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.An oak tree can be seen as an ecosystem in which each zone of the tree is a habitat for a distinct community of organisms

An ecosystem describes a natural biological unit that is made up of both living and non-living parts. It is made up of a number of:

  • habitats - the place where an organism lives
  • communities - all the living organisms that live within a habitat

A community can contain a number of different species.


A species is a group of organisms that can interbreed to produce fertile offspring. For example, a horse and a donkey can interbreed with one another but their offspring (a mule) will be unable to reproduce as mules are infertile. The fact that offspring cannot reporduce shows that horses and donkeys belong to two different species.


Biodiversity is the degree of variation that exists among all living organisms on Earth.

When examining the biodiversity on Earth it is useful to consider the populations of organisms in a species. These can fluctuate in an ecosystem depending on factors such as:

  • competition between organisms for resources
  • predator numbers
  • disease

All living things within an ecosystem are interdependent. A change in the size of one population affects all other organisms within the ecosystem. This is shown particularly clearly by the relationship between predator and prey populations.