Common sense dictates that the smaller the size of a species population, the more vulnerable it is to extinction. Strictly speaking, though, no species population has a 100% probability of surviving over any period of time. A severe, unanticipated catastrophe can wipe out the largest population of a species. Even if a catastrophe does not strike a small population, chance variations in births and deaths can drive it to extinction.
Mark Shaffer thus emphasized that the continued survival of a population can be expressed only in probabilistic terms within a certain time frame. The "minimum viable population" (MVP) size for a species thus depends on one's definition of the MVP. To the biologist who takes a long-term view of the evolutionary potential of the species, the MVP may be one that has a 99% probability of surviving for 1,000 years. A reserve manager, on the other hand, may be willing to commit resources to conserving a population that has a 90% chance of persisting for 100 or even 50 years. Shaffer further pointed out that there were four major considerations for estimating MVP: (1) demographic sto-chasticity, chance variations in births and deaths; (2) environmental stochastic-ity, which arises from interannual fluctuations; (3) genetic stochasticity, the loss of variation from drift (chance); and (4) catastrophes, such as disease epidemics or major climatic events.
Population viability analysis (PVA) is a process that evaluates data and models for a species population to estimate a probability that it would survive for a chosen period of time into the future. A PVA using computer simulations also helps determine the MVP for a species in accordance with an agreed-upon definition. Related to this is the viability of the habitat for the elephant; the two processes can be combined into population and habitat viability analysis (PHVA), an approach championed by the IUCN/SSC Conservation Breeding Specialist Group.
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