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Jennifer B. Hughes,* Gretchen C. Daily, Paul R. Ehrlich
Genetically distinct populations are an important component of biodiversity. This work estimates the number of populations per area of a sample of species from literature on population differentiation and the average range area of a species from a sample of distribution maps. This yields an estimate of about 220 populations per species, or 1.1 to 6.6 billion populations globally. Assuming that population extinction is a linear function of habitat loss, approximately 1800 populations per hour (16 million annually) are being destroyed in tropical forests alone.
Much of the current scientific and public concern over the extinction crisis centers on the loss of species globally (1). Most of the benefits biodiversity confers on humanity, however, are dependent on large numbers of populations of species, because each population ordinarily provides an incremental amount of an ecosystem good or service. Examples of these goods and services are seafood, timber, water purification, generation of soil fertility, pest control, mitigation of floods and droughts, and regulation of biogeochemical cycles (2). Populations also supply the genetic diversity that is crucial for the development and improvement of pharmaceuticals and agricultural crops (3).
Here we make a crude first approximation of population diversity (defined as the number of populations on the planet) and then estimate the extinction rate at this level of biodiversity. We reviewed the literature on population differentiation from a variety of taxa and estimated the average number of mendelian populations per unit area for a species. We then estimated the average range size of a species from a sample of distribution maps. The product of these two numbers is an approximation of the average number of populations per species, which, multiplied by the total number of species, yields an estimate of the number of populations on Earth (4).
Populations are normally defined as geographical entities within a species, distinguished either ecologically or genetically (5). We adopted the genetically based definition, or mendelian population (6), defined here as a group of individuals evolving independently of other groups because of limited gene flow and genetically distinguishable from other populations.
To estimate the number of populations per unit area, we searched 15 journals from 1980 to 1995 for genetic studies on population...