Life on Earth faces a crisis of historical and planetary proportions (Pimm and Brooks, 1997). Unsustainable consumption in many northern countries and crushing poverty in the tropics are destroying wild nature. Biodiversity is besieged. Extinction is the gravest aspect of the biodiversity crisis: it is irreversible. While extinction is a natural process, human impacts have elevated the rate of extinction by at least a thousand, possibly several thousand, times the natural rate (Pimm and Brooks, 1997). Mass extinctions of this magnitude have only occurred five times in the history of our planet; the last brought the end of the dinosaur age (Jablonski, 1986).
The problem of stemming the extinction crisis can best be framed by a question: In which areas would a given dollar contribute the most towards slowing the current rate of extinction? To accomplish this we first need to understand species’ distributions. This requires that we measure endemism: the degree to which species are found only in a given place. This can be thought of as a measure of “irreplaceability”. Since endemic species cannot be found anywhere else, the area where an endemic species lives is wholly irreplaceable. Our ultimate goal is to keep nature intact, which means that we must stop anthropogenic species extinctions. To approach this goal, we must slow the rate of species extinction as much as possible with whatever conservation resources we have at our disposal, which requires incorporating threats (or “vulnerability”) and costs into priority setting. Generally, the more threatened an area is, the more it will cost to conserve. However, because economic opportunity costs vary dramatically, there do still exist areas of relatively low cost all over the globe.
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