Living things can be studied at different levels of organisation. The smallest level is a single living individual, called an organism. A cat, an oak tree and a single bacterium are each one organism.
All the organisms of the same species living in the same area at the same time make up a population. All the oak trees in a wood form one population, and all the foxes in the same wood form another.
The populations of different species that live together in the same habitat make up a community. The foxes, oak trees, insects, birds, fungi and bacteria in the wood are all part of one community.
An ecosystem is the community together with the non-living, or abiotic, parts of its surroundings, such as the soil, water, light and temperature. The living parts are called biotic factors.
