Levels of Organisation

The levels form this sequence:

individual organism → population → community → ecosystem

Each new level contains the level before it but adds something more.

Individual organism

An individual organism is one living thing. One frog beside a pond, one oak tree in a woodland and one bacterium in soil are each individual organisms.

An organism belongs to a species. Members of the same species can reproduce with one another to produce fertile offspring. Identifying the species matters when deciding whether several organisms form one population or belong to different populations.

From one organism to a population

A population consists of organisms of the same species living in the same area at the same time. For example, all the common frogs living around one pond at a particular time form a population.

The place and time are important. Common frogs beside two widely separated ponds would normally be counted as different populations, while frogs and newts in one pond are not one population because they are different species.

From populations to a community

A community contains the populations of different species living and interacting in the same area. A pond community might include populations of frogs, newts, fish, pondweed, algae and microorganisms.

The word interacting is important. Organisms may feed on one another, compete for resources or provide food, shelter or pollination. These relationships connect the different populations into a community. The particular effects of competition, predation and other interactions are studied separately; here, the key point is that a community contains many populations rather than just one.

From a community to an ecosystem

To describe the whole ecosystem, include both the community and the non-living environment with which it interacts. Living parts, such as plants, animals and microorganisms, are biotic components. Non-living parts, such as light, water, air, soil and temperature, are abiotic components.

A pond ecosystem therefore includes its entire community, the pond water, dissolved substances, light, temperature and the pond bed, together with the interactions among them. Plants use light, water and mineral ions; animals depend on water conditions and other organisms; decomposers act on dead biological material. Matter and energy can therefore move between the living and non-living parts.

An ecosystem is not defined by one fixed physical size. A garden pond can be treated as an ecosystem, while a woodland or a much larger region can also be studied as one. What matters is that the chosen area includes organisms, their non-living surroundings and the interactions between them.

One pond at every level

Consider a pond containing several common frogs, pondweed, insects and bacteria:

• one common frog is an individual organism;
• all the common frogs in that pond at that time are a population;
• the frog, pondweed, insect and bacterial populations together form a community;
• that community interacting with the water, light, temperature and pond sediment forms the ecosystem.

This example shows the change in focus clearly: one organism; one species; many interacting species; then the living community plus its abiotic environment.