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A study of an endangered turtle population suggests that a commonly used generic estimate for the smallest sustainable population of a species — 5,000 adults — is far too high. The finding suggests that some species, thought to be on an irreversible path to extinction, might actually be safe for generations.

The minimum viable population (MVP) is an estimate of the smallest number of individuals required for a population to survive environmental changes such as natural disasters. Researchers sometimes assume MVPs of 5,000 in deciding which species need urgent protection. But a study of the North American bog turtle (Glyptemys muhlenbergii) — which is listed as critically endangered by the International Union for Conservation of Nature — finds that just 15 breeding females are sufficient for a bog-turtle population to survive the next 100 years. The study is published this month in Conservation Biology1.

“We have to keep conservation theory somewhat grounded in reality,” says Kevin Shoemaker, a conservation biologist at the State University of New York in Syracuse, who led the study. “People in my circles have to almost laugh at the idea that bog turtle populations have to be 5,000.”

Shoemaker and his colleagues tracked several populations of the turtles — which can live for up to 70 years and breed almost every year during adulthood — in New York bogs between 2001 and 2010. The researchers found that, on average, female turtles produced nearly one baby a year, and these offspring had a 33% chance of surviving their first year of life. Using those rates of reproduction and survival, the researchers then estimated how a population would fare with various starting numbers of turtles. They found that a total population of about 40, with as few as 15 breeding females, had a more than 90% probability of persisting for 100 years.

The researchers note that they did not account for the complications of inbreeding, which can substantially lower a small population’s viability. Nonetheless, they suggest that generic MVPs should be ditched.

Other researchers suggest that population size is not the most important factor. More important are a population's dynamics and environment, says Steven Beissinger, a conservation biologist at the University of California, Berkeley. He recommends that conservation managers base decisions on factors such as the health of a species’ habitat and rates of population decline.

And oversimplified — and overestimated — MVP thresholds could erode the credibility of conservation efforts, says Philip Stephens, an ecologist at Durham University, UK.

But others argue merely for better, species-specific estimates of MVP. Tim Tear, a conservation biologist at the Nature Conservancy, a conservation-advocacy group in Arlington, Virginia, says that MVPs are useful for simplifying complex conservation management decisions. “I still think the concept is extremely valuable,” he says.