minimum viable population

minimum viable population (MVP), ecological threshold that specifies the smallest number of individuals in a species or population capable of persisting at a specific statistical probability level for a predetermined amount of time. Ecologists seek to understand how large populations must be in order to establish population-size benchmarks that help to keep species from going extinct. These benchmarks can vary according to species and according to measures set by the ecologist performing the study, but one that is commonly used in ecological investigations involves establishing greater than 95 percent probability of survival for more than 100 years. Since different species have different life spans, however, a time benchmark of 40 generations also may be used, especially when making comparisons between species.

Estimating MVP

Estimates of MVP have their greatest value in the field of conservation biology, which combines genetic and ecological theories to address global declines in biodiversity. One of the goals of conservation biology is to prevent extinction, which requires managing the small populations that are at greatest risk. To manage such endangered species over decades and centuries, researchers must identify the MVP necessary for the species’ long-term survival. Although ecologists have attempted to define a general MVP estimate that can be applied to numerous species for the purpose of simplifying ecological management, research shows that the MVP estimate for one species differs from that of another because of differences in reproductive rates, habitat requirements, and other factors.

The probability for long-term persistence of a species depends on whether the species can avoid the erosion of genetic variability that can occur in small populations. When genetic variation is reduced, the ability of a species to adapt to environmental change may become restricted. In small populations the genetic diversity of the gene pool may be reduced further by limited mating opportunities, such as when only low numbers of adults or adult members of one sex or the other are present. In these cases, genetic variability can be substantially reduced through inbreeding (mating between close relatives) and genetic drift (random changes in gene frequencies). Inbreeding and genetic drift both can result in an increased chance for the transmission of harmful traits to subsequent generations, which ultimately affects population and species viability (see population ecology).

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One of the earliest attempts to define a minimum lower threshold that would prevent the loss of genetic variability in a species was made in 1980 by Australian geneticist Ian Franklin and American biologist Michael Soulé. They created the “50/500” rule, which suggested that a minimum population size of 50 was necessary to combat inbreeding and a minimum of 500 individuals was needed to reduce genetic drift. Management agencies tended to use the 50/500 rule under the assumption that it was applicable to species generally. Many experts, however, questioned its validity.

With advances in technology and mathematical theory throughout the 1970s and ’80s, a computer simulation model known as population viability analysis (PVA) was developed to estimate the MVP of a species. The method was later found to be useful for providing more-sophisticated estimates of extinction risk and long-term persistence. PVA can be customized by the researcher to incorporate various data related to a natural history of the species, including its reproduction and dispersal behaviour (movement of individuals among populations). Researchers can also incorporate into their PVA studies factors related to the current genetic context of a species (such as evidence of inbreeding depression, which is the overall decrease in ecological fitness as a result of inbreeding).

In general, the results of PVA modeling indicate that species with high reproductive capacities, such as arthropods and rodents, can accommodate lower MVPs than species with lower reproductive capacities, such as redwood trees and large mammals and some birds. High MVPs typically are found for species that are sedentary (e.g., trees), that do not breed until individuals are several years old, that have mating behaviours in which only a few individuals account for most of the mating, or that show high levels of inbreeding (such as elephants, California condors, and cheetahs).

The PVA model also incorporates environmental and demographic stochasticity. Environmentally stochastic events are random events, such as severe weather, floods, fires, and other ecological disturbances. Demographically stochastic events are random fluctuations in population variables, such as sex ratios and number of births or deaths. Depicting such events with PVA has the effect of increasing the model’s MVP estimate, because both types of phenomena have the potential for reducing population size, either by increasing the death rate or increasing the annual variability with respect to the birth rate.

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Estimating MVP with PVA allows scientists to determine which biological parameters (e.g., hunting pressure, disease, habitat loss, inbreeding) will have the greatest impact on the extinction probability of a given species. This information can provide environmental managers with a set of quantitative targets for the minimum critical area required to support a viable population.

One major limitation of PVA is that it requires large amounts of data to make realistic predictions. Therefore, some researchers argue that using a single, universal MVP (such as the 50/500 rule) would streamline conservation efforts. Others, however, maintain that MVPs must be carried out in a case-by-case fashion, because the circumstances that characterize extinction risk differ among species.

MVP and species management

A classic example of an early management plan based on estimates of MVP is that for the northern spotted owl (Strix occidentalis caurina), which is found in the coniferous and mixed-hardwood forests of the northwestern United States and of British Columbia. The owl depends on old-growth trees with hollows for nesting sites, but heavy logging in the region during the 1970s and ’80s reduced its nesting habitat. In 1986 several environmental groups sought to add the owl to the U.S. endangered species list, demanding protection for the owl and its remaining habitat. Conservationists used the mandate for maintaining MVPs found in U.S. Forest Service (USFS) regulations associated with the National Forest Management Act of 1976 to force the USFS to maintain sufficient old-growth forest to support an MVP of northern spotted owls. In 1990 the northern spotted owl was granted protection as a threatened species under the Endangered Species Act, and logging companies were forbidden from removing more than 60 percent of old-growth forest within 2.1 km (1.3 miles) of each nesting site.

Since then, owl-management plans have added PVA data-collection variables into their monitoring, and studies performed during the first part of the 21st century showed that spotted owl populations continued to decline across their range because of habitat loss and competition with barred owls (S. varia). Some researchers claimed that the earlier-implemented management plan, which focused on habitat maintenance and restoration, helped prevent the northern spotted owl from going extinct, and they suggested that an additional measure focused on control of the barred owl would help to stabilize the northern spotted owl’s population.