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Understanding how living organisms interact with each other and their environment is fundamental to ecology. This lesson covers the key vocabulary and concepts you need for AQA GCSE Biology Topic 7: Ecology. You will learn what ecosystems are, how communities are structured, and how organisms relate to the non-living world around them.
An ecosystem is the interaction of a community of living organisms (biotic factors) with the non-living (abiotic) parts of their environment. Ecosystems can range in size from a small rock pool to an entire ocean. Every ecosystem, no matter how large or small, contains both living and non-living components that interact with one another.
| Component | Meaning | Examples |
|---|---|---|
| Biotic | The living parts of an ecosystem | Plants, animals, fungi, bacteria |
| Abiotic | The non-living, physical parts | Temperature, light, water, soil pH, minerals |
The key idea is that biotic and abiotic factors are connected. A change in one will cause changes in others. For example, a rise in temperature (abiotic) may allow new plant species (biotic) to grow in an area, which then provides food for different animals.
Exam Tip: Make sure you can clearly define biotic and abiotic and give at least two examples of each. This is a very common short-answer question worth easy marks.
Ecologists organise the living world into a hierarchy. You need to know the following terms and how they fit together, from smallest to largest:
| Level | Definition | Example |
|---|---|---|
| Individual | A single organism of one species | One oak tree |
| Population | All the organisms of the same species living in an area at a given time | All the oak trees in a forest |
| Community | All the populations of different species living and interacting in an area | All the plants, animals, fungi and bacteria in a forest |
| Ecosystem | The community of organisms plus the abiotic factors of their environment | A forest ecosystem including soil, climate, water |
graph LR
A[Individual] --> B[Population]
B --> C[Community]
C --> D[Ecosystem]
style A fill:#e8f5e9,stroke:#2e7d32
style B fill:#c8e6c9,stroke:#2e7d32
style C fill:#a5d6a7,stroke:#2e7d32
style D fill:#81c784,stroke:#2e7d32
Exam Tip: Questions sometimes ask you to arrange these terms in order, or to identify the correct term from a description. Remember: a community only includes living things, whereas an ecosystem includes the abiotic environment as well.
Abiotic factors are non-living conditions that can affect the distribution of organisms — that is, where organisms are found and how many there are. You need to know the following abiotic factors for AQA GCSE Biology:
| Abiotic Factor | How It Affects Organisms |
|---|---|
| Light intensity | Plants need light for photosynthesis; more light usually means more plant growth |
| Temperature | Affects the rate of enzyme-controlled reactions; most organisms have an optimum temperature range |
| Moisture level | Water is essential for life; organisms in dry environments must be adapted to conserve water |
| Soil pH | Affects which plants can grow; most plants grow best in slightly acidic to neutral soil (pH 5.5–7) |
| Soil mineral content | Plants need minerals such as nitrates for protein synthesis and phosphates for DNA and energy transfer |
| Wind intensity and direction | Affects transpiration rate in plants, and the distribution of wind-dispersed seeds and spores |
| Carbon dioxide levels | Higher CO2 may increase the rate of photosynthesis in plants |
| Oxygen levels (for aquatic organisms) | Fish and other aquatic organisms need dissolved oxygen to respire |
Biotic factors are living factors that can affect the distribution of organisms. You need to know these for AQA GCSE Biology:
| Biotic Factor | How It Affects Organisms |
|---|---|
| Availability of food | More food supports larger populations; lack of food leads to competition and population decline |
| New predators arriving | A new predator can reduce prey populations; prey may move to new areas |
| New pathogens (diseases) | Disease can wipe out large numbers of a species, reducing population size drastically |
| One species outcompeting another | If a new species is a better competitor, the original species may decline or be forced out entirely |
Exam Tip: In longer-answer questions, explain the mechanism behind the biotic factor. Do not just say "more predators means fewer prey" — explain that predators eat the prey, reducing prey numbers, which then means less food for the predators themselves (a negative feedback loop).
All species within a community depend on each other for resources such as food, shelter, pollination and seed dispersal. This is called interdependence. If one species is removed from a community, it can have knock-on effects on many other species.
Consider what happens if bee populations decline:
This chain of events demonstrates why biodiversity is so important for stable ecosystems.
A stable community is one where all the species and environmental factors are in balance so that population sizes remain relatively constant over time. Examples include mature tropical rainforests and ancient oak woodlands.
An unstable community is one where factors are changing, causing population sizes to fluctuate. This might happen after a natural disaster, the introduction of a new species, or human interference such as deforestation.
| Feature | Stable Community | Unstable Community |
|---|---|---|
| Population sizes | Relatively constant | Fluctuating and unpredictable |
| Species diversity | Usually high | May be low or declining |
| Environmental conditions | Relatively unchanging | Changing or disrupted |
| Example | Mature tropical rainforest | Recently cleared farmland |
A habitat is the place where an organism lives. For example, the habitat of a woodlouse is under logs and stones in damp, dark conditions.
A niche describes the role of an organism within its ecosystem — what it eats, what eats it, how it interacts with abiotic factors and other species. No two species can occupy exactly the same niche in the same habitat for a prolonged period; one will outcompete the other (this is known as the competitive exclusion principle).
Exam Tip: When answering questions about ecosystems, always think about cause and effect. Examiners reward chains of reasoning — show how one change leads to another, and then to another. Never leave your answer as a single isolated statement.