Population viability analysis

This chapter incorporates stochastic dynamics and statistical uncertainty into population viability analysis using population prediction intervals. For most species insufficient information exists to construct an accurate quantitative population model, but qualitative assessment of extinction risk can be performed using objective, population-based systems such as the International Union for Conservation of Nature and Natural Resources (IUCN) Red List criteria. IUCN lists substantial proportions of animal taxa as threatened with extinction during the next 100 years. Quantitative population viability analysis (PVA) uses a stochastic population model to estimate the probability of extinction of a population or species before a certain time. A population prediction interval (PPI) is the stochastic interval that includes the unknown population size at a specified future time with a given probability or confidence. The concept of population viability can be generalized to incorporate uncertainty and the Precautionary Principle, by employing the upper PPI with a specified probability. For example, the IUCN category of Vulnerable is defined by Criterion E as a 10% probability of extinction within 100 years. With uncertainty this naturally generalizes to categorizing a species as Vulnerable if the upper 90% PPI at 100 years includes extinction. As a direct statement about a future population size, a PPI is easier to interpret and communicate to population managers and decision makers than the alternative approach of estimating a confidence interval for the probability of extinction at a future time.