To inform conservation policy, scientists rely on a measure known as minimum viable population (MVP)—the smallest population size required for a species to persist over a given interval of time. The MVP threshold commonly used to assess the long-term persistence for any species is 5,000 adult individuals. Once the number of individuals in a population drops below this threshold, the population’s risk of extinction increases and policies to protect the population are considered. But a recent study, in which scientists reexamined the applications of the MVP concept, has challenged the utility of the threshold figure and its generalization to all threatened species.

The study, published in the journal Trends in Ecology and Evolution, determined that no single population size can be used as a general guideline to save endangered species and that, in fact, population sizes required for long-term viability vary widely. As an example, in the early 1800s several billion passenger pigeons thrived in North America, but in 1914 the species went extinct, having been overhunted, slaughtered, and overexploited by humans. For other species currently undergoing similar declines, at such a blistering pace, considering conservation policies once a population reaches a threshold of 5,000 may be too late, providing insufficient time to mitigate major threats to a species' survival or to even reach consensus on and enact protection policies.

Controversy surrounding MVP and the concept of a target population size for species conservation is not new. Implemented following the National Forest Management Act of 1976, the measure was criticized in the following decades by scientists who argued that small populations of wild species could be viable in the long term, if afforded legal protection, and that even large populations could be pushed to the brink of extinction over a relatively short interval of time when faced with threats associated with human activity. In addition, while the MVP threshold does take into account generalized, foreseeable variations in environmental, demographic, and genetic factors, it does not consider the precise impact of human activity on a species' survival, nor does it consider species’ life history or taxonomy—factors that can influence extinction risk.

Still, today the MVP threshold is used widely as a guideline for understanding extinction dynamics. For instance, it is used by the IUCN for its Red List of Threatened Species, one of the best-known assessment systems for classifying the status of threatened species. Indeed, simply because the MVP concept provides a general target for species recovery, which could otherwise be difficult and time-consuming to determine for each threatened population, it can help conservationists and policy makers prioritize species recovery efforts.

Although the utility of the MVP concept is likely to remain a source of controversy in the years to come, the latest study serves as a reminder that population size is not a safeguard against extinction, that not even large a population—like that of the passenger pigeon—is guaranteed long-term survival in the context of increasing human activity. Furthermore, species extinction is inevitable if population viability requirements are ignored by policy makers or if conservation efforts fail to mitigate the pressures causing a population's decline. Hence, MVP assessments form only part of the equation for conservation efforts. Planning, cooperation, and, perhaps above all else, recognition of the importance of species biodiversity to human life and the health of our planet all are needed for conservation efforts to prove successful.
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Kara Rogers is the senior editor of biomedical sciences at Encyclopaedia Britannica, Inc. She is also a member of the National Association of Science Writers and a contributor to the Britannica Blog, where she runs a series called Science Up Front. She holds a Ph.D. in Pharmacology/Toxicology, but enjoys reading and writing about all things science. You can follow her on Twitter at @karaerogers.

This post also appears on the Britannica Blog.