Form 5 · Paper 2 essay guide

Ecosystem, Paper 2 essay guide

How Ecosystem appears in Paper 2 essays: themes, a planning grid, a model answer structure and the keyword list.

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Essay themes this chapter supports

Describing feeding relationships and constructing food chains and food webs: Define producer, consumer and decomposer and state each one's role. → Explain the correct direction of arrows in a food chain. → Construct a food chain from a given set of organisms. → Explain why a food web is a more realistic model than a single food chain. → Classify given organisms as producers, named consumers or decomposers.
Explaining energy flow and comparing ecological pyramids: Describe how energy enters an ecosystem and flows through the food chain. → Explain the causes of energy loss at each trophic level. → Explain why food chains rarely exceed four or five levels. → Compare pyramids of numbers, biomass and energy. → Explain why only the pyramid of energy is always a true pyramid.
Comparing nutrient cycling with energy flow: State that energy flows one way while nutrients are recycled. → Describe the roles of photosynthesis and respiration in the carbon cycle. → Describe the role of decomposers and nitrogen-fixing bacteria in the nitrogen cycle. → Explain why nutrients can be reused but energy cannot. → Link nutrient cycling to the overall stability of an ecosystem.
Explaining habitat, niche, interspecific relationships and population growth: Distinguish habitat from niche using a named organism. → Define competition, predation and symbiosis with an example of each. → Describe the shape of a typical population growth curve. → Define carrying capacity and explain what limits population growth. → Distinguish density-dependent from density-independent limiting factors.

How to structure your essay

Open with one or two sentences that define the topic and set the scope.
Devote one paragraph to each key concept, in a logical order.
Use precise biological terms and, where relevant, a labelled diagram.
Give an example or state where the process happens in the body or plant.
Close by linking the ideas back to the question.

Planning grid

Question	Answer
Introduction	State what the question asks and define the key term it uses, such as 'ecosystem', 'food web' or 'carrying capacity', in one sentence.
Point 1	Name the organisms, levels or factors involved, and state what each one is.
Point 2	Develop the explanation or comparison the question asks for, such as energy loss or the shape of a growth curve.
Point 3	Support the answer with a correctly labelled example, such as a named food chain or a named symbiotic pair.
Conclusion	Draw the ideas together, linking the process to why it matters, such as ecosystem stability or sustainable population size.

Key terms to include

population
community
ecosystem
food chain
food web
trophic level
energy flow
decomposer
carbon cycle
nitrogen cycle
ecological pyramid
habitat
niche
carrying capacity
density-dependent

A model essay outline

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.

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.