Individuals form populations

1. One oak tree, one fox or one bacterial cell can each be described as an individual organism.
2. A population includes all the individuals of one species within a stated area at the same time.
3. For example, all the common frogs in one pond form a population, whereas one frog is an individual.
4. A population must refer to a particular species, so all the animals in the pond do not form a single population.
5. The area and time must be clear because the same species can occur in several places and its numbers can change.
6. Population size is the number of individuals in that population, rather than the number of species present.

Populations form a community

1. A pond can contain separate populations of common frogs, great pond snails, duckweed and many other species.
2. These populations together make up its community.
3. A community includes plants, animals, fungi and microorganisms, including organisms that are too small to see without a microscope.
4. The word community refers to the living organisms, so the pond water, rocks and light are not themselves members of the community.
5. A species represented by only one individual in the area still contributes to the community.
6. When naming a community, use the different populations together, rather than listing only the most visible animals.

An ecosystem includes the environment

1. An ecosystem contains a community and the non-living environment with which that community interacts.
2. The living organisms are its biotic components, while water, air, soil and physical conditions are its abiotic components.
3. Including interactions matters because an ecosystem is more than a list of organisms and objects in the same place.
4. For example, pond plants take in dissolved substances from the water, and their activity changes the substances present in that water.
5. The levels of organisation progress from individual to population to community to ecosystem.
6. An ecosystem can be relatively small, such as a rock pool, or much larger, such as a woodland.
7. Its defining feature is the living community interacting with its surroundings, rather than a fixed size.

Habitats are places within ecosystems

1. A woodland contains several habitats, including leaf litter, tree bark and the canopy, even though they belong to the same wider ecosystem.
2. Different organisms can share a habitat, so a habitat does not correspond to only one population.
3. A microhabitat is a small, localised part of a habitat, such as the damp space beneath a log.
4. The same tree can provide living space for several bird species, as shown in the retained illustration.
5. The birds occupy different parts of the tree and use its resources differently, illustrating that a habitat describes a place while a niche describes an organism’s role there.
6. When identifying organisational levels, keep where an organism lives separate from whether the description concerns an individual, population, community or ecosystem.