Isle Royale wolves decline; moose numbers double | MPR News
MOOSE OFFER CLUES. TO THE CAUSE OF. ARTHRITIS IN HUMANS Institutional Support for the Wolf-Moose Project: Website Design and Construction by. The moose and wolves of Isle Royale have been vigorously studied and The wolf and moose relationship is unique in that wolves are the. function of total wolf and moose density on Isle Royale. each pack) studied in relation to pack size and moose abundance .. advice on the statistical analysis.
Established inthe Isle Royale archipelago is a uniquely isolated ecosystem that lies in Lake Superior. In fact, the only way to reach the park is by ferry or floatplane. The mile long island is surrounded by smaller island segments creating a mosaic of some of the most secluded and untouched hardwood forests in Michigan. Along with this unique ecosystem comes the longest continuous study of predator-prey relationships in the world.
The moose and wolves of Isle Royale have been vigorously studied and observed for more than five decades. Being as isolated as it is, the island initially had neither wolves nor moose residing on it. At this time the island was not an established National Park so it is possible that they were brought over for recreational hunting purposes which would now be illegal.
It is also possible that they swam from the mainland of Minnesota, as moose are surprisingly excellent swimmers. Wolves, however, did not arrive untilwhen a single breeding pair crossed over an ice bridge from Ontario during a very harsh winter.
Following the first two wolves, more began to cross ice bridges and populate Isle Royale. However, this did not happen. 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 Population Biology of Isle Royale Wolves and Moose: An Overview
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. 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.
Welcome to The Wolves and Moose of Isle Royale | The Wolves and Moose of Isle Royale
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.Isle Royale
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.
Listen: Scientists turn wolf-moose relationships on Isle Royale into music | Michigan Radio
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.
Isle Royale wolves decline; moose numbers double
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. At some point, they may reach the point where they're severely over-browsing island vegetation, which has happened before. In the s, following another wolf decline, moose numbers reached about 2, leading to mass starvation during the harsh winter ofwhen about two-thirds of the moose died.
He and Peterson are lobbying the National Park Service to bring more wolves to the island to refresh the gene pool. They say there is growing evidence that the wolves have lasted because newcomers periodically crossed ice bridges and mated with island wolves. Analyzing decades of field notes, they recently concluded that a pack of seven or eight wolves migrated to Isle Royale in A lone male immigrant in had such an effect that bymost of the island's wolves were his descendants.
But ice bridges are forming less frequently, lowering the odds that more wolves will arrive on their own, the scientists said. Although ice provided a pathway for 25 days this winter, no wolves are believed to have traveled to the island and one used the occasion to escape, only to be shot dead on the mainland.
Park Superintendent Phyllis Green announced this month that officials had decided not to intervene as long as a breeding wolf population remains.