The Princeton Guide to Ecology

Population viability analysis (PVA) is the use of quantitative models to predict future population growth and extinction risks. PVA includes a variety of methods to gauge the sensitivity of population viability to natural and human-caused impacts and to estimate the efficacy of management interventions in promoting population growth and safety from extinction. PVA began as a field that borrowed tools from basic population ecology and applied them to conservation questions. From those beginnings, PVA has matured into a discipline that drives innovations in analysis methods and tries more generally to address the processes of conservation planning and priority setting. Because of their wide usage, in particular for assessing management actions, PVA approaches have been closely scrutinized, and the field continues to refine its methods to tackle key criticisms. In summary, PVA has provided specific guidance that has aided the recovery of scores of endangered species and has helped to crystallize several general principles in conservation.

demographic stochasticity. Unpredictability through time in a population’s demography (how many individuals die, how many reproduce, etc.) caused by the randomness of individual fates. This type of stochasticity is usually important only at very small population sizes.

environmental stochasticity. Unpredictable changes through time in average demographic rates of a population. These changes can be caused by vacillations in weather, food, predators, or other biotic and abiotic forces influencing individuals in a population and can exert strong effects on the dynamics of populations.

genetic stochasticity. Unpredictable changes in gene frequencies as a result of processes such as random genetic drift. This type of stochasticity is usually important only at very small population sizes.

inbreeding depression. The decline in measures of individual performance (e.g., survival, growth, or reproduction) sometimes seen in offspring of parents that are closely related to one another.

lambda (λ). Annual population growth rate.

metapopulation. In general, a collection of populations that are connected by movement. More specifically, the term is usually reserved for a collection of populations each of which has reasonably high probabilities of local extinction and also of recolonization.

N_t. Population size in year t.

parameters. Values used to describe population dynamics in models, such as the mean or variance in fecundity or survival rate.

population viability. The probability of continued existence of a population. Viability is the converse of the risk of extinction (often defined in terms of quasiextinction rather than complete extinction) over some time period.

quasiextinction threshold (N_qe). The minimum number of individuals below which a population is likely to be critically and immediately imperiled.

The International Union for Conservation of Nature (IUCN) currently recognizes over 15,000 species as threatened with extinction worldwide (http://www .iucnredlist.org/). However, given the uncertainty surrounding the status of numerous species or even how many species exist, the number of imperiled species on a global scale is almost certainly considerably more

-521-