Wolf Ecology and Prey Relationships on Isle Royale (Chapter 1)
In our view, media attention and the cultural history of the wolf-moose relationship on Isle Royale have outweighed most scientific qualms about. IN Lake Superior lies a remote island, Isle Royale National Park, , acres that will test our ideas about wilderness and our relationship with nature. As the lead researchers in the study of wolves and moose, we favor. Isle Royale has not always been known for its wolf population. A long-term study of wolf-moose relationships on Isle Royale began in .. In this test, a calculated G value is compared to a critical chi-square value to.
Conversely, if environmental stochasticity causes unexpected population decline, the growth rate in the subsequent year would be positive and the population would tend to increase, again returning to the equilibrium.
The population is stable. This stability is also represented by the blue arrows showing how abundance is attracted to the equilibrium.
Any population that persists for any length of time must have density dependence - it is a mathematical consequence of persistence. This means when a population is perturbed from the equilibrium it will have a very strong tendency to return rather quickly to the equilibrium. For the broadest context, consider panel C. As before, the population would not tend to increase or decrease when it existed at its equilibrium abundance.
However, the population would tend to increase forever if some perturbation increased its abundance above the equilibrium. And the population would tend to decline to extinction if some perturbation caused the population to decline below the equilibrium.
Such populations are unstable, and may be prone to extinction. So, how does the Isle Royale moose population compare with these theoretical possibilities? For a wide range of moose abundance i. For this range of abundances, the population is unstable. But we get a different sense is if we also consider the highest density of moose ever observed on Isle Royale 4. This observation is represented by the point on the lower right portion of the graph.
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.
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.
Wolf-Moose Drama, Wolves Recover from Disaster
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.
The Population Biology of Isle Royale Wolves and Moose: An Overview
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. 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.
Listen: Scientists turn wolf-moose relationships on Isle Royale into music
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.
In each pack,the dominant, or alpha, male and female were distinguished by the fact that they never displayed submissive behavior toward other wolves, while other wolves were submissive in their presence. East Pack alpha male urinating in posture unique to males; alpha female stands behind him. Identification of pups in midwinter presented a problem.
Sometimes pups could be distinguished by their behavior. Pups seemed to be more playful, were sometimes hesitant in situations such as crossing glare ice, and behaved inconsistently toward dominant wolves during group greetings.
Sometimes pups were distinguished by a lighter, uniform color, slim body, and guard hairs that stick up along their back, creating a scraggly outline Fig. At other times, however, pups and adults are indistinguishable. For example, in we examined the carcass of a wolf that appeared to be an adult.
Unworn canine teeth and lack of epiphyseal closure in the radius Rauschhowever, showed that it was actually a pup. Underdeveloped pups may be more pup-like in appearance Van Ballenberghe and Mechleading one to conclude that pups are plentiful and show high survival in years when actually they were identified only because they were underdeveloped.
Pups may appear scraggly, as three wolves on left, or may be indistinguishable from adults. Winter Moose Census The present phase of the Isle Royale studies was initiated in with a firm foundation of aerial moose-inventory techniques which have been steadily refined.
The first aerial censuses on Isle Royale used a transect, or strip, method, in which all the moose seen in midwinter during many parallel flight lines were tallied Aldous and Krefting Fig. Cole and Mech employed a variation of this strip method by "buzzing" moose that were spotted in order to flush any nearby animals.
Mech's census was the last attempt at a complete count of the Isle Royale herd. Weather factors and moose-distribution patterns in subsequent years were such that stripwise coverage of the island would have produced a far less meaningful estimate. In addition, confidence intervals cannot be constructed for such an inventory.
Aerial counts were used to determine moose population size and composition. Afterthe island was divided into several zones of relative moose densities. The plots occurring in each zone were used to calculate mean moose density and variance for that zone Wolfe and Jordan, unpubl. I used a similar aerial-plot method. Since the population estimate depends on the area included in each zone of moose density, it is important that the zone assignments accurately reflect current moose distribution.
For this reason I made some modifications in zones in both and Winter moose censuses in both years were flown in a hp Aeronca Champion.
We flew overlapping circles a few hundred yards wide over a given plot until it had been covered completely. Most plots were flown at an altitude of m; the "open" character of burned-over areas permitted a censusing altitude of about m.
Moose densities were highest in coniferous cover where visibility was poor, so these areas were circled most intensively. Counting in both years continued throughout February. Although the counting period was long, no noticeable variations in moose distribution invalidated the zone assignments.
Within each zone, we attempted to sample plots randomly, but in practice this plan was modified considerably by the weather and a need to minimize time spent flying from one plot to another. Plots were counted in areas where flying and counting conditions were near optimum: Allocation of sampling among the zones was such that sampling error would be minimized at whatever point the work was terminated.
Censusing in both years was discontinued early in March after moose were confined to areas of conifer cover by strong crusts on the snow surface.
Inexperienced observers saw less than half of the moose in experimental enclosures, even under good conditions. The flight patterns were either narrow transect or concentric circles of ever-decreasing radii. Compared to the relatively open cover of the Alaska study area, Isle Royale has large areas of dense conifer cover which make moose counting more difficult.
In addition, moose concentrate in these conifer areas, further reducing accuracy. To compensate somewhat, we spent a greater amount of time circling each plot.
Listen: Scientists turn wolf-moose relationships on Isle Royale into music | Michigan Radio
The Isle Royale plots were kept small km2 to reduce observer fatigue and resultant errors. Factors responsible for the assumed high level of accuracy are the flight pattern of overlapping circles, the intense circling, and the long experience of the pilot. Aerial moose counts probably can do little more than provide a rough index of trends in the population. Summer Ecology of the Wolf Most of the spring-fall field work consisted of ground searches for moose remains and wolf sign Fig. Since wolves made extensive use of the km of hiking trails during spring and fall, when visitation is lowest, we also walked the trails at these times monitoring wolf sign and checking for the occasional kill made on a trail.
When human activity caused the wolves to abandon the trails, we generally did likewise. Open ridges, shorelines, creek beds, swamp edges, and animal trails then became our travel routes. Summer field headquarters on Rock Harbor. Howling responses were sometimes useful in locating wolves. Human imitations of howls were frequently broadcast over the island in and through a portable megaphone with electronic amplification. However, success was limited because we could move about only on foot. By checking known travel routes and reports of howling, we finally located wolf activity areas den and rendezvous sites in the summer of The presence of wolves at summer homesites was then monitored by camping near enough to hear spontaneous howling.
In order to minimize disturbance to the wolves, human howling was employed to elicit wolf responses only when the location of wolves was uncertain. We collected a large sample of wolf droppings scats in summerto determine food habits at this time of year. All samples were autoclaved to avoid contamination by Echinococcus, a tapeworm whose eggs are passed with feces.
Scats were pulled apart under a binocular scope, and the incidence of prey remains in each scat recorded.
Hair was examined under reflected light and magnification. Identification was aided by photographs and descriptions provided by Adorjan and Kolenosky Moose Mortality Patterns The collection of skeletal remains of Isle Royale moose over a year period provided a unique opportunity to investigate mortality patterns in a naturally regulated moose herd and to document changes in the type of moose killed by wolves from one year to the next.
In addition to the winter-spotted kills, other carcasses and skeletal remains located at random during summer field work provided data on year-round mortality Fig. Information on date and cause of death, age, sex, skeletal abnormalities, and marrow condition was recorded whenever possible. Extensive carcass examinations revealed moose mortality patterns. Sincea major effort has been made each winter to locate wolf kills, primarily by following wolf tracks.
Sometimes this has led to carcasses of old kills or moose that died of causes other than wolf predation. Almost all of these dead moose were subsequently ground-checked, either in winter or as soon as possible the following spring.