closedpopulation

A closed population is a population defined within a specific geographic area that experiences no immigration or emigration during a designated study period. In population genetics, the term also refers to a population with no gene flow from outside populations. The concept is often used as an assumption in statistical and mathematical models of population size, structure, and dynamics.

In ecology and wildlife management, closed-population models assume that the number of individuals remains constant during the sampling window, aside from births and deaths. This simplification enables certain estimators to infer abundance from encounter or capture histories. Common methods include the Lincoln–Petersen estimator for two sampling occasions and the Schnabel estimator for multiple occasions. Violation of the closure assumption—through immigration, emigration, births, or deaths—can bias estimates and lead to erroneous conclusions.

In population genetics, a closed population excludes migrants, so allele frequencies change only by genetic drift, mutation, and selection, not by gene flow. Closed populations can diverge from connected populations more rapidly, particularly when small, increasing risks of reduced genetic diversity and inbreeding.

Implications for conservation and management include heightened extinction risk for small, isolated populations and considerations regarding local adaptation versus genetic diversity. Assessing whether a population is truly closed is challenging; researchers employ genetic markers, assignment tests, and migration-rate estimates to evaluate openness. Related concepts include open populations and metapopulations, which acknowledge ongoing movement of individuals between subpopulations.