Geoscience Reference
In-Depth Information
Chapter 9
Population Viability Analysis: Data Requirements
and Essential Analyses
GARY C. WHITE
The biological diversity of the earth is threatened by the burgeoning human
population. To prevent extinctions of species, conservationists must manage
many populations in isolated habitat parcels that are smaller than desirable. An
example is maintaining large-bodied predator populations in isolated, limited-
area nature reserves (Clark et al. 1996).
A population has been defined as “a group of individuals of the same
species occupying a defined area at the same time” (Hunter 1996:132). The
viability of a population is the probability that the population will persist for
some specified time. Two procedures are commonly used for evaluating the
viability of a population. Population viability analysis (PVA) is the method of
estimating the probability that a population of a specified size will persist for a
specified length of time. The minimum viable population (MVP) is the small-
est population size that will persist some specified length of time with a speci-
fied probability. In the first case, the probability of extinction is estimated,
whereas in the second, the number of animals that is needed in the population
to meet a specified probability of persistence is estimated. For a population
that is expected to go extinct, the time to extinction is the expected time the
population will persist. Both PVA and MVP require a time horizon: a specified
but arbitrary time to which the probability of extinction pertains.
Definitions and criteria for viability, persistence, and extinction are arbi-
trary, such as a 95 percent probability of a population persisting for at least 100
years (Boyce 1992). Mace and Lande (1991) discussed criteria for extinction.
Ginzburg et al. (1982) suggested the phrase “quasi-extinction risk” as the prob-
ability of a population dropping below some critical threshold, a concept also
