Could hunting or U.P. wolves solve Isle Royale moose problem? | Great Lakes Echo
The single predator-single prey relationship between wolves and moose on Isle Royale in Lake Inbreeding leads to inbreeding depression and fitness problems, often accompanied by violent social rejection by other wolves. When study. Only two wolves likely remain on Isle Royale; the population crashed The study's report marks the project's 58th year of monitoring wolves and moose in Isle Royale. Geneticists measure the severity of incestuous relationships with damage than cannot be undone – a kind of Humpty-Dumpty problem. Michigan Technological University's annual Winter Study of the wolves and moose of Isle Royale National Park counted eight wolves on the.
This observation is represented by the point on the lower right portion of the graph.
- The Wolf and the Moose: Natural Enemies That Need Each Other
- Could hunting or U.P. wolves solve Isle Royale moose problem?
- The Population Biology of Isle Royale Wolves and Moose: An Overview
The collapse was caused by a combination of events - most severe winter in a century, outbreak of ticks, lack of forage, and high moose density. When we consider this extreme observation, then the most parsimonious relationship between moose abundance and population growth rate is a complicated curve 3rd order polynomial.
That curve indicates the moose population is, overall - across the full range of possible densities - density dependent. That is, the population will tend to increase when abundance is very low and decrease when abundance is very high.
So, while Isle Royale moose are density dependent, in the big picture, they are inversely density dependent, or unstable for a wide range of abundances. This instability is manifest as wide ranging fluctuations in moose abundance see graph in section 1. Is predation driven by wolves or severe winters? So far, we know that annual fluctuations in predation rate impact on moose population growth rate Section 6predation is a potentially destabilizing force section 5and that the moose population is, in fact, quite unstable section 7.
There is one more possibility to assess. Specifically, what causes predation rate to fluctuate from year to year? One might presume it is caused by annual fluctuations in wolf abundance Scenario A. However, it is possible that severe winters are responsible. Perhaps the direct effect of a severe winter is to weaken the condition of moose, which makes it easier for wolves to kill more moose Scenario B.
In this case, we might say winter weather is the ultimate cause of fluctuating predation rate i. We can use data to test whether Isle Royale is more likely characterized by scenario A or B. To do so, we need to compare two graphs - a graph showing how predation rate is related to wolf abundance, and another showing how predation rate is related to winter severity. The graph to the left shows how wolf abundance has a reasonably important influence on predation rate.
The next graph requires more explanation. Measuring winter severity is very complicated. Severity depends on the amount of snow, whether the snow is wet and heavy or light and fluffy, how many months the snow is on the ground, how frequently snow crusts form, etc.
Ecologists have learned that a useful, overall index of winter severity for eastern North America and Western Europe is the North Atlantic Oscillation index. For details on that click here or check out this paper, Ottersen et al. The graph to the left suggests that predation rate has only a slight tendency to be greater during severe winters.
Summing it up, so far Moose are more than merely a food supply for wolves. Wolves are more than simply a source of mortality for moose. These processes - food for wolves, mortality for moose - are both important, and despite being related to one another, they do not operate in complete synchrony. The result is a complicated set of dynamics. Consequently, the abundance of wolves and moose are not related in any simple manner Section 2.
For 80 years, predation theory has guided the observations that field ecologists make about the predation in the real world. The center pieces of that theory are the functional response and numerical response.
The functional response reveals the extent to which per capita kill rate varies over time as the density of prey varies Section 3. The numerical response reveals the extent to which the predator abundance increases or decreases as the kill rate food supply varies from one year to the next Section 4.NPS debating wolf reintroduction on Isle Royale
Together, the numerical and functional responses aim to explain the causes and consequences of fluctuation in per capita kill rate. The wolves and moose of Isle Royale show us that these ideas are important, but explain only a limited portion of the dynamics that occur between Isle Royale wolves and moose. The most important predictor of whether predation rate will be high or low is the abundance of moose Section 5. Specifically, predation rate tends to be highest when moose are least abundant.
That is, predation is inversely density dependent. That makes predation a potentially destabilizing force. Predation is also largely additive rather than compensatory with respect to moose growth rate Section 6.
Consequently, the moose population exhibits only very weak density dependence Section 7. Finally, it seems that fluctuation in wolves from year to year, not winter severity, is the primary ultimate cause of fluctuations in predation rate Section 8. That is, wolves have an important destabilizing impact on moose population dynamics. Nevertheless, there is no worry that wolves would drive Isle Royale moose to extinction. If wolves drove moose to particularly low levels of abundance, the wolf population would be at much greater risk of extinction, due to lack of food.
If wolves went extinct, the moose population would increase greatly and be governed by a different set of relationships - forage and climate would become the most important determinants of moose abundance. Predation rate, kill rate, additive predation, stability, the influence of climate The wolves and moose of Isle Royale are also influenced by the age structure of the moose population. The age structure of a population refers to the proportion of individuals in a population belonging to different age groups.
These changes are depicted in the graph above. Each vertical bar in the graph corresponds to a different year. The three portions of each bar, from bottom to top, represent the portion of the moose population that is comprise of calves, prime-aged moose, and senescent-aged moose. The first important lesson about age structure is that it can fluctuates greatly over time.
Age structure is important for a second reason. That is, the ecology of an individual varies greatly with its age. Prime-aged moose have the highest rates of survival and reproduction, senescent moose have lower rates of survival and reproduction, and calves have the lowest rate of survival and do not reproduce. These age-specific differences have an important influence on overall moose population dynamics. In particular, population growth rate tends to be lower during years when the average age of a moose is greater see graph to left.
Food might be plentiful, predation might be low, and winter may have been mild. Nevertheless, if the moose population is comprised mostly of very old individuals that are likely to die anyways, then the population might still decline, or at least not increase as much as would otherwise be expected. Different-aged moose also exhibit different vulnerabilities to wolf predation. Calves are vulnerable because they are small, and senescent aged moose are vulnerable cause they are often weakened by arthritis, jaw necrosis, or malnutrition.
Similarly, if prime-aged moose are rare in the diet, that rarity might not indicate that wolves avoid prime-aged moose, it might simply indicate that prime-aged moose are rare in the environment.
Larger values, indicating preference, mean that kind of prey is more common in the diet than would be expected given its frequency in the environment. Values smaller than 0. The strongest preference is 1, and the strongest avoidance is 0. Those calculations were made for each year between and The three bars represent preference for each age group, averaged across these 32 years.
The small vertical lines at the top of each bar represent the standard deviation. These calculations show that wolves avoid prime-aged moose very strongly.
It also shows wolves have a slightly higher preference for calves than senescent-aged moose. Behavioral ecology can sometimes seem a world apart from population ecology. However, the two are connected, and ecologists are keen to understand how behaviors affect population processes. In years when calves are more common, kill rates are greater. Frequency of calves is the second most important predictor of kill rate The ratio of moose to wolves is the most important predictor, see section 3.
For more, see Sand et al. The importance of age structure is manifest in a complex relationship between the abundance of wolves and senescent moose. For the first two decades of observationopen circles in graph to the leftwolf abundance tracked quite closely the number of senescent moose.
Restocking wolves on Isle Royale raises questions about which species get rescued
If the abundance of senescent moose is a good indicator of food availability then the Isle Royale system was strongly bottom-up during those years see Section 2. Then, inthe wolf population crashed due to disease Section 1 and inbreeding took its toll on wolves see section 12 below.
From onward, Isle Royale shifted from being bottom-up to something else. The abundance of wolves was completely unrelated to the number of senescent moose from Moose are in the middle of a food chain.
They are supported by the abundance of forage below. Wolves represent a pressure from above. And climate is a force that can lead to either increases or declines in population abundance. Of all the annual fluctuations we observe in the moose population, what portion of those fluctuations can be attributed to fluctuations in wolves, forage, and climate? For the 22 years between andwolves had the greatest influence on moose abundance, and climate and forage abundance were similarly important.
Then for the next two decades, the decades which followed the disease-induced crash of the wolf population, variation in winter severity from year to year replaced wolves as the most important influence on moose population dynamics.
What explains this shift in population dynamics? We consider the most likely explanation in the next section.
Quite aside from understanding the cause of this shift, there is an important lesson. But then after watching for another 20 years, we got an entirely different answer. Inbreeding depression and genetic rescue in the wolf population. The Isle Royale wolf population was founded when wolves crossed an ice bridge from Canada in about They were believed to have been isolated ever since.
Comprised typically of just a couple dozen wolves, the population is also small. Small, isolated populations exhibit high rates of inbreeding, which means to mate with close relatives. Inbreeding accumulates over the generations, and that accumulation is quantified by the inbreeding coefficient, which is denoted by the symbol F and ranges from zero completely outbred to one completely inbred. Values of F greater than 0. By the late s, F for Isle Royale wolves had increased to nearly 0.
For many decades, the wolves of Isle Royale had been taken as an example of a very small, isolated and highly inbred population which showed no signs of inbreeding depression, the negative impact of inbreeding.
But we had it wrong, very wrong. In fact, the population dynamics of Isle Royale wolves have been affected by genetic processes in ways that have been as important as they are subtle. A surprising number of these wolves suffered from several different kinds of congenital malformity in the spine. Left, is an image of the ventral side of a wolf pelvis and sacral vertebrae.
The red line highlights a gross asymmetry. This wolf would have likely suffered damage to nerves that control its tail and hind legs. A particular kind of deformity, known as a lumbosacral transitional vertebrae LSTVis particularly well studied in dogs and wolves.
Among healthy, outbred populations LSTV occurs in one out of a wolves. On Isle Royale, a third of the wolves suffered from this malformity. Not only did Isle Royale wolves exhibit LSTV at a high rate, but the rate of malformities had once been relatively low and increased over the decades, as the population became increasingly inbred. The curve represents logistic regression, which predicts the incidence of malformities for each year, based on the observed data. Isle Royale wolves had been suffering from inbreeding depression all along, we just never knew it.
Then something remarkable happened. In a wolf from Canada walked across the frozen ice bridge that had formed that winter. He was physically large and light in the color of his coat see image below.
And through that genetic analysis, we learned that no. He represented a badly needed infusion of new genes. Within about five years of his arrival, the inbreeding coefficient for Isle Royale wolves dropped well below 0. The fitness of the inbred Isle Royale wolves was so inferior to the fitness of no. He began mating with his daughter in Over the next several years they produced 21 offspring.
Incidences of inbreeding like this caused F to increase again soon after no.
Within a decade of his arrival, 7 of the 8 breeding wolves on Isle Royale were either no. His genes were taking over the entire population. The graph to the left shows how the ancestry of wolf no. Bymore than half the genes in the Isle Royale population were descended from this single immigrant wolf.
So, how has inbreeding and the subsequent genetic rescue affected population dynamics on Isle Royale? A precise understanding is beyond us. However, we have some important general understandings. First, we have already indicated how the disease-induced population crash of seemed to trigger an important change in population dynamics for wolves.
During the s and early s, wolf abundance never really recovered. Prior to wolves are an important influence on wolf population dynamics section 11 and senescent-aged moose were an important predictor of wolf abundance section However, afterclimate replaced wolves as the dominant predictor of moose dynamics, and wolf abundance became completely uncoupled from the abundance of senescent-aged moose.
In retrospect, these changes were likely the result, at least in part, of inbreeding depression. The answer is complicated.
The shaded portions of these graphs highlight the period of time after the arrival of no. Recall that the number of moose per wolf is an important indicator of food availability for wolves section 3.
Given that circumstance, the wolf population should have declined, and probably by quite a bit, at the time when the immigrant arrived. The arrival of wolf no. Yet this does not mean that he had no effect. Vucetich, an assistant professor of wildlife ecology at Michigan Technological University. 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. 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.
Wolves and moose on Isle Royale
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 0. Before wolves hunted them to extinctioncoyotes 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. They are as easy prey for wolves and they create aquatic macrophytesvery nutritional plants for moose, although the macrophytes are also consumed by the beavers. 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.
Wolves do not commonly hunt foxes, though wolves have been observed killing foxes when they attempt to feed on an animal carcass. 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. Otherwise, they die out. Hot summers also lead to moose resting in the shade, or in the water to keep cool, making them easier prey for wolves.
Wolves and moose on Isle Royale - Wikipedia
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.
When there is a significant amount of snow, moose stay in conifer swampsmaking them easier prey for wolves, because they are more confined, and immobilized due to the snow. 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.