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WWRC 92-18
Population Viability Analysis


Population viability analysis (PVA) is a process. It entails evaluation of data and models for a population to anticipate the likelihood that a population will persist for some arbitrarily chosen time into the future (125, 128). A closely related concept is minimum viable population (MVP) analysis. An MVP is an estimate of the minimum number of organisms of a particular species that constitutes a viable population. Reference is also made to population vulnerability analysis which is a negative appellation for PVA. PVA embraces MVP, but without seeking to estimate the absolute minimum population necessary to keep a species viable (136).

In the United States, the US Forest Service has a mandate to preserve viable populations on its lands under the National Forest Management Act (158). Likewise, the US Fish and Wildlife Service and the National Marine Fisheries Service have been evaluating PVAs for many species or populations proposed for listing under the Endangered Species Act (152). Establishing criteria for what constitutes a viable population is no longer strictly an academic pursuit.

PVAs have been attempted for at least 35 species; perhaps the most celebrated are those for the grizzly bear,(Ursus arctos horribilis) (126, 129, 144), and the northern spotted owl (Strix occidentalis caurina) (18, 79, 95, 98a). Most PVAs are simulation studies that remain unpublished, or when published, they may only include outlines of model structure (95, 126, 131). Others invoke analytical methods or "rules of thumb," always burdened with severe assumptions (31, 152). PVAs vary according to the ecology of the species, the expertise of the modelers, and the extent of available data.

There are no guidelines on what constitutes a valid PVA, and because each case is unique, I am loathe to devise any. Any attempt is qualified that involves a population simulation or analysis with the intent of projecting future populations or estimating some extinction or persistence parameter, e.g. time to extinction, probability of extinction, probability of persisting for 100 years, etc. Definitions and criteria for viability, persistence, and extinction are arbitrary, e.g. ensuring a 95% probability of surviving for at least 100 years. Discussion of such criteria can be found in Mace & Lande (92) and Thompson (152).

Collecting sufficient data to derive reliable estimates for all the parameters necessary to determine MVP is simply not practical in most cases. It is further questionable how well ecologists can predict the future (34), particularly over time horizons necessary to project extinctions. On the other hand, Soule (l36) suggests that managers have the right to expect population biologists to project the number of animals necessary to ensure the long-term viability of a population. But to do so has proven to be dangerous ground (83) which risks damaging the credibility of conservation biologists.

My purpose in this review is an attempt to niace PVA and MVP on more comfortable ground by identifying a realistic domain. I maintain that PVA ought to be an integral part of any species management plan, but rather than being so presumptuous as to claim that we can actually use modeling to define a MVP, or to estimate the probability of extinction, I use it as a forum to champion the adaptive management approach of Holling (67) and Walters (162). For those all-too-frequent cases that cannot wait for a full-blown PVA, I review empirical evidence suggesting that use of rules-of-thumb for MVPs may not be unrealistic.

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