The last two wolves known to live on Isle Royale/Rolf Peterson, Michigan Tech University

Just two aging wolves remain at Isle Royale National Park, while the park's moose population has swelled to nearly 1,500, numbers that stand to change if the National Park Service moves forward with a plan to bring wolves from Canada to the park.

While formal approval of the Park Service's plan has yet to be made, Balsam fir forests on Isle Royale, an island in Michigan's Lake Superior, are vanishing in large part due to heavy browsing by moose, according to the latest ecological study of the two species. Without intervention, the biologists who wrote the study -- Rolf O. Peterson, John A. Vucetich, and Sarah R. Hoy -- predict the park's wolves will vanish and the island ecosystem will suffer.

"The wolf population is also almost certainly headed for extinction, and wolf predation has been effectively absent as an ecological process for the past seven years," reads the summary of Ecological Studies of Wolves on Isle Royale, an annual report of which the latest edition was released Thursday.

"Moose abundance probably increased over the past year even though the most recent point estimate declined from 1,600 to 1,475 moose based on the moose census in 2018. In the absence of wolf predation, moose abundance may double over the next four or five years. If that happens, it will be the largest number of moose ever observed during the six-decade history of the wolf-moose project."

Park Service officials outlined a plan in March in which they would bring 20-30 wolves from Canada into Isle Royale over a three-year period. When final approval might come is unknown.

Chronic inbreeding has impacted the wolf population. There was hope that "ice bridges" that formed between Isle Royale and the Canadian mainland during the winter of 2013-14 would enable wolves to arrive from Canada with new genes. But no wolves reached the island, while one female left and was killed by a gunshot wound in February 2014 near Grand Portage National Monument in Minnesota.

Isle Royale wolves have been in decline for more than a decade. In recent years, park managers have discussed island and wolf management with wildlife managers and geneticists from across the United States and Canada, and have received input during public meetings and from Native American tribes of the area. Those discussions have examined the question of whether wolves should be physically transported to Isle Royale, in large part due to concerns that a loss of the predators would lead to a boom in the moose population that likely would over-browse island vegetation.

Across Isle Royale balsam fir has declined about 75% since 1846, reduced from 36% in the original land survey to 9% in 2010 (U.S. Forest Service data). Moose browsing is the primary driver of this decline on the west end of Isle Royale. Under the dense hardwood canopy of the western half of the island, where fir is further limited by light and germination sites, moose browsing largely eliminated growth of regenerating fir trees in the past 100 years. In the absence of regeneration, balsam fir would eventually disappear. In fact, over 90% of a sample of west-end fir trees tagged in 1988 have now died without replacement by new fir. In the late 2000s, quite unexpectedly, long-suppressed fir saplings at the west end of the island, often decades-old but still less than a meter tall, began to grow because browsing by moose was reduced. The moose population was reduced because wolves, buoyed by genetic rescue a decade earlier, preyed on moose at a higher level than previously seen on Isle Royale. -- Ecological Studies of Wolves on Isle Royale

The study of wolf-moose interactions on Isle Royale marked their 60th year this past winter. From those six decades of study biologists have determined that, during healthy times, the wolf population "typically varies from 18 to 27 animals, organized into three packs. The moose population usually numbers between 700 and 1,200 moose."

Without wolf predation, Isle Royale's moose population has risen to nearly 1,500, or nearly 3 per square kilometer/Michigan Tech University, Rolf Peterson

But inbreeding woes and the lack of long-standing ice bridges have conspired to bring down the wolf population to just the two aging animals seen this past winter.

"The pair are closely related—both as siblings and as father-daughter—and the inbreeding within the island's isolated wolf population is what contributed to their demise," this year's study said. "The wolves’ numbers started plummeting in 2009, declining by 88 percent from 24 to 2 wolves for that period; historical levels of wolves typically varied between 18 and 27. The pair, aged eight and ten years old, may have produced a pup several years ago but the female has continued to reject the male as a mate."

While an ice bridge did form briefly this past February, it tied the island to Ontario for just a week before breaking up.

"With fewer ice bridges and warmer winters, the chances of wolves recovering naturally are slim," the study said.

An interesting aspect of the study is that moose on Isle Royale are shrinking in size, something researchers believe is tied both to warmer winters and population density.

In a 2017 paper published in Global Change Biology, we demonstrated that moose who experience warm winters during their first year of life end up later in life with smaller skulls – an indication of smaller overall body size. We also found that skull size was smaller for moose born in years when moose were more abundant – because more moose means less food for each moose. Between 1960 and 2000, mean skull size has declined by 16%.  Approximately a fifth of that decline is attributable to climate and much of the rest was due to increasing density. That increase in density was largely due to the collapse of predation. It is widely known among moose biologists that smaller moose tend not to live as long. -- Ecological Studies of Wolves on Isle Royale

Moose have decimated Balsam fir on the island through their browsing/Michigan Tech University, Rolf Peterson