapid phase, and a final levelling-off phase at the carrying capacity.	Label all three phases of the growth curve, slow, rapid and levelling-off, when describing or sketching it.

How to avoid these mistakes

Practise drawing food chains and webs with correctly directed arrows, labelling each organism's trophic level.
Explain energy loss between trophic levels using the idea of heat loss and unconsumed material.
Compare pyramids of number, biomass and energy, noting when each pyramid can take an unusual shape.
Learn to distinguish habitat and niche using one example organism you can describe in both terms.
Learn one clear example each of mutualism, commensalism and parasitism.
Practise reading a population growth curve, labelling the slow, rapid and levelling-off phases and identifying the carrying capacity.

Frequently asked questions

Why does energy decrease along a food chain?

At each level of a food chain, only some of the energy is passed on to the next level. Much of the energy is used for the organism's own life processes and lost as heat during respiration, and some is in parts that are not eaten. Because only about a tenth is passed on each time, the energy decreases sharply, which is why food chains rarely have more than four or five levels.

What is the difference between a food chain and a food web?

A food chain shows a single pathway of energy from a producer through a chain of consumers, for example grass to grasshopper to frog to snake. A food web is a network of interconnected food chains, showing that most organisms eat more than one type of food and are eaten by more than one predator, which gives a more realistic picture of an ecosystem.

Why are nutrients recycled but energy is not?

Nutrients such as carbon and nitrogen are chemical elements that are passed between organisms and the environment and used again and again in cycles. Energy, however, enters an ecosystem from the Sun, flows one way through the organisms, and is gradually lost as heat, so it cannot be reused and must be constantly resupplied by sunlight.