These are important key terms:

  • An ecosystem is the interaction between a community of living organisms and their environment.
  • A community is two or more populations of organisms.
  • An ecosystem is two or more populations of organisms (usually many more) in their environment.
  • A population is all the organisms of the same or closely-related species in an area.

Levels of organisation within an ecosystem

Organisms within an ecosystem are organised into levels.

ProducerProducers are plants and algae, which photosynthesise.
Primary consumerPrimary consumers are herbivores, which eat producers.
Secondary consumerSecondary consumers are carnivores, which eat primary consumers.
Tertiary consumerTertiary consumers are also carnivores. They eat secondary consumers.
Oak tree environment. Root and litter zone has bacteria, earthworms, wood lice and funghi. Trunk has insects and larvae. Branches and leaves have bees, wasps, moths, squirrels, bluetits and hawks.A woodland ecosystem showing the habitats of different species.


All organisms in an ecosystem depend upon each other. If the population of one organism rises or falls, then this can affect the rest of the ecosystem.

A simple food chain is:

grass → rabbit → fox

If the foxes in the food chain above were killed, the population of rabbits would increase because they are no longer prey to the foxes. As a result the amount of grass would decrease because the increased population of rabbits would be eating it.

Often very small changes to ecosystems have large consequences, which can be difficult to predict. This means that all the organisms in an ecosystem are dependent upon each other. We call this interdependence.


All photosynthesising plants and algae in an ecosystem compete for light, space, water and minerals from the soil. Animals in an ecosystem compete for food, mates and their territory. Organisms which have more of these resources tend to grow more healthily and are more likely to have offspring. Competition can be interspecific or intraspecific depending on whether organisms from different species or the same species are competing for resources.

Stable communities

A stable community is one in which the size of the populations of all species remain relatively constant over time. In the example above the amount of grass, and the numbers of rabbits and foxes all remain relatively constant. The different populations are living in a healthy balance with their environment.