Communities and Ecosystems

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.

What Is an Ecosystem?

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.

Levels of Organisation

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

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

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).

Interdependence

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.

Example: The Removal of Bees

Consider what happens if bee populations decline:

Plants that rely on bee pollination produce fewer seeds and fruit
Herbivores that feed on those plants have less food available
Carnivores that eat those herbivores are also affected
Decomposers have less dead organic matter to break down
Soil quality declines because fewer nutrients are recycled

This chain of events demonstrates why biodiversity is so important for stable ecosystems.

Stable and Unstable Communities

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

Habitat and Niche

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).

Summary

An ecosystem is a community of organisms interacting with the abiotic environment.
Abiotic factors include light intensity, temperature, moisture, soil pH, wind, CO2 and oxygen levels.
Biotic factors include food availability, new predators, new pathogens and competition.
A community is all the populations of different species in an area; a population is all the organisms of one species.
Interdependence means species in a community rely on each other; removing one species has knock-on effects.
A stable community has balanced populations; an unstable community has fluctuating populations.
A habitat is where an organism lives; a niche is the role it plays.

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.