Ecologists study living things at different levels of organisation. The smallest level is a single living thing, an individual organism, such as one frog. All the organisms of the same species living in one area make up a population, for example all the frogs in a pond.
Populations of different species do not live alone. All the populations living together in the same area form a community. A pond community includes the frogs, fish, insects, pondweed and bacteria that live there.
The community is only part of the picture. It depends on the non-living, or abiotic, parts of the environment such as temperature, light and the water itself. A community together with the abiotic parts of its environment is an ecosystem. The levels from smallest to largest are therefore organism, population, community and ecosystem.
