Form 5 · Worked answers

Ecosystem, worked answers

Fully worked answers for Ecosystem, original structured and essay questions with mark-scheme keywords highlighted.

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How this topic is examined

Constructing a food chain or food web from a list of given organisms, using arrows in the correct direction.
Explaining why a food chain rarely has more than four or five trophic levels.
Interpreting a pyramid of number, biomass or energy, including an unusual shape.
Distinguishing habitat from niche using a named organism as a worked example.
Identifying the type of interaction shown in a described relationship between two species, such as mutualism or parasitism.
Interpreting a population growth curve and identifying the carrying capacity from the graph.

Model answer structure

Read the command word and answer to the marks, one clear point per mark.
Define the key biological term precisely before you explain it.
Explain the process or reason in the correct sequence, using the right terms.
Where useful, add a labelled diagram or a worked example.
End with the link the question asks for (cause → effect, structure → function).

Fully worked answers

Define the terms population, community and ecosystem, using a pond as your example.

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A population is all the individuals of one species living in the pond at the same time, such as all the tilapia. A community is all the different populations living together in the pond, including the fish, plants, insects and other organisms. An ecosystem is that community together with the pond's abiotic factors, such as water temperature, light penetration and dissolved oxygen, with which the community interacts.

populationcommunityecosystemabiotic factorspond

Construct a food chain from the following organisms: hawk, grasshopper, grass, frog. State which organism is the producer.

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The food chain is grass → grasshopper → frog → hawk, with each arrow pointing from the organism being eaten to the organism eating it. Grass is the producer because it makes its own food by photosynthesis; the grasshopper is the primary consumer, the frog is the secondary consumer, and the hawk is the tertiary consumer.

food chainproducerprimary consumerarrow directionhawk

Explain why energy decreases at each trophic level of a food chain.

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At each trophic level, only a fraction of the energy taken in is passed on to the next level. Much of the energy is used for the organism's own life processes, such as movement and growth, and lost as heat during respiration, while some remains in parts of the organism that are not eaten by the next consumer. Because only a small proportion is transferred each time, the total energy available falls sharply along the chain, which is why food chains rarely extend beyond four or five levels.

energyrespirationheattrophic levelnot eaten

Explain why nutrients such as carbon and nitrogen are described as being recycled, while energy is not.

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Carbon and nitrogen are chemical elements that pass repeatedly between living organisms and the non-living environment through processes such as photosynthesis, respiration, decomposition and nitrogen fixation, so the same atoms are reused again and again. Energy, by contrast, enters an ecosystem from the Sun, flows one way through the food chain, and is gradually lost as heat at every transfer, so it cannot be reused and must be constantly resupplied by sunlight.

carbonnitrogenrecycledenergyone wayheat

A pyramid of numbers for a food chain based on one large oak tree appears inverted, while the pyramid of energy for the same food chain is a normal pyramid shape. Explain this apparent contradiction.

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A pyramid of numbers simply counts individual organisms, so one large oak tree supporting many small herbivorous insects gives a narrow base and a wide second level, making the pyramid appear inverted. A pyramid of energy instead measures the energy available at each level over a fixed time, and energy is always lost between levels through respiration and uneaten material, so the amount of energy available to consumers must always be less than that available to the producer. The pyramid of energy is therefore always a true pyramid shape even when the pyramid of numbers is not.

pyramid of numbersinvertedpyramid of energytrue pyramidenergy loss

Distinguish between habitat and niche using a named organism as an example.

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A habitat is the physical place where an organism lives; for example, the habitat of a clownfish is a coral reef, or more specifically among the tentacles of a sea anemone. A niche is broader, describing the organism's functional role: the clownfish's niche includes feeding on small invertebrates and algae, sheltering within an anemone that stings its predators, and being partly protected from those predators by a mucus coating. Two organisms can share the same habitat while occupying different niches.

habitatnichefunctional roleclownfishexample

A population of deer introduced to an island grows following an S-shaped curve. Explain the shape of this curve and state what happens if the population briefly exceeds the island's carrying capacity.

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The population grows slowly at first because there are few breeding individuals, then rapidly as resources such as food and space remain abundant, and finally levels off as the population approaches the carrying capacity, the maximum size the island can sustainably support. If the population briefly exceeds the carrying capacity, limited resources cause the death rate to rise and the birth rate to fall, and the population falls back towards a size the island's resources can sustain.

S-shaped curvecarrying capacitydeath ratebirth rateovershoot

Phrasing that earns marks

Community and ecosystem: A population is all the individuals of one species living in an area; a community is all the populations of different species living together in that area; and an ecosystem is the community together with the non-living, abiotic factors of its environment, such as light, temperature, water and soil, with which it interacts.
Feeding relationships: Producers make their own food by photosynthesis, primary consumers eat producers, and secondary or tertiary consumers eat other consumers; decomposers break down dead organisms and waste, returning nutrients to the environment. These feeding relationships link organisms together into food chains, and overlapping food chains together form a food web.
Energy flow: Energy from the Sun enters an ecosystem through photosynthesis and flows one way through the food chain from producers to consumers, decreasing at every trophic level because much of it is lost as heat during respiration, used for the organism's own life processes, or contained in parts that are not eaten.
Nutrient cycles: Unlike energy, which flows through an ecosystem only once, nutrients such as carbon and nitrogen are recycled repeatedly between living organisms and the non-living environment through processes including photosynthesis, respiration, decomposition, and, for nitrogen, fixation by bacteria, so the same atoms are used again and again.
Population ecology: A population's size at any time depends on the balance between the processes that add individuals, birth and immigration, and the processes that remove them, death and emigration, together with limiting factors in the environment such as food supply, disease, predators and competitors.
Ecological pyramids: A pyramid of numbers, biomass or energy represents each trophic level of a food chain as a bar, usually narrowing towards the top because both the amount of living material and the amount of available energy decrease at each successive level, though a pyramid of numbers can occasionally have an unusual shape.
Habitat and niche: A habitat is the physical place where an organism lives, such as a pond or a forest floor, while a niche is the organism's functional role within its ecosystem, including what it eats, what eats it, and how it interacts with other organisms. Two species can share a habitat but rarely occupy exactly the same niche for long.
Interspecific relationships: Organisms interact in defined ways: competition occurs when two organisms require the same limited resource, predation occurs when a predator captures and consumes prey, and symbiosis describes a close, long-term relationship between two species that is classified as mutualism when both benefit, commensalism when one benefits and the other is unaffected, or parasitism when one benefits at the other's expense.

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.