A population is a group of individuals of the same species that live together in the same place, and that posess an average set of properties, such as birth rates and death rates. This definition recognizes that populations are made up of individual organisms but does not require that we know which individuals give birth or die, or where they are located in space. Instead the population is characterized by average birth and death rates, and variability in these averages is treated as a statistical property of the population. We sacrifice knowledge about the individual in order to have a practical theory that does not require us to have information about the inclination or location of individual organisms.
Most definitions of population have some kind of spatial reference. The simplest and least restrictive of these is that a population is a group of individuals of the same species that live together in a particular area (e.g., Roughgarden 1989). However, even though this definition is widely used by ecologists, it gives rise to serious difficulties and misinterpretations. A more rigorous definition should define the spatial dimension more precisely; for example, a group of individuals of the same species that live together in an area of sufficient size that all the requirements for reproduction, survival and migration can be met (e.g., Huffaker et al. 1984). The problem is, how does one define "an area of sufficient size"? Sometimes this area can be quite obvious as, for example, a population of elk inhabiting a particular drainage. In others it is less so. It may be helpful to conceive of an area of sufficient size such that the rates of emigration out of the area and immigration into the area are roughly balanced. Another approach is to imagine a circle large enough that an organism placed near its center would have a very low probability of exiting the circle during its lifetime. Whatever method is used to define the appropriate size of an area within which the population of a particular organism exists, it is important that most of the change in population size or density is due to births and deaths rather than immigration and emigration because the theory of population dynamics is based on this assumption. For a detailed discussion of the population concept, see Andrewartha and Birch (1984).
Local population is a group of organisms of the same species that live together in an area where there is a high probability of interbreeding. Also called a subpopulation or, in systematics, a deme. Emigration and immigration from local populations need not necessarily be balanced and there may be a fairly high probability of local extinction.
Metapopulation is a group of populations that share occasional migrants.
Absolute population is an estimate of the total number of organisms in an area.
Population density is an estimate of the number of organisms per unit area (e.g., hectare) or unit of habitat (e.g., kilogram of soil).
Relative population is an estimate of the number of organisms caught in nets or traps but which cannot be related to area in any way.
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©1997 Alan A. Berryman