An isolated population of wolves living on an island in Lake Superior is down to just three animals, leading conservationists to express concern for the lone group's survival.

Just two adults and one pup make up the last remaining pack on Michigan's Isle Royale National Park, say researchers at Michigan Technological University conducting their yearly Winter Study.

Wolves and moose on Isle Royal have been the subject of 57 continuous years of observation in the world's longest running prey-predator study.

In addition to the three wolves and an estimated 1,250 moose, observers noted two visiting wolves that came to the island over a temporary ice bridge but later returned to the mainland. Isle Royale is one of the islands in the park of Michigan's Lake Superior.

Researchers say they have tracked a growing gap between the wolf predators and their moose prey, with wolf numbers dropping from 24 in 2006 to just the three recently spotted. There were 9 wolves counted last winter.

And of those three there is concern over the pup, which at 9 months old appears sickly, displaying a hunched posture and deformed tail.

"Those observations suggest that the pup is not well off," says study leader John Vucetich, an MTU wildlife ecologist. "It would not be surprising if the pup was dead a year from today."

The pup's condition is probably the result of ongoing inbreeding, which researchers say has been largely responsible for the pack's diminishing numbers. The wolf numbers fell after 2006 when the moose population declined. By 2011, only 15 wolves were alive, with just two adult females in the population.

With just three survivors this winter, a natural recovery of the pack is not likely without the introduction of new genetic material, they say, but even that would be difficult.

"There is now a good chance that it is too late to conduct genetic rescue," Vucetich says.

The visiting wolves, which could have been a source of that genetic improvement, only stayed on the island for two weeks, leaving over the same ice bridge they came on, he points out.

Even if the two visiting wolves had been aware of the presence of Isle Royale wolves, which is far from certain, there is no guaranteed genetic rescue would have happened, he says, because the adult wolves on the island are probably a mated pair who would not be interested in mating with the visitors.

Meanwhile, with the decline in the wolf population has come an associated boom in numbers of moose on the island, which are thriving in the absence of predators.

"It's not the presence of wolves that matters so much, it's whether wolves are performing their ecological function," Vucetich explains.

The increase in the island's moose population, now that the balance of predator to prey has clearly tipped, could have a negative impact on the island's forest vegetation, he says.

 "Concerns remain that the upcoming increase in moose abundance will result in long-term damage to the health of Isle Royale's vegetative community," he says.

Scientists have to consider whether to reintroduce wolves to the island, or to leave nature take its course and see what happens.