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Isle Royale, the principal island of Isle Royale National Park in Michigan in the United States, is home to a predator-prey relationship between wolves (Canis lupus) and moose (Alces alces) that is unlike any other in the world. Because Isle Royale is a small, isolated island, the two species are able to be studied with virtually no factor of human interaction, as the island is protected by its national park status, and there is little migration of animals into and out of the island. The unique single predator-single prey relationship, where wolves are the only predator of moose, and moose are almost exclusively the only prey for wolves, allows for precise research on predator-prey relationships, and of the fluctuation of the two populations.
The relationship between wolves and moose on Isle Royale has been studied and researched for an unprecedented amount of time, which provides a rich background of research that aids in the better understanding of the two species and their interaction. The ongoing study, begun in 1958, is the longest study of either wolves or moose.
Isle Royale National Park is made up of about 400 islands, and is in the northwest portion of Lake Superior. It is about 50 miles (80 km) from Michigan's shore, and 12 miles (19 km) from the Canadian shore. The main island is about 45 miles (72 km) long, and nine miles (14 km) wide at the widest point, with an area of 205 square miles (530 km²). There are no roads, and no motorized vehicles are allowed on the island. Because there are no roads, nor any residential area, virtually all of the land on the island is wilderness. The remoteness of Isle Royale leads to it having fewer visitors in one year than Great Smoky Mountains National Park has in just one day. The park is closed from September to May during which the wolf-moose study personnel are the only humans resident on the island.
Because of Isle Royale being an isolated island, neither wolves nor moose were there initially. Moose swam across Lake Superior from Minnesota in the early 1900s. For nearly fifty years the moose thrived, having no predator to dampen their population growth. In 1949 a pair of wolves wandered across an ice bridge made to the island during a harsh winter from Ontario. But because only one pair of wolves migrated to the island, inbreeding has posed problems to the wolves' existence. 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." In fact, all of the wolves' DNA on Isle Royale can be traced back to one ancestor. The lack of genetic variability leads to inbreeding, and inbreeding in turn leads to mutations and fitness problems. Wolves with mutations often are violently harassed, and sometimes killed because of their differences.
When initially studied in 1958, many researchers believed the two species would eventually reach a population equilibrium (which is believed to be about twenty-five wolves, and 1,500 moose). They have not yet shown any sign of settling into one, instead tending to fluctuate unpredictably. The highest number of moose on Isle Royale since being researched with the interactions of wolves was 2,450 in 1995. The highest number of wolves ever recorded on the island was fifty in 1980. The most dramatic decrease in the wolf population occurred when the canine parvovirus was spread to the wolves on the island, introduced by a park visitor's dog (breaking the rules of the national park) in 1980 or 1981, causing a crash in the population; there were fifty wolves in 1980, but only fourteen by 1982. The population of the two species as of 2005 was 540 moose, which is the lowest recorded, and thirty wolves, which is considered rather high. In 2008, the number of moose was 700, and the number of wolves were 23.
The density of the two species is very much dependent on the density of proper forage. Because a moose's diet is 59% balsam fir, any change in its density affects the density of moose. The consistent presence of moose on Isle Royale since 1900 has led to a dramatic decline in understory growth of balsam fir. As of 2002, understory growth of balsam fir was at 5%, which is down 40% from 19th century rates, when it was recorded at 46%. The reliance of moose on balsam fir has led to a much higher concentration of the species on the east side of the island, where the plant is more plentiful. Birch and aspen trees, which are much healthier for moose, used to grow plentifully on the island, but have now mainly been replaced by the less nutritious balsam fir. Because balsam fir does not fulfill the amount of moisture moose need in their diet, they have recently been spotted eating snow, which is a very rare occurrence. They have also been sighted eating lichens, which is another very strange occurrence; researcher Rolf Peterson has compared it to eating dust. When the moose population grows too high, the balsam fir population crashes, which leads to a crash in the moose population. This leads to the continuing "see-sawing" of the moose population on the island. Crashes in balsam fir growth lead to malnutrition, which is one of the most common killers, both directly and indirectly of moose. When there are too many moose for the amount of balsam fir, then many moose, especially young moose become plagued with malnutrition, which leads to arthritis. Moose commonly die either directly from malnutrition, or, because they become emaciated, and slowed down by arthritis, they are picked out by a wolf pack as easy prey, and are killed. The most common cause of malnutrition in moose is when calves are born during a winter with deep enough snow that browsing for proper forage is very difficult. The moose population should be able to rebound soon, because there are not as many moose as there have been in the past, which will allow for the balsam fir to grow back.
Wolves use their intelligence to pick out easy moose prey, which they can outrun, and bring down. They do not go after healthy moose, unless they are completely desperate; a healthy moose in its prime years is very tough for any wolf pack to bring down. Also, moose in their prime years commonly outrun wolves in a hunt; moose can cross snow two feet deep at 20 mph (32 km/h), faster than the best cross-country skiers, and outrun wolves if the snow is soft. Even if wolves can catch up to a moose, they cannot always bring it down; researchers have found many bruises and scars on wolves that have not been successful when trying to kill healthy moose. Wolves pick out young moose calves, old moose, or moose plagued with disease or injury in order to increase their chances in successfully killing one. The typical moose that a wolf kills is about 12 years old, and suffers from arthritis, osteoporosis, or peridontitis, and in some cases all of them. Though moose are often infected by various diseases, approximately 80% to 90% of moose deaths on Isle Royale can be attributed to wolves. The number of calves and old moose has declined recently, placing the future of the wolves in doubt. Because moose make up nine-tenths of an Isle Royale wolf’s diet (the remainder being snowshoe hare and beaver), finding a young, old or sickly moose is imperative for survival. The average number of moose a wolf kills a month is between 0.44 and 1.69.
The wolf population of Isle Royale is, with the exception of a 10 loners and separate pairs, formed into three packs; the east pack, the middle pack, and the Chippewa Harbor pack. 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.
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 believes that the Chippewa Harbor pack is now in grave danger of dying off without their leader.
During the winter of 1997 a particularly virile wolf (later named "Old Gray Guy" and wolf No. 93) crossed the 15 miles of ice to Isle Royale. Old Gray Guy was larger and more territorial than the other Isle Royale wolves. His own pack grew to an unusually large 10 wolves, and displaced and drove to extinction one of the other 4 packs. He was so-named because as he aged his fur became very pale, an unusual phenomenon. It was determined that by 2009, 56% of the wolves on Isle Royale had descended from Old Gray Guy.
“We don’t know of any other instance — except when they first came — of wolves crossing the ice,” said John A. Vucetich, the lead author of a study of the wolves published online in The Proceedings of the Royal Society B last week. “The entire population is descended from a single female.” 
The wolf population on Isle Royale is small, averaging only about 23 wolves. By the end of his eight years of breeding, he produced 34 pups, those had produced an additional 45 pups.
Such a genetic impact is evidence for substantially greater fitness of Old Gray compared to the other inbred wolves on the island. Scientists expected that such an introduction would create a "genetic rescue" population boom, but it did not happen. "A co-author of the study, Rolf O. Peterson, a research professor at Michigan Technological University, said that the population of Isle Royale hangs on by a thread, as it has for decades. The average reproduction after the Old Gray Guy arrived was no different from before. Yet this does not mean that he had no effect."
“The simple interpretation is that genetic rescue doesn’t work,” said Dr. Vucetich, an assistant professor of wildlife ecology at Michigan Technological University. “But what happened here is that when the immigrant came in 1997, in the decade that followed, the moose population declined radically. It’s plausible that we didn’t see an effect because the wolves were suffering from some other trouble that disguised the benefit. What if wolf No. 93 had never arrived? Dr. Vucetich said that it is impossible to know for sure, but the Isle Royale wolves might have disappeared completely. It may be that the Old Gray Guy arrived just in time."
Once a moose is brought down and killed, wolves have to compete with scavenging ravens. Ravens are tenacious scavengers that can easily dodge the strike of a wolf and are unbothered by them. Ravens can eat and store up to two pounds (1 kg) in a few days, which is minuscule compared to the storage capacity of wolves, which is up to eighteen pounds (8 kg) in just a few hours.
Other anti-social species have an effect, though rather small, on the relationship between wolves and moose on Isle Royale. Before wolves hunted them to extinction, coyotes used to inhabit the island. Beavers and snowshoe hares also have an effect on both populations, because beavers and snowshoe hares are the only two animals that wolves prey on excluding moose, constituting a tenth of an Isle Royale wolf's diet. The beaver population has sharply declined since the arrival of wolves, but they still are present, and though they are not a prime food source for typical wolves, they are the second to moose as the most consumed animal by the wolves on the island. Beavers benefit both species, because they create aquatic macrophytes, which are very nutritional plants for moose, because they are a key source of sodium, and also because they serve as easy prey for wolves. Unfortunately for the moose, beavers themselves also consume the macrophytes they create.
Though wolves are thought to contribute to the decline in beaver, researchers believe that the decline of aspen, the primary food source of beavers, which used to be plentiful, could have led to their decrease. Beavers have been exposed to predation by having to travel long distances to find only parts of the island where aspen remains. Snowshoe hares, the third most consumed animal by wolves on Isle Royale constitute a very small portion of the wolves' diet, because snowshoe hares are so difficult to catch. Researchers have found that wolves do not show much interest in preying on hares, and only feed on them incidentally. Snowshoe hares have a negative effect on moose as they eat some of the same vegetation that moose eat, which only contributes to the decline in appropriate forage for moose. The red fox is yet another animal that inhabits Isle Royale; red foxes mainly feed on snowshoe hares and occasionally scavenge on moose, or any other meat a wolf leaves behind. Wolves do not commonly kill foxes, though wolves have been observed killing foxes when they attempt to feed on an animal carcass.
Climate plays a major role in the moose-wolf relationship as well. Since El Niño hit in 1998, the climate has warmed up, which has significantly affected the moose population across North America. The warmer climate in recent years has produced more Moose winter ticks, which consume the blood of animals, making them more susceptible to anemia, and induce the Moose to scratch off their hair, exposing them to hypothermia in cold weather. A moose can have tens of thousands of ticks feeding on its blood at one time, each sucking up to one mililiter of blood. The biting ticks cause a lot of discomfort for the moose, so they try to get the ticks off their bodies by biting off their hair, and rubbing up against trees. This preoccupies moose, and keeps them from browsing for food, which can lead to malnutrition. Compounded with blood loss, moose weakened by ticks are easier for wolves to kill. Ticks are more prominent in years where spring arrives earlier than usual, because when they fall on ground not covered in snow, then they can reproduce. Otherwise, they die out. Then, if the summer is hot, ticks are able to reproduce at a higher rate. Hot summers also lead to moose resting in the shade, or in the water to keep cool, making them easier prey for wolves. Also, hot summers lead to tougher foraging for moose which makes them less prepared and more vulnerable to the winter. Not only has the recent warming of Isle Royale hurt the moose, but completely opposite problems harm them also. Harsh winters pose significant problems to moose, because moose have problems finding food when there is too much snow on the ground. The less snow there is, the more freely moose can move around the island. When there is a significant amount of snow, moose stay in conifer swamps, making them easier prey for wolves, because they are more confined, and immobilized due to the snow. Deep or heavy snow decreases the speed and agility of moose that is necessary to evade wolf attacks, and calves born during a winter with particularly deep snow are more vulnerable to being weaker prey for wolves later on in their lives because of foraging problems that occur when the snow is deep. The keen survival instincts of moose have been clearly evident from the studying of their actions on Isle Royale. Female moose (called "cows") have been spotted on nearby smaller islands, around the main island of Isle Royale, because they swim across to give birth. This allows for them to give birth and raise their young without the threat of wolves preying on their young when they are vulnerable. This also causes trouble for moose that are born in the winter, because they can no longer swim across the water to another island, and must raise the new calf in the snow. Once the calves are physically mature, they are able to swim back, and are then able to better protect themselves from wolves, as they are then in their prime years. This practice should theoretically help the moose population rebound in the future.
One of the first publications on the subject was the book "The Wolves of Isle Royale" by L. David Mech which led to the prominence of both the author and the topic. The book was published in 1966 by the Department of the Interior, having evolved from his doctoral thesis.
Because the moose population is at an all time low since being studied, scientists expect that the wolf population will decline due to the lack of prey, which will lead to a rebound in the moose population. The normal ratio of moose to wolves is forty or fifty to one, whereas in 2005 it was fifteen to one. This imbalance should allow the moose population to increase. Also, because of the low moose population, foliage is given the opportunity to grow back, which will help the moose population to rebound.