Ecosystems: Scales & Interactions Revision Notes | GCSE AQA Higher Geography

1. What Is an Ecosystem?

Picture a pond. There are fish, water plants, insects, algae, mud on the bottom, sunlight filtering through the surface, and water at a particular temperature. Every single one of those things — living and non-living — is part of one system. That system is an ecosystem.

An ecosystem is a community of living organisms interacting with each other and with their non-living environment. The key word is interaction — nothing in an ecosystem exists in isolation. A change to one part ripples through the whole system.

To understand how ecosystems work, you need to separate their ingredients into two groups: the biotic (living) components and the abiotic (non-living) components. Once you can identify both, you can start to explain how they interact — and that is exactly what the exam will ask you to do.

2. Biotic and Abiotic Components

Think of the biotic components as all the living things sharing a space: plants, animals, fungi, bacteria, and other microorganisms. They interact through feeding (predator and prey), competition for resources, pollination, and decomposition.

Abiotic components are the non-living physical and chemical conditions of the environment. They set the stage — they determine which organisms can survive there in the first place.

• Temperature — affects enzyme activity and metabolic rates in organisms
• Rainfall and water availability — determines which plants can grow
• Sunlight intensity — drives photosynthesis and therefore the whole food chain
• Soil type and pH — influences which nutrients are available to plants
• Wind speed — affects moisture loss and seed dispersal
• Altitude — affects temperature and oxygen availability

Notice how each abiotic factor directly affects the biotic components. Low rainfall means fewer plants can survive; fewer plants means less food for herbivores; fewer herbivores means less prey for carnivores. The whole community shifts because one abiotic factor changed.

3. How Biotic and Abiotic Components Interact

The real power of ecosystem thinking is understanding that biotic and abiotic components do not just coexist — they constantly influence each other in both directions. Abiotic conditions shape which organisms survive; those organisms then modify the abiotic environment around them.

Consider a forest floor. The trees (biotic) intercept rainfall and reduce the amount of sunlight reaching the ground (abiotic). That reduced light means only shade-tolerant plants can grow beneath them (biotic response to abiotic change). Those plants then add organic matter to the soil when they die, changing the soil's nutrient content and pH (biotic components altering abiotic conditions).

This two-way relationship is what makes ecosystems dynamic. They are not static pictures — they are constantly adjusting. When you explain interactions in an exam, always try to show this chain: abiotic factor affects biotic component, which in turn affects another component.

4. Ecosystems at a Range of Scales

One of the most important ideas in this topic is that ecosystems exist at many different scales. The same principles — biotic and abiotic components interacting — apply whether you are looking at a tiny rock pool or the entire Amazon rainforest.

At every scale, the same question applies: which biotic components are present, which abiotic conditions define the environment, and how do they interact? Examiners often give you an unfamiliar ecosystem and ask you to apply this framework — so practise thinking in these terms rather than memorising specific examples.

5. What Happens When One Component Changes?

Because all components are linked, a change to any one of them triggers a chain of effects throughout the ecosystem. This is sometimes called the interdependence of components, and it is a key idea for both describe and explain questions.

Here is a worked example using a freshwater pond. Follow the chain carefully — each step connects one component to the next.

Worked example: Tracing the Impact of an Abiotic Change Through an Ecosystem

A prolonged drought reduces the water level in a freshwater pond. Describe and explain the likely effects on the ecosystem.

1. Water level falls due to reduced rainfall (abiotic component changes). — Always start by naming the abiotic factor that has changed. This earns the first descriptive mark.
2. Less water means reduced habitat space and lower dissolved oxygen levels, so aquatic plants and algae decline. — Connect the abiotic change to a specific biotic response. Use 'because' to show the causal link — this is what turns a describe answer into an explain answer.
3. Fewer aquatic plants means less food and shelter for invertebrates such as water fleas and pond snails. — Show how the effect moves through the biotic community. Examiners reward chains of reasoning, not isolated points.
4. Reduced invertebrate populations mean less food for fish and amphibians, so their numbers also fall. — A strong answer traces the effect at least two or three steps through the food web.
5. Fewer plants means less transpiration, which may further reduce local humidity and rainfall — reinforcing the drought conditions. — For top-band marks, show that biotic changes can feed back into abiotic conditions, demonstrating the two-way nature of ecosystem interactions.

Answer: A fall in water level (abiotic) reduces aquatic plant growth (biotic), which reduces food for invertebrates, then fish and amphibians. Reduced vegetation also lowers transpiration, potentially worsening the drought — showing how biotic and abiotic components are interdependent.

6. Exam Technique: Getting Full Marks on Ecosystem Questions

Ecosystem questions on Paper 1 tend to use the command words describe, explain, and suggest. Each one demands a slightly different approach, and mixing them up is one of the most common ways students lose marks.

When a question says describe, state what happens — in sequence, without explaining why. When it says explain, you must give reasons using the word 'because' or 'therefore'. When it says suggest, apply your knowledge to an unfamiliar scenario; there may be more than one valid answer, so commit to a logical chain rather than hedging.

The central idea of this topic is straightforward: every ecosystem, at every scale, is defined by the two-way interaction between its biotic and abiotic components. Nail that principle and you can apply it to any ecosystem the examiner puts in front of you.