A viable population is a population capable of maintaining itself, without significant manipulation.

Impetus for interest: The National Forest Management Act (1976) required that all forests maintain "viable" populations of all vertebrate species on their lands.

A minimum viable population is an estimate of the smallest viable population that will persist

- for a specified length of time (e.g., 500 yr) and
- with a specified level of certainty (e.g., 95%).


Growth rate (r)

Modeling points to the importance of r and its variance.

How the population is regulated.

A. Density-independent factors

- kill the same proportion of a population at all densities (e.g., catastrophic mortality related to weather)

B. Density-dependent factors

- kill a larger proportion of a population at higher densities (e.g., shortage of resources, predation, parasitism, disease increasing intraspecific social interaction)

Small populations are fragile because of chance events, or stochasticity.

- Environmental stochasticity (e.g., weather)
- Demographic stochasticity (e.g., sex ratio)

Simulation models can be used to estimate persistence times

(E.g., spotted owls, acorn woodpeckers)


PVA is the study of the ways in which habitat loss, environmental uncertainty, demographic stochasticity, and genetic stochasticity interact to determine extinction probabilities for an individual species.

Note: all populations go extinct; the question is whether they are going extinct due to natural or human-caused factors


Rate of decline in S (# Species) depends on:

- area
- degree of isolation
- time since isolation


A. Demographic Uncertainty - uncertainty resulting from the effects of random events on survival and reproduction of individuals (e.g., skewed sex ratio of dusky seaside sparrow in 1980; six individuals remained and all were male)

1. Variation in birth and death rates

2. Variation in size of reproducing population

3. Metapopulations

a. extinction and colonization rates
b. rescue effects

4. Dynamics of source and sink populations (immigration & emigration)

B. Genetic Uncertainty - Genetic diversity is necessary for populations to adapt to environmental change. Loss of genetic diversity reduces future evolutionary options.

1. Causes of loss of genetic diversity

a. habitat loss and reduction in population size
b. genetic drift (random changes lead to fixation of alleles)
c. inbreeding depression from fixation of deleterious alleles

2. Actual population size vs. effective population size (Ne)

Ne is the size of an "ideal" population that would have the same amount of genetic drift as the actual population

Ne affected by

- variance in number of offspring

Ne =        4N          , approximately
      2 + sigma squared

where N is the number of breeding adults and sigma squared is the variance of the number of offspring contributed to the next generation by each mated pair

- sex ratio among breeders (skewed in species with harems, for example)

Ne = 4 Nm Nf 
     Nm + Nf

- population fluctuations

1/Ne = (1/N1 + 1/N2 + ... 1/Nt)/t

where t is the number of generations over which Ne is calculated

Ne often a fraction of the actual size

3. Genetic structure and movement among populations

Structured populations lose genetic variation over time more quickly than panmictic ones of the same total size.

(1 migrant/generation between populations will maintain similar allele frequencies)

4. Measuring genetic diversity

Heterozygosity, the average proportion of alleles that are heterozygous, is a common index of genetic variation withing a population.

F-statistics quantify how much genetic variation is within and among (genetic structure) populations.

C. Environmental Uncertainty - unpredictable events such as changes in weather, food supply, competitors, predators, or parasites (temporal and spatial component).

D. Natural Catastrophes - extreme cases of environmental uncertainty (floods, fires, etc.; infrequent in time, short in duration, but widespread in impact)


Generally, genetic and demographic uncertainty are only important in affecting viability of very small populations (e.g., endangered vertebrates). Demographic uncertainty usually kicks in first.

Invertebrates often have a different set of problems in that they are restricted to a few habitat patches, but have high population densities within those patches; in these cases PVA's should emphasize environmental uncertainty and the effects of catastrophes.

In general, PVA's cannot be conducted for a single isolated population. Regardless of the species, analyzing its distribution across the landscape and its mechanisms for dispersal are important. Linking subpopulations (e.g., through the use of corridors) is integral for most species.

Last Updated on November 18, 1998 by Rolf Koford