After surviving a parvovirus epidemic, bitter winters, hunger and warfare between packs, the gray wolves that roam Michigan's Isle Royale National Park may go extinct because of what amounts to an unlucky roll of the biological dice: They're running out of females as the overall population slides.
The number of wolves on the Lake Superior island chain dropped to 16 over the past year, according to a Michigan Tech University tracking report obtained by The Associated Press. Scientists who study the predator-prey relationship between wolves and moose say the number of wolves is the lowest since 1998, when it hit 14 following a huge moose die-off that left the wolves short of food. Earlier that decade, the parvovirus-decimated total was a dozen after peaking at 50 in the early 1980s.
They've always bounced back from such calamities since their ancestors migrated to Isle Royale across an ice bridge from Canada more than six decades ago, producing new generations that defied tough odds imposed by the harsh climate and geographical isolation. But this time, their prospects for reproduction are in serious doubt. Genetic analysis of their droppings, which scientists have dutifully collected and preserved, suggests the 16 remaining wolves include just one or two adult females.
"If both of them were to die before successfully raising pups, that would be the end," John Vucetich, a Michigan Tech wildlife biologist, said Tuesday. "The population would persist for a few years but its fate would almost certainly be sealed."
Random chance appears to be the only reason for the gender imbalance, Vucetich said, adding that researchers believe the long-term ratio has been fairly evenly split. Animal populations that remain small for many years can go through periods when the male-female ratio is skewed. The Isle Royale wolf population has averaged about 23.
Their plight resurrects a question that has been debated over the years: whether to bring in more wolves from the mainland. Vucetich and Michigan Tech colleague Rolf Peterson say it's time to consider the matter more seriously in view of a recent discovery: A male wolf made its way to the island from Canada in the late 1990s, sired offspring and reinvigorated the gene pool. Previously, it was believed that the wolves' only migration to Isle Royale happened in the late 1940s.
The park is 15 miles from the Ontario shore, far enough to prevent other species such as deer and coyotes from arriving and complicating the wolf-moose relationship. But it's just close enough for moose to have swum to the island around the turn of the last century.
Moose give wolves a steady food source. Wolves help prevent moose from starving themselves by over-browsing vegetation at the park, which consists of one 45-mile-long island and 450 smaller ones.
Scientists began observing their interactions in 1958. Vucetich and Peterson now lead what has become one of the world's longest-running studies of a relationship between predator and prey species in a closed environment.
Wolves that survive to adulthood usually live only four to six years. Even so, Peterson and Vucetich say, things now are especially dire. Only two pups appear to have been born this winter and their condition is unknown.
A couple of years ago, there were four packs. Two died out in 2010. This year, the strongest remaining pack _ dubbed Chippewa Harbor _ killed the alpha male of the rival Middle Pack, scattering its remaining members.
That leaves just one well-organized pack for the first time in four decades. Because only the alpha male and female in a pack tend to mate, the outlook for replenishing the population is grim.
"It's as precarious as it was during the parvovirus days _ or more so," Peterson said.
He and Vucetich said any decision about importing wolves from the mainland would be made by the National Park Service. The subject has come up before because inbreeding is believed to be shortening the wolves' life span and hampering reproduction.
Some scientists have argued that because Isle Royale is a federal wilderness area where human influence is kept to a minimum, nature should take its course _ even if the wolves become extinct. Others say their disappearance would harm the ecosystem. The moose population, currently estimated at 515, probably would explode if their only natural predator on the island died off, leading to depletion of vegetation such as balsam fir on which the big herbivores thrive.
Peterson and Vucetich said the case for intervening may be bolstered by their research on the surprise arrival of the Canadian male wolf, discussed in an article being published Wednesday by the British journal Proceedings of the Royal Society B.
In the paper, they say observation and genetic analysis of collected wolf droppings, or "scat," pinpoint the male's arrival to 1997. Larger and lighter-colored than the others, he became an alpha and eventually sired 34 offspring before dying in 2006. More than half the genes in the current wolf population trace back to him.
Vucetich and Peterson had noticed the wolf during their annual winter study observation flights and dubbed him "Old Gray Guy," but hadn't realized he was a migrant until a colleague examined his preserved feces collected years ago.
His positive influence suggests that introducing new members to an inbred population _ which scientists call "genetic rescue" _ can improve succeeding generations, although any individual's benefit probably will be short-lived, their paper says.
That's an important finding because scientists are considering genetic rescue of other species, said Phil Hedrick, an Arizona State University biologist and co-author of the paper. The importation of Texas pumas to Florida in the 1990s is regarded as a success story, helping save endangered Florida panthers.
"These immigrants that come in on their own like at Isle Royale or are brought in can really change the whole population dynamic for the better, as long as it's done carefully to avoid unintended consequences such as bringing in diseased animals," Hedrick said.