Empirical estimates of minimum viable population sizes for primates: tens to tens of thousands?

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

An important goal for conservation is to define minimum viable population (MVP) sizes for long-term persistence of a species. There is increasing evidence of the role of genetics in population extinction; thus, conservation practitioners are starting to consider the effects of deleterious mutations (DM), in particular the effects of inbreeding depression on fitness. We sought to develop methods to account for genetic problems other than inbreeding depression in MVP estimates, quantify the effect of the interaction of multiple genetic problems on MVP sizes, and find ways to reduce the arbitrariness of time and persistence probability thresholds in MVP analyses. To do so, we developed ecoevolutionary quantitative models to track population size and levels of genetic diversity. We assumed a biallelic multilocus genome with loci under single or multiple, interacting genetic forces. We included mutation-selection-drift balance (for loci with DM) and 3 forms of balancing selection for loci for which variation is lost through genetic drift. We defined MVP size as the lowest population size that avoids an ecoevolutionary extinction vortex. For populations affected by only balancing selection, MVP size decreased rapidly as mutation rates increased. For populations affected by mutation-selection-drift balance, the MVP size increased rapidly. In addition, MVP sizes increased rapidly as the number of loci increased under the same or different selection mechanisms until even arbitrarily large populations could not survive. In the case of fixed number of loci under selection, interaction of genetic problems did not always increase MVP sizes. To further enhance understanding about interaction of genetic problems, there is need for more empirical studies to reveal how different genetic processes interact in the genome.

The minimum viable population (MVP) size has been compared for a wide range of organisms in conservation biology, but a limited number of studies investigated it for freshwater fishes, which exhibit diverse life history strategies. In this study, the MVP size and population growth rate of 36 fish species in the Yangtze River were estimated and compared with their life-history traits. The results indicated that the MVP size ranged from 42 to 320 individuals, and instantaneous per-capita population growth rate ranged from 0.009 to 0.188 per year. MVP size and population growth rate were significantly associated with three life history traits: the age at maturity, generation time, and fecundity. Long-lived species with delayed maturation, long generation time, and high fecundity had a greater MVP size and a lower population growth rate than short-lived species. Therefore, our results emphasize a need for prioritizing our conservation effort more on long-lived species.

There are two broad concepts of a minimum viable population (MVP) size. The first is a genetic concept, based on the rate at which genetic variation in a population is lost, and hence fitness decreased, through random genetic drift. The second is a demographic concept and is concerned with the probability of complete extinction of a population through random demographic forces. Although at an overall level these concepts are related, since inbreeding decreases fecundity and increases the death rate, present theory treats these as distinct concepts, since normal practice has been to assume the population size constant in defining and calculating the genetic MVP. For convenience, we also preserve this distinction in this chapter, and note that until a generalized theory covering both concepts is attempted, confusion may arise by the loose transfer of a numerical value of the MVP from the genetic to the demographic case, particularly the genetically derived values offered by Franklin (1980) and Soule (1980). For each of these two MVP concepts, the numerical value for the MVP eventually reached will depend on two assumptions. The first is the criterion chosen to define an MVP; for example, using the demographic concept, the size of the population which guarantees 95% probability of survival for y years clearly depends on the value chosen for y .

In Memoriam : Edward O. Wilson (1929-2021): It All Started with Ants.

Estimates of minimum viable population sizes for vertebrates and factors influencing those estimates

Minimum viable population size: A meta-analysis of 30 years of published estimates

A procedure is described to estimate viable population size and the area of habitat needed to support an endangered stream fish population. Monte Carlo simulations were used to evaluate population fates with stochastic demography and random variation simulated to cause age class 0 failures in some years. Viable population was defined in this paper to be large enough to have less than a 10% chance of extinction in 100 years and to have a long‐term effective size of at least 500 breeding adults, although the method could be applied for any assumed extinction rate and effective size. Using data for Rio Grande cutthroat trout (Oncorhynchus clarkii virginalis) in an age class model, it was inferred from simulations that minimum viable population size was 2750 fish, which would require 2.2 ha of habitat at median density of the subspecies in New Mexico streams. Minimum viable population size occurred at the highest survival rate of young of year and no population‐wide year class failures. Viable population size, and hence required habitat size, increased as the failure rate for age class 0 increased or when the survival rate from age 0 to 1 declined. This suggested that managers should avoid managing for smallest possible viable population size and instead plan for much larger population sizes to accommodate temporal variation in demography and habitat quality. Decreased survival rate of young of year caused the stable age class distribution to be skewed toward the age class 0, which profoundly reduced effective population size. This suggested that habitat restoration that improves survival of young of year would be a good strategy to increase effective size over the long term. Estimating required habitat size could improve introductions of fish to create new populations. Although requiring time‐consuming simulations, the procedure can be used to estimate required habitat size for any species for which population and demographic data are available. Copyright © 2007 John Wiley & Sons, Ltd.

Using ecological theory to develop recovery criteria for an endangered butterfly

Estimating the minimum viable population size of kaka (Nestor meridionalis), a potential surrogate species in New Zealand lowland forest

Management of nature reserves, of multiple-use lands, and of captive breeding programs requires knowledge of the minimum population sizes below which the combined effects of random genetic changes and demographic variation would likely result in extinction. One prerequisite to estimating such minimum viable population sizes is the determination of the effects of inbreeding on fitness. Two hypotheses make distinct predictions about the relative tolerance of populations to inbreeding: If inbreeding depression results primarily from the expression of deleterious recessive alleles, then selection would have removed most such genes from populations with long histories of inbreeding, and those populations would be resistant to further inbreeding impacts. If inbreeding depression occurs because of a general selective advantage of heterozygosity throughout the genome, then previously inbred populations would have reduced fitness presently and would fare no better under future inbreeding than would large and heterogeneous populations. We tested the hypothesis that small, isolated populations of Peromyscus mice would show less depression in fitness when inbred than would large, central populations. Remnant, insular populations had one-quarter to one-third the genie diversity of large, central populations. Although the populations varied greatly in the rate of loss of fitness (measured as infant viability) when experimentally inbred, the severity of inbreeding depression did not correlate with initial genie diversity of the stocks or, therefore, with the size and degree of insularity of the wild populations. Neither simple theory of inbreeding depression could account for the varied responses of the populations. It remains an important task for conservation biologists to discover phylogenetic, ecological, or genetic predictors of genetically minimum viable population sizes.

The concept of minimum populations of wildlife and plants has only recently been discussed in the literature. Population genetics has emerged as a basic underlying criterion for determining minimum population size. This paper presents a genetic framework and procedure for determining minimum viable population size and dispersion strategies in the context of multiple-use land management planning. A procedure is presented for determining minimum population size based on maintenance of genetic heterozygosity and reduction of inbreeding. A minimum effective population size (N e ) of 50 breeding animals is taken from the literature as the minimum shortterm size to keep inbreeding below 1% per generation. Steps in the procedure adjustN e to account for variance in progeny number, unequal sex ratios, overlapping generations, population fluctuations, and period of habitat/population constraint. The result is an approximate census number that falls within a range of effective population size of 50–500 individuals. This population range defines the time range of short- to long-term population fitness and evolutionary potential. The length of the term is a relative function of the species generation time. Two population dispersion strategies are proposed: core population and dispersed population.

Shoemaker et al.’s (2013) estimate of the minimum viable population size (MVPS) of the bog turtle (Glyptemys muhlenbergii) is 3 orders of magnitude below population sizes typically viewed as minimally viable (Reed et al. 2003; Trail et al. 2007). Even Flather et al. (2011), who question the utility of MVPS for conservation planning, concede that long-term persistent populations will require thousands of individuals. Rather than conclude their result is incorrect, Shoemaker at al. suggest that MVPS has been overestimated for long-lived species. Although we agree in principle that small populations, especially of long-lived species, have conservation value, we do not agree that Shoemaker et al.’s result provides evidence that small populations are viable. This position is inconsistent with existing empirical evidence of persistence of small populations (Simberloff & Gibbons 2004; Fagan et al. 2005; Fagan & Holmes 2006). We suggest that Shoemaker et al.’s estimate is a gross underestimate, emanating from their definition of viability, which is too narrow, not biologically meaningful, and ignores factors such as demographic and environmental stochasticity, loss of genetic variability, and catastrophes. Their conclusion that populations of long-lived species can be orders of magnitude smaller than currently believed does a disservice to species conservation and raises the longterm discussion in conservation biology about whether we should be focusing on the minimum for management. Shoemaker et al. used too short a time frame, 100 years. Frankham and Brook (2004) and O’Grady et al. (2008) identify the importance of setting the proper time frame for conservation questions and point out that generation length is often the appropriate scale. It can be shown easily that even a strongly declining population can be viable over the short time frame of 10 generations ( 100 years for long-lived species) and that the

Effects of patch connectivity and arrangement on animal metapopulation dynamics: a simulation study

How does the 50/500 rule apply to MVPs?