Why Do Populations Matter?
Populations matter because they are the fundamental units of species management. A population is an interbreeding group that has a degree of reproductive isolation and demographic cohesiveness. Population members share key demographic features, including fecundity (i.e., individual reproductive output), sex ratio, survivorship, and recruitment. In fishery science, these populations may be called stocks, and in conservation, they are often termed management units. The terms may not be synonymous, but they all entail the key feature of reproductive and demographic independence. Isolation of populations can be spatial, temporal, or behavioral. Nesting populations may aggregate with others during periods of their life history but retain their integrity when breeding. In that case, the nesting populations form a metapopulation—a group of interconnected populations that have some genetic exchange (Kritzer and Sale, 2006). A metapopulation may also qualify as an evolutionarily significant unit in conservation; a distinct population segment (DPS) under the Endangered Species Act; and a regional management unit (RMU), a term developed to fit the natural history of sea turtles (Wallace et al., 2009a, b). Populations and metapopulations are also important because they are potential reservoirs of genetic diversity that retain local or regional adaptations (Jones, 2006) in that they may evolve somewhat separately, providing a source of genetic diversity that can give a species greater resilience in the face of environmental challenges.
Population genetic studies in migratory marine animals have emphasized one important lesson in the last decade: primary sampling needs to be at or as close as possible to the breeding and birthing site because these samples are not subject to dispersive life-history stages that may confound population-genetic analyses. For example, genetic surveys of bluefin tuna (Thunnus thynnus) across the North Atlantic yielded contradictory results but no consistent evidence of population structure. However, when young-of-year were sampled near the spawning areas, substantial evidence of population structure emerged, indicating homing by reproductive adults (Carlsson et al., 2007). Such population separation is obscured on feeding grounds by mixing of multiple populations.