The single predator-single prey relationship between wolves and moose on Isle Royale in Lake Superior is unique, and has been the subject of detailed study since 1958. Isle Royale, the principal island of Isle Royale National Park in Michigan in the United States, is an isolated island with little migration of animals into and out of the island, and as a U.S. national park, human interaction and impact on the two species is also limited. Both the wolves and the moose first became established populations on Isle Royale in the 1900s. The populations of both moose and wolves have shown repeated spikes and declines and have not settled to a balanced relationship. The moose populations have ranged from 500 to 2500 while the number of wolves has ranged from almost 50 to down to two. From 2018 to 2019, 19 wolves were released at Isle Royale in hopes of bringing stability to the ecosystem, and , there are estimated to be 14 wolves remaining on the island.
The relationship between wolves and moose on Isle Royale has been the subject of the longest predator-prey research study, begun in 1958. The wolves have been subject to inbreeding and carry a spinal deformity.Isle Royale pack down to 2 female wolves by John Myers March 30, 2011 Duluth News Tribune newspaper As of the 2014 count, there were only 9 wolves on the island, with the 2015–2017 counts showing only 2. A review completed in 2014 determined that new wolves would not be introduced into the park to attempt a genetic rescue, but as of December 2016, the National Park Service had instead decided to introduce 20 to 30 wolves to the island. In 2018, three females and one male wolf from Minnesota were transferred to the island system.
Isle Royale National Park is made up of about 400 islands, and is in the northwest portion of Lake Superior. It is about from Michigan's shore, and from the Canadian shore. The main island is about long, and wide at the widest point, with an area of . There are no roads, and no motorized vehicles are allowed on the island. The park is closed from September to May, during which the wolf-moose study personnel are the only human residents on the island.
Wolf-moose dynamics interactions
As an isolated island, Isle Royale initially had neither wolves nor moose. The moose are believed to have either swum across Lake Superior from Minnesota in the early 1900s or were stocked on the island by humans for the purpose of recreational hunting. In 1949 a few wolves, possibly only a single pair, crossed an ice bridge from Ontario to the island during a harsh winter. But because only one pair of wolves migrated to the island, they have suffered from severe inbreeding. According to Rolf Peterson, a professor at Michigan Technological University and the lead wolf-moose researcher, "Moose were isolated here 100 years ago. Most of the genes are still here, but they have enough population (to compensate). There are so few wolves that they have lost genetic variability. The scientific dogma suggests that they are not going to make it."Weier, Anita. "Isle is Perfect Laboratory for Moose and Wolves – Their Interactions Studied Since 1958. (Saturday Extra)" The Capital Times (Madison, WI) (May 31, 2003): 1B. In fact, all of the wolves' DNA on Isle Royale can be traced back to one ancestor. Inbreeding leads to inbreeding depression and fitness problems, often accompanied by violent social rejection by other wolves.
When the study began in 1958, many researchers believed the two species would eventually reach a population equilibrium of about 25 wolves and 1,500 moose; but there has been no sign of this, with populations fluctuating unpredictably. The highest number of moose observed since the arrival of wolves was 2,450 in 1995. The highest number of wolves observed was 50 in 1980 followed by a population crash to 14 by 1982. As of 2005, there were 540 moose, the lowest recorded, and a relatively high population of 30 wolves. In 2008, there were 700 moose and 23 wolves.
The density of the two species depends strongly on the density of forage. Moose prefer birch and aspen trees, which used to grow plentifully on the island, but over a century of moose browsing have been largely replaced by the less nutritious balsam fir, which now makes up 59% of a moose's diet.McLaren, B.E., and R.O. Peterson. "Wolves, moose, and tree rings on Isle Royale. (Isle Royale National Park, Michigan)." Science 266.n5190 (Dec 2, 1994): 1555(4). Even this has declined dramatically: as of 2002, understory growth of balsam fir was at 5%, down 40% from 19th century observations. The plant is more plentiful on the east side of the island, which draws a higher concentration of moose. Because balsam fir does not give sufficient moisture, moose have recently been spotted eating snow, a very rare occurrence. They have also been sighted eating lichen, which researcher Rolf Peterson has compared to eating dust. When the moose population grows too high, the balsam fir population crashes, leading to a crash in the moose population, in a continuing population "see-saw". Moose mostly die from the consequences of malnutrition: they become emaciated and slowed down by arthritis, until they are easy prey for a wolf pack. Also, calves suffer from malnutrition when they are born during a winter with snow too deep for easy foraging.
Moose make up nine-tenths of an Isle Royale wolf's diet (the remainder being snowshoe hare and beaver). Moose in their prime years commonly outrun wolves in a hunt, especially on soft snow: moose can cross snow two feet deep at . Even if wolves can catch up to a moose, they cannot always bring it down; researchers often find wolves with hunting bruises and scars. To improve their chances, wolves pick out moose which are calves, old, diseased, or injured. The typical moose killed is about 12 years old and suffers from arthritis, osteoporosis, and/or periodontitis."Isle Royale is a study in moose and wolves." Akron Beacon Journal (Akron, OH) (Dec 29, 2005): NA. Eighty to ninety percent of moose are brought down by wolves rather than directly by disease,Peterson, R.O., R.E. Page, and K.M. Dodge. "Wolves, moose, and the allometry of population cycles." Science 224 (June 22, 1984): 1350(3) and each wolf kills an average of between 0.44 and 1.69 moose per month.Vucetich, JA & RO Peterson. 2004. Long-term population and predation dynamics of wolves on Isle Royale. Pages 281–292 in Biology and Conservation of Wild Canids, edited by D. Macdonald & C. Sillero-Zubiri, Oxford University Press.
Wolf population dynamics
Wolves on the island have, historically, been separated into three or four packs, with each pack usually having between three and eight members, including two or three pups. The number of wolves in a pack depends mainly on the amount of snow that fell in the previous winter. In winters with light snow, pups tend to leave the pack to find mates, so packs run at four or five members; in heavily snowy winters, the pups stay with the pack, which can reach ten to twelve members. If many members of a pack die, the pack dissolves and a new one forms within a year. One pack will dissolve about once every thirty years.
In 2006, the wolf population, with the exception of 10 loners and separate pairs, formed three packs; the East pack, the Middle pack, and the Chippewa Harbor pack."Moose population decline leaves Isle Royale wolves scrapping for food." The America's Intelligence Wire (March 11, 2006): NA. Wolf packs on the island have been known to fight to try to extend their territory and, thus, supply of moose. In 2006, the East pack killed Chippewa Harbor pack's alpha male, as witnessed by John Vucetich, a professor at Michigan Technological University and one of the lead researchers on the island, who believed that the Chippewa Harbor pack may die off without their leader.
Old Gray Guy In early 1997, a particularly virile male wolf that had not been born on Isle Royale was observed by researchers on the island. This was the first time that an adult wolf had entered the island's isolated population since the study began, and he is assumed to have crossed over the ice from mainland Ontario, 15 miles away. The wolf, designated M93 by Isle Royale researchers and later nicknamed "Old Gray Guy" because of his pale fur, was larger and more territorial than the other Isle Royale wolves. Old Gray Guy's pack grew to an unusually large 10 wolves, and drove one of the other four packs to extinction. Over the course of eight years of breeding, the wolf produced 34 pups, which in turn produced an additional 45 pups. It was determined that by 2009, 56% of the wolves on Isle Royale were descended from Old Gray Guy.
Scientists expected that the introduction of Old Gray Guy's genes to an inbred population would create a "genetic rescue" population boom, but the overall wolf population did not substantially increase over time. In an interview with The New York Times in 2011, wolf researcher John A. Vucetich said that this should not be taken as evidence against the "genetic rescue" theory, suggesting that other factors may have kept the wolf population from growing further and that the Isle Royale population may have disappeared entirely if Old Gray Guy had never arrived. Furthermore, Old Gray Guy's offspring soon began breeding with one another, leading a new set of traits to become reinforced by heavy inbreeding. A 2023 study found that Old Gray Guy's impact on the gene pool had been partially reversed as his inbred descendants died without offspring.
Trends Initially, it was thought that the wolf and moose populations would reach a stable balance. However, during the nearly sixty years of the study, the populations of both species have fluctuated up and down with the number of moose ranging from a high of nearly 2500 down to 500 and the number of wolves ranging from a high of 50 down to one in 2017/18.
During 2016, the wolf population was nearly extirpated with only two severely inbred wolves present. The moose population was about 2/3rd of its historical maximum with ample forage and growing rapidly. Absent a new infusion of migrant wolves, or human intervention, the original situation of a high moose population limited only by starvation is the prospect.
In December 2016, the National Park Service (NPS) put forward a plan in which they would add 20 to 30 wolves to the island in order to prevent the pack from disappearing completely.
In December 2017, Sarah Hoy et al. of Michigan Technological University published results of a 40-year study showing a decrease in the size and lifespans of moose. Analysis of moose skulls documents a 16% shrinkage likely consistent with warming winters which are correlated with smaller brain size in one-year-old moose. The moose population has tripled in the past decade, reaching about 1,600 in the 2017 survey, but as wolf die off approaches, competition for food due to overpopulation will become a further stress on moose.