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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.
A student samples a moorland ecosystem with a 0.25 m² quadrat placed at 12 random coordinates. They record the number of heather plants in each quadrat: 4, 6, 5, 7, 3, 5, 6, 4, 8, 5, 4, 7. The study area is 10,000 m².
Because sample counts cluster tightly around the mean (low spread), the student can describe this moorland as a relatively stable community with high heather dominance. A much wider spread of values would suggest patchy distribution driven by changing abiotic factors (e.g. soil moisture, pH) across the site.
Common mistake: Confusing the biotic factor (heather itself) with the abiotic factor (soil pH) that allows heather to thrive. Always label whether a factor is living or non-living when you describe it. Another frequent error: treating correlation (heather density falls near the footpath) as causation (trampling causes the decline) without measuring the abiotic variable directly.
| Feature | Biotic Factors | Abiotic Factors |
|---|---|---|
| Definition | Living components of the ecosystem | Non-living, physical/chemical components |
| Examples | Predators, pathogens, food, competitors | Light, temperature, pH, moisture, CO₂, O₂ |
| How measured | Quadrats, transects, capture-recapture | Light meter, thermometer, pH probe, moisture probe |
| Typical effect | Changes population size via predation/competition | Changes distribution via physiological tolerance |
| Response speed | Can be rapid (e.g. new disease outbreak) | Often seasonal or long-term (e.g. climate shift) |
Common mistake: Listing water as biotic — water itself is abiotic, but the availability of water can be influenced by biotic factors such as plant transpiration. Be precise.
Question: A woodland pond is colonised by a new species of aquatic snail. Explain how the introduction of this snail could affect the community of organisms already living in the pond. (4 marks)
Grade 4–5 answer: "The snail will eat the plants in the pond, so there will be less food for other animals. Some animals might die because they have no food. The snail could also be eaten by fish, so there will be more food for the fish. This means fish numbers will go up."
Why this is Grade 4–5: Correct direction of reasoning but imprecise — no use of the terms biotic factor, interdependence or competition, and no distinction between interspecific and intraspecific effects.
Grade 8–9 answer: "The snail acts as a new biotic factor in the community. As a primary consumer, it competes interspecifically with existing herbivores such as tadpoles for the same aquatic plants, reducing their population size through resource competition. Simultaneously, the snail provides an additional food source for secondary consumers (e.g. fish and newts), which may increase in number. Because species within the pond show interdependence, these changes propagate through the food web: plant biomass falls, decomposer activity rises as snail faeces accumulates, and the ecosystem may shift from a stable to an unstable community until a new equilibrium is reached."
Why this is Grade 8–9: Uses precise terms (biotic factor, interspecific, interdependence, unstable community), traces a chain of reasoning through multiple trophic levels, and distinguishes short-term from long-term effects.
AQA alignment: This content is aligned with AQA GCSE Biology (8461) specification section 4.7 Ecology — specifically 4.7.1.1 Communities, 4.7.1.2 Abiotic factors, 4.7.1.3 Biotic factors, and 4.7.2.1 Levels of organisation. Assessed on Paper 2.
Consider a rockpool on a rocky shore as a small, self-contained ecosystem. Within a 1 m² rockpool you might find:
Every one of the levels of organisation discussed earlier is present: each periwinkle is an individual; all the periwinkles together are a population; all the populations form the rockpool community; and once we add the abiotic factors (salinity, temperature, pH, wave exposure, oxygen level), we have a full ecosystem.
The rockpool is a useful illustration of interdependence: remove the seaweed (perhaps through excessive grazing or pollution) and limpets have no food, so their population crashes; whelks lose their prey; decomposers lose the detritus supply. In other words, every biotic interaction is underpinned by abiotic limits — salinity must stay roughly constant; oxygen must be replenished by wave action; temperature must remain within tolerance. This is why ecologists measure both the organisms and the physical environment during fieldwork.
Common mistake: Describing the rockpool as a "community" when the question asks for an ecosystem. A community is the living organisms only — the ecosystem also includes the salinity, temperature and wave energy. Swap these terms accidentally and you can lose an easy mark.
Exam Tip: Try to memorise one small ecosystem example (rockpool, hedgerow, or pond) with named producers, consumers and decomposers. You can then adapt it to many exam questions about biotic/abiotic factors, interdependence and community stability.