Biologists use at least 30 words for a group of animals
Population sounds like the simplest idea in biology: a bunch of the same species living in one place. In practice, biologists disagree on the definition so often that more than 30 alternative terms have sprung up, from deme to metapopulation, largely because the basic word kept proving too vague to do the job.
The word comes from Late Latin populatio, a multitude, from populus, a people. In its loosest biological sense it means individuals of one species occupying a particular space at a particular time, set apart from other groups by some break in location, genetics or demographic structure. Until the 1940s and 1950s, ecologists also spoke of a population of birds covering many species; that usage has faded, and a mix of species in one area is now called a community.
Two outlooks compete. Ecologists see individuals interacting and competing within a geographic area, and study each population as part of a wider community of interdependent plants and animals. Evolutionary biologists focus on genes and breeding: a population is a set of individuals that can interbreed, each with its own gene pool adapting over time, which makes it a central organising unit of biology. Where barriers stop groups from interbreeding, they are reproductively isolated, and a species can be defined as one or more interbreeding populations cut off in this way. Geneticists call a breeding group whose members can all pair up a gamodeme.
The extra vocabulary fills the gaps. A deme is a semi-isolated or genetically similar group; clusters or aggregations are individuals sitting closer to each other than to anyone else; a metapopulation links spatially separated groups of the same species.
Counting them is its own challenge. A complete census is rarely practical for large populations, though it can work for small, easily counted ones in applied ecology. Otherwise ecologists estimate totals from a sample of observed animals using methods such as the Lincoln index.
Source: Population (biology)
