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This lesson covers the key concepts of ecosystems and communities as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand the organisation of life within ecosystems, the difference between biotic and abiotic factors, and how organisms within communities are interdependent.
An ecosystem is a community of living organisms interacting with each other and with the non-living (physical) components of their environment. Ecosystems can be large (e.g. tropical rainforest, ocean) or small (e.g. a pond, a rotting log).
Every ecosystem contains two types of component:
| Component | Meaning | Examples |
|---|---|---|
| Biotic | Living factors | Plants, animals, fungi, bacteria, competition, predation |
| Abiotic | Non-living factors | Temperature, light, water availability, soil pH, wind |
Exam Tip: The examiner may describe a scenario and ask you to identify biotic and abiotic factors. Always check — if something is alive or produced by a living organism, it is biotic.
You must be confident with the following definitions:
| Term | Definition |
|---|---|
| Habitat | The place where an organism lives, e.g. a pond, a woodland, a rock pool |
| Population | All the organisms of one species living in a habitat at a given time |
| Community | All the populations of different species living and interacting in an area |
| Ecosystem | A community of organisms plus the abiotic (non-living) conditions in which they live |
| Niche | The role an organism plays within its ecosystem, including what it eats and how it interacts with other species |
graph LR
A["Individual organism"] --> B["Population (one species)"]
B --> C["Community (all species)"]
C --> D["Ecosystem (community + abiotic environment)"]
Organisms within a community are interdependent — they depend on each other for survival. Changes in one species can affect others.
Examples of interdependence:
Exam Tip: If asked to explain the effect of removing one species from a food web, trace the knock-on effects both up (predators lose food) and down (prey populations may increase).
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:
In a stable community:
Organisms in a community compete for limited resources. Competition can be:
Competition between organisms of the same species, e.g. two oak trees competing for light.
Competition between organisms of different species, e.g. red squirrels and grey squirrels competing for food.
| Organisms compete for... | Plants | Animals |
|---|---|---|
| Light | Yes | — |
| Water | Yes | Yes |
| Minerals / nutrients | Yes | — |
| Space / territory | Yes | Yes |
| Food | — | Yes |
| Mates | — | Yes |
Every ecosystem relies on energy flow through trophic levels:
| Trophic Level | Name | Description |
|---|---|---|
| 1 | Producer | Organisms that make their own food by photosynthesis (e.g. plants, algae) |
| 2 | Primary consumer | Herbivores that eat producers (e.g. rabbit, caterpillar) |
| 3 | Secondary consumer | Carnivores or omnivores that eat primary consumers (e.g. frog, robin) |
| 4 | Tertiary consumer | Top predators that eat secondary consumers (e.g. hawk, fox) |
Decomposers (bacteria and fungi) break down dead organisms at every trophic level and recycle nutrients.
graph TD
A["Sun (energy source)"] --> B["Producers (plants, algae)"]
B --> C["Primary consumers (herbivores)"]
C --> D["Secondary consumers (carnivores)"]
D --> E["Tertiary consumers (top predators)"]
B --> F["Decomposers"]
C --> F
D --> F
E --> F
F --> G["Nutrients returned to soil"]
G --> B
Exam Tip: Always remember that the Sun is the ultimate source of energy for almost all ecosystems. Producers transfer light energy into chemical energy via photosynthesis.