2 - The demography of chance extinction

Summary

With many sorts of habitats and some entire ecosystems dwindling in extent, the extinctions of many species are imminent. Attempts at saving some of these species as ecologically functioning members of more or less natural communities, rather than zoo populations, involve the establishment of reserves whose extent is very modest in comparison to the original range of the species, and which, for that reason, can only maintain comparatively small relict populations. We wish, therefore, to estimate the viabilities of these small populations, and to learn what management measures and reserve-design features will enhance their viabilities. Furthermore, since there is inevitably pressure, in a crowded world, to encroach upon reserves, we should like to estimate the minimum extent of a reserve that will suffice to confer upon a population an expected time of extinction that is, by some criterion, acceptably remote. We shall scale this measure of reserve extent in units of population size. This is the minimum viable population problem.

At the most elementary level, the minimum viable population problem can be framed in demographic terms; but the magnitudes of the demographic variables will depend on a variety of factors, such as habitat quality, environmental variation, and genetic composition of the population (Shaffer, 1981; Soulé, 1980).