Minimum Viable Population Size of the Red-Cockaded Woodpecker

Abstract

We calculated a genetically based minimum viable population size of the red-cockaded woodpecker (Picoides borealis) using a formula derived from Hill (1972) and life history data from a long-term study. Based on published criteria for maintenance of genetic variability, a red-cockaded woodpecker population must contain 509 breeding pairs to be considered viable. It is likely that no existing population contains 509 breeding pairs. Genetically based estimates of population viability may not be valid, but if they are adopted as in the recovery plan for the red-cockaded woodpecker, the area required for a viable population would be >25,450 ha. The estimates of population size and area required for a viable population are considerably higher than those contained in the species' recovery plan (U.S. Fish and Wildl. Serv. 1985). J. WILDL. MANAGE. 52(3):385-391 A viable population is self-sustaining over a long period. Population viability is a critical issue in conservation, but how viability should be assessed is unclear. The most commonly used methods are (1) determining the probability, based on demography and population size, that a population will become extinct over a predetermined number of years (Shaffer 1983, Shaffer and Samson 1985); (2) determining the size of naturally occurring, stable populations of a species (Shaffer 1981); and (3) determining if a population is large enough to maintain its genetic variability (Franklin 1980, Lehmkuhl 1984). These 3 methods address different aspects of viability and are not interchangeable. Genetic variability contributes to viability because it provides potential for populations to adapt to changing environments. An excessive loss of genetic variability therefore reduces the chances of a population persisting. In small populations, loss of genetic variability may be caused by inbreeding (Ballou and Rails 1982, Ralls and Ballou 1983) or genetic drift (Wright 1931, 1948). Franklin (1980) suggested that an effective population size (N,) of 50 would be adequate to avoid losses from inbreeding and an N, of 500 would avoid loss of genetic variability caused by drift. Frankel and Soule (1981) and Frankel (1983) provided further support for these recommendations. There are, however, serious problems in determining viable population size using genetic criteria. Although the relationship between N, and loss of genetic variability is fairly well understood, the relationship between genetic variability and population viability is not. Franklin's (1980) recommendations are based on the former relationship. Moreover, N, required to maintain genetic variability may vary from the standard of 500 individuals because of differences in inherent variability among species, demographic constraints, or evolutionary history of a population's structure (Frankel 1983, Lande and Barrowclough 1987). Therefore, Franklin's (1980) estimate of 500 should be considered an approximation subject to a variety of errors in any application. Furthermore, maintainin a certain N, does not guarantee the longterm preservation of genetic variability. Consider a s ochastic event, such as a hurricane or severe winter, that kills a large portion of the breeding population and causes poor reproduction the following year. Even if the population recovers to its original size within 2 years, the loss of genetic variability caused by the stochastic event is not recovered immediately (Franklin 1980). Such rare events are unlikely to be incorporated into calculations of N,. These considerations lead some to favor demographically based estimates of minimum viable popl ion size over genetically based estimates (Shaffer 1983), and to argue against the use of genetic models of population viability in conse vation (W. R. Dawson et al., Report of the advisory panel on the spotted owl, Natl. Audubon Soc., unpubl. rep., 1986). Although we share these reservations about genetic models of population viability, we recognize certain realities. Genetically based estimates of minimum viable population size are already being incorporated into recovery plans for endangered species. Because the National