Those are among the findings in a scientific report ordered by Parks Canada, the newly appointed custodian of the historic sand crescent that lies more than 100 kilometres off the Atlantic coast of Nova Scotia.
The herd, left to fend for itself since 1960, now numbers more than 500 animals but the population could drop precipitously after just one harsh winter, with food hard to access under heavy snow or ice, says the study.
The ponies also may suffer from low genetic diversity, making them less resilient to disease and prone to reproductive failure.
The combination of factors raises the prospect of extinction for the feral horses, a romantic element of Canada's storied past, though scientists say more research is needed to be certain.
"Obtaining this (additional) information would be relatively easy, in the form of a series of population viability analyses, to assess the level of risk of population extinction in the near future," says the $35,000 report, completed in September and obtained by CBC News under the Access to Information Act.
Graveyard of Atlantic
Last December, Parks Canada formally relieved the province of Nova Scotia of its stewardship over Sable Island, sometimes called the "graveyard of the Atlantic" because of the more than 350 ships wrecked off its wave-washed beaches in the last 400 years.
To assess its new responsibilities, Parks Canada hired Bill Freedman, a respected ecologist with Dalhousie University in Halifax, to write and edit a comprehensive science volume, drawing on expert contributors and leading-edge research.
The 400-page report examines a wide range of topics, from lichens and beetles to the number of tarballs washing ashore.
But the chapter of perhaps the broadest interest is on the horses, first introduced to the island in the 1760s and now "naturalized" or "wild" after more than 250 years.
The herd last faced extinction in 1960, when the federal government ordered all the animals removed from Sable Island, some of them apparently destined to become dog food. The move was justified as humanitarian, as many of the horses had died of starvation and other causes following a harsh winter.
But a public outcry, which included plaintive letters from schoolchildren and articles in the New York Times, prompted Prime Minister John Diefenbaker to rescind the removal order and make it illegal to interfere with the herd.
Fast forward to 2014, and the extinction threat may be back – though not through any deliberate government policy.
Freedman says many scientists regard the horses as ecologically intrusive, and best eliminated from the treeless, 30-square-kilometre island.
"I also know that what little research has been done suggests that the horses aren't the greatest threat to the ecological integrity of the place," he said in an interview.
"Society definitely isn't ready to do anything of consequence to reduce their abundance. I wouldn't recommend that Parks Canada jump into that one now – in a couple of decades, they can reconsider it."
In the meantime, if bad weather and inbreeding starts to reduce numbers to unsustainable levels, most ecologists would want a "do not resuscitate" order for the herd, Freedman says.
Biologist Ian Jones says he's not convinced the herd is headed for extinction, noting the numbers are higher than ever, and says Parks Canada needs to remove them as an invasive species that is damaging the local ecology.
Most endangered place
"The most endangered ecosystems on the planet are remote offshore islands," he said in an interview from St. John's, where he teaches at Memorial University.
"And the reason for the endangerment is the introduction of domestic animals onto these islands. … This is just the kind of place national parks were intended to preserve."
Jones slams Parks Canada for allowing the horses to remain, accusing the agency of using weasel words such as "wild" and "naturalized" to disguise the invasive, damaging nature of the herd.
"Parks Canada is completely out to lunch in relation to the science," he said, noting that some bird species are directly threatened by the heavy hooves and foraging of the horses.
Parks Canada did not respond to a request for interviews, but sent an email saying the Freedman document "will inform the upcoming management plan (for Sable Island), which will also be based on public consultation and other science-based input from the research community."
Spokeswoman Theresa Bunbury said the agency is still reviewing the content of Freedman's report, but spoke in support of the horses remaining.
"The horses have been living on the island since the mid-1700s and are considered iconic to Sable Island," she said.
"The horses will be protected by Parks Canada as wildlife under the Canada National Parks Act and the National Parks Wildlife Regulations."
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