Topic 19.4 · Core
Population, Community and Ecosystem
These three words get used loosely in everyday speech, which is exactly why Cambridge tests them precisely. Each one describes a different scale — one species, several species together, or those species plus their surroundings — and mixing them up is a quick way to lose an easy definition mark.
Ecology can be studied at several connected levels: an individual belongs to a population, different populations together form a community, and a community interacting with its environment forms an ecosystem.
Population
A population is a group of organisms of one species living in the same area at the same time. All three conditions matter together — same species, same area, same time — and dropping any one of them changes what you’re describing. All the rabbits living in one particular grassland at a particular time form a population; rabbits and foxes together in that same grassland do not, because they belong to different species. If a question describes organisms from two species, the word you want is not “population.”
Community
A community consists of all the populations of different species in an ecosystem. A grassland community might include populations of grass, insects, rabbits, birds, foxes, fungi and microorganisms all at once — a community is many populations together, not a single species, however large that one species’ population happens to be.
Ecosystem
An ecosystem is a unit containing the community of organisms and their environment, interacting together. Set alongside one another, the distinction is: a population is organisms of one species; a community is populations of different species; and an ecosystem is that community together with its environment and the interactions between them. This is also why a food web properly belongs at the ecosystem level rather than the community level on its own — the populations in it are interacting with one another specifically through feeding relationships, which is an interaction, not just a list of species present.
Keeping the three levels straight in an answer
A reliable way to check your own answer is to ask what the question is actually counting. If it’s counting individuals of one named species, you’re at the population level. If it’s describing several different species coexisting, without yet mentioning their surroundings, you’re at the community level. If it brings in the physical environment — soil, water, climate, or the interactions between organisms and that environment — you’ve moved up to the ecosystem level. That scale is also what determines how a population can change in the first place, which is covered on the population growth page.