The big idea: Ecology builds up in levels, from small to large.
Start with a population (all the individuals of one species in an area). Put many different populations together and you have a community (the living part).
Add the abiotic (non-living) surroundings — light, temperature, water, soil — and the community plus its environment becomes an ecosystem.
- Species
- A group of organisms that can interbreed and produce fertile offspring.
- Population
- All the individuals of the same species living in the same area at the same time (for example, all the rabbits in a meadow).
- Community
- All the populations of the different species living together and interacting in the same area.
- Habitat
- The place (the type of environment) where a species or community normally lives.
- Ecosystem
- A community of organisms together with the abiotic (non-living) environment it interacts with.
- Species richness
- The number of different species present in a community (a simple count — it ignores how many individuals of each there are).
| Level | What it includes | Living, non-living, or both? |
|---|---|---|
| Population | All the individuals of ONE species living in the same area at the same time | Living only (one species) |
| Community | ALL the populations of all the different species living together and interacting in an area | Living only (many species) |
| Habitat | The place (the type of environment) where a species or community lives | The environment / place |
| Ecosystem | A community PLUS the abiotic (non-living) environment it interacts with | Both living and non-living |
The 'nesting' order: Each level contains the one before it:
population → community → ecosystem.
A population is one species. A community is many populations. An ecosystem is a community plus its non-living surroundings.
A habitat is simply the place an organism or community lives in.
A community is not just a list of species that happen to share an area.
The populations in a community are related — they interact and depend on one another. They feed on each other, compete for the same resources, and provide shelter or pollination.
This interdependence is exactly what an exam means when it asks how individuals in a community are 'related'.
A community = interacting populations: The key idea is interaction. A community is made of many populations that interact through feeding relationships, competition and other links.
Because the populations depend on one another, a change in one population (say, fewer rabbits) ripples out and affects the others (foxes that eat them, the grass they grazed).
A community is many populations living together and interacting. Each labelled organism is a separate population; the arrows are feeding relationships that link them into one interdependent community.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
What a community is made of — autotrophs and heterotrophs: Every community contains two basic kinds of organism:
Autotrophs (producers) make their own food, usually by photosynthesis — grass, trees, algae.
Heterotrophs (consumers and decomposers) take in food made by others — rabbits, foxes, and the bacteria and fungi that break down dead matter.
So a community is the autotrophs and heterotrophs of an area, living together as interacting populations.
| Make-up of a community | What they do | Example populations |
|---|---|---|
| Autotrophs (producers) | Make their own food, usually by photosynthesis | Grass, clover, trees, algae |
| Heterotrophs (consumers) | Take in food made by other organisms | Rabbits, foxes, hawks, frogs |
| Heterotrophs (decomposers) | Feed on dead matter and recycle nutrients | Many bacteria and fungi |
Community vs ecosystem — the abiotic part: A community is the living organisms only.
An ecosystem is that community plus the abiotic (non-living) environment — the light, temperature, water, oxygen, carbon dioxide and soil — and the way the living and non-living parts interact.
This is the favourite diagram-identification trap: an oval drawn around autotrophs, heterotrophs and the abiotic environment is labelling an ecosystem, not just a community.
| Feature | Community | Ecosystem |
|---|---|---|
| What it is | all the interacting populations (the living part) | the community PLUS its abiotic environment |
| Includes non-living factors? | no — living organisms only | yes — light, temperature, water, soil, air |
| Biotic / abiotic | biotic factors only | biotic AND abiotic factors together |
| The diagram clue | the oval holding autotrophs and heterotrophs | the oval that ALSO encloses the abiotic environment |
Community
- All the interacting populations in an area
- Living organisms only (biotic)
- Autotrophs and heterotrophs together
- Held together by feeding and other interactions
Ecosystem
- A community plus its abiotic environment
- Both living (biotic) and non-living (abiotic)
- Adds light, temperature, water, soil, air
- Living and non-living parts interact
A memory hook: Community = the community of living things only.
Ecosystem = community + environment (the abiotic bit).
If you can see non-living factors in the description, it is an ecosystem.
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How this is tested: These terms are tested as precise definitions. On Paper 1B or Paper 3 a 1-mark Define or State question can ask for the meaning of habitat, community, ecosystem or species richness — you need the exact distinguishing idea, not a vague gesture.
On Paper 1A a diagram-identification item shows an oval enclosing autotrophs, heterotrophs and the abiotic environment and asks what level it represents — the answer is the ecosystem.
A short Describe question may ask how the individuals in a community are related — the marks are for interaction / interdependence between the populations.
IB-style question — define community and ecosystem
Define the terms community and ecosystem, and state the key difference between them. [3]
How to score all three marks
- Define community. A community is all the populations of different species living together and interacting in the same area.
- Define ecosystem. An ecosystem is a community together with the abiotic (non-living) environment it interacts with.
- State the difference. A community is the living organisms only, whereas an ecosystem also includes the non-living (abiotic) factors. (Mark 1: community = interacting populations. Mark 2: ecosystem = community + abiotic environment. Mark 3: the difference is the abiotic / non-living part.)
Final answer
A community is all the interacting populations of different species in an area; an ecosystem is that community plus its abiotic environment. The difference is that an ecosystem also includes the non-living (abiotic) factors.
✓ Why this scores full marks: Each definition carries one distinguishing idea — 'interacting populations' for the community, 'plus the abiotic environment' for the ecosystem.
The third mark is the contrast: the ecosystem adds the non-living part. Writing the two definitions without naming the difference would lose that mark.
| Level | What it includes | Living, non-living, or both? |
|---|---|---|
| Population | All the individuals of ONE species living in the same area at the same time | Living only (one species) |
| Community | ALL the populations of all the different species living together and interacting in an area | Living only (many species) |
| Habitat | The place (the type of environment) where a species or community lives | The environment / place |
| Ecosystem | A community PLUS the abiotic (non-living) environment it interacts with | Both living and non-living |