Imagine a lake that is full of perch. Each perch individual belongs to the species Perca fluviatilis. All the fish that belong to this species and live in the lake form the perch population of the lake. The individuals that belong to the same population are capable of reproducing with each other. In contrast, the perch from two different lakes belong to two different populations, as they cannot move between the two lakes.

The lake is also the home of many other species that form their own populations. All of these different populations living in the same enviroment form the lake's organism community. In communities, the different populations interact with each other.