Populations of different species form a community.

The idea

A population is all the organisms of one species living in the same place at the same time that can interbreed. A community is all the populations of all the different species living in that same place at the same time, and the point of the word is that those populations interact: they compete for the same resources, eat one another, pollinate one another, parasitise one another. Community is a purely biotic term, so soil, temperature and water are not part of it. Communities are described by sampling them. Species richness counts how many different species are present; an index of diversity goes further and uses the number of individuals of each species as well, so it distinguishes a community with one dominant species from one where individuals are spread evenly.

What earns the marks

• Defining a population as all the individuals of one species living in the same area at the same time.
• Defining a community as all the populations of different species living in the same area at the same time.
• Stating that the populations in a community interact, with a named interaction such as competition or predation.
• Stating that data collected at a single site describe only that community and may not represent other communities.
• Describing a community using species richness and/or an index of diversity.

Common mistakes and how to put them right

• Including soil, temperature or water in the definition of a community, which describes an ecosystem instead.

• Using the word population for a mixed group containing several species.

• Concluding that results from one sampled site apply to every community of that type, when the mark scheme credits the opposite point.

• Describing a community by species richness alone when the index of diversity also needs the number of individuals of each species.

• Treating habitat and community as interchangeable words.

Exam tips

• Learn population, community, habitat, niche and ecosystem as one set and check which one the stem is actually asking for.
• In a data question, name the community exactly as the stem names it, for example the fish community at that river site.
• If asked to criticise a conclusion about a community, look first for one site, one date or one season.