Listen: Scientists turn wolf-moose relationships on Isle Royale into music | Michigan Radio
The Isle Royale Wolf-Moose Project has been tracking the rare ecosystem on Isle Royale for almost 60 years. What makes Isle Royale rare is. Michigan Radio's Rebecca Williams and Mark Brush traveled to Isle Royale to meet the researchers who have been watching how wolves and moose interact. More than 60 years ago, a couple of wolves wandered across a frozen channel on Lake Superior and settled on moose-rich Isle Royale in.
Any observation below the dotted line represents a year when the population declined. More specifically, the vertical axis reflects the percentage by which the wolf population grew each year. In that year, the kill rate was 0.
Lessons from Isle Royale's Wolves and Moose
This observation is far to the left side of the graph and a little high. Most important is the overall trend revealed by this graph. That trend is for the population to grow more during years when food was more plentiful, i.
Although this trend is pretty much what you might expect, there is something as subtle as it is important. There is a simple, but important idea, that the population dynamics of a top predator should be determined primarily by its food supply.
This graph shows that most of the variation in wolf growth rate is not explained by variation in food supply. There are many explanations for what might be going on here, but the list of important factors include disease, inbreeding depression, and demographic stochasticity. Predation rate is the proportion of moose each year that are killed by wolves.
It is useful to think of a population processes as a balancing act. Predation takes some moose away and the population will decline, unless for example, something like the birth or recruitment of new moose offset that loss. You might think that kill rate and predation rate would be pretty well correlated - that years of high kill rate would tend to be years of high predation rate.NPS debating wolf reintroduction on Isle Royale
You might think that a good year for wolves is a bad year for moose, and vice versa. Knowing kill rate is only half the story. You also have to investigate the predation rate. The graph above shows that predation rate has a pretty strong tendency to be greatest during years when moose density or abundance is lowest.
This trend represents an an important idea. This trend suggests that predation is inversely density-dependent. Suppose the moose population experiences a series of good years, maybe mild winters or lots of food. Consequently, moose abundance increases and according to the trend on this graph predation rate declines.
As moose abundance increases, predation becomes a less powerful force, which could allow moose abundance to increase further. Alternatively, suppose the moose population experiences some difficult years and declines, perhaps because of severe winters or lack of forage. As the moose population declines, predation becomes according to the trend on this graph an increasingly powerful force, which can cause the population to decline even further. By this reasoning, predation would be a destabilizing force.
We can expect that predation fuels much of the fluctuations we observe in moose abundance. But before we can come to this conclusion, however, we need to consider a few more ideas.
How does predation affect the moose population? This is one of the oldest questions in ecology. The simplest answer is, it depends. On one hand, predators could focus on prey that would have died anyways - prey that are sick or old. Alternatively, a prey population might respond to increased predation with increased birth rates. In either case, we say predation is compensated by some other process. The result is that increased predation rate has no effect on prey growth rate panel A. In this case, every one percent increase in predation leads to a one percent decrease in the growth rate of the prey population panel B.
Other intermediate circumstances are also possible, e. So how is it on Isle Royale? Overall, predation rate is pretty variable from year-to-year. For example, predation rate is 2 to 4 times greater during years of high predation, compared to years of low predation. Moreover, predation for Isle Royale moose is strongly additive, with the slope not significantly different from negative one see graph at left. Consequently, annual variation in predation rate has a big impact on whether the moose abundance will increase or decline.
In fact, annual variation in predation rate is one of the most important influences on moose population growth rate i. Is the moose population unstable, and what does that mean? So far, it seems that predation rate is an important predictor of whether the moose population will grow or decline section 6.
And, earlier we concluded that predation is a destabilizing force section 5. All populations fluctuate in abundance. Or, for example, environmental stochasticity might, also for example, be manifest as an outbreak of disease, causing an unexpected population decline. That is, after a population increases or declines, is there a strong tendency for the population to return to its previous abundance? If so, the population is stable.
A population is stable when it tends to return to some long-term average abundance after any environmental perturbation. This kind of stability promotes long-term population persistence. Without this kind of stability a population would grow to infinity which is impossibleor risk declining to extinction which is possible. Stable populations are also said to exhibit density dependent fluctuations.
These ideas about stability and density dependence can be represented graphically by a relationship between population density horizontal axis and population growth rate vertical axis. The red line indicates that population growth rate tends to decline with increasing abundance. Moreover, there is a certain level of abundance where growth is zero red dot. When the population is at this level of abundance the population will not grow or shrink. This is the equilibrium abundance.
If environmental stochasticity causes the population to unexpectedly increase from this equilibrium i. 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. 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.
Save the Wolves, Control the Moose
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 Rolf Peterson is a research professor at Michigan Tech and he just spent his 44th winter on the island. I called him up to find out how the animals are doing. This year, the team counted nine wolves, up from eight last year.
Researchers were unsure how the wolf died at the time, but a necropsy found that the five-year-old female wolf was shot with a pellet gun.
Isle Royale out of balance: The wolf-moose dilemma | Toledo Blade
Lee Berquist of the Milwaukee Journal-Sentinel has more: Michigan Technological University's Rolf Peterson confirmed it in an e-mail to me last night. Friday, February 7, The ice bridge to Isle Royale has formed. See our post here. In Michigan we plan on killing wolves because some feel there are too many.
It's a different story on Isle Royale where the wolf population is hanging on by a thread. But because Isle Royale National Park is a designated wilderness area, we, as humans, have pledged not to intervene. So what should we do? The National Park Service has a big decision to make. The folks who have been studying this place for a long time share their thoughts in this op-ed piece.
The number of predators on the island has been sinking fast. The Park is a dedicated wilderness area, so managers do their best to keep it as untouched by humans as possible. But people might need to step in. Phyllis Green is the park's superintendent. There are just eight wolves left on Isle Royale. This is the first year that Michigan Technological University researchers were unable to document any pups born to the wolves.
Scientists reported last year that only nine wolves remained on the Lake Superior island chain - the lowest total in more than 50 years. They said just one was known to be a female, raising doubts about the predator's long-term prospects for survival in the wilderness park. But Superintendent Phyllis Green said Thursday that genetic analysis of wolf excrement and additional observations suggest that four or five of the animals are females.
Even so, Green says the wolves' situation remains tenuous and experts are studying how climate change may affect them.
Michigan Technological University biologists are conducting their annual winter study at Isle Royale and are expected to release updated wolf and moose numbers next month. The total population is now down to nine wolves. That's the lowest number recorded by researchers who have been studying the Isle Royale wolf population for the last 54 years.
It's the longest continuous predator-prey study in the world. He told us they didn't know what happened to them, "but we will know," he said. Well, now they know how three of the seven wolves died. One was a young female wolf. Researchers like Durwood Allen, and Michigan Tech's John Vucetich and Rolf Peterson have been keeping a close eye on the animals on the island for more than five decades. Peterson has been doing it the longest. He's been watching and documenting things on Isle Royale for 42 years.
So they have a program called Moosewatch. For more than five decades, researchers have been closely watching the ebb and flow of wolves and moose on Isle Royale. To do their work, wolf biologists Rolf Peterson and John Vucetich of Michigan Tech lean on those willing to pitch in and help. Moosewatch volunteers hike off-trail for miles with their backpacks getting heavier as they pick up moose bones along the way. They get bitten by bugs, scratched by branches, and soaked by the rain as they make their way through Isle Royale's boreal forest.
And they pay for the experience. The researchers have been relying on these summer volunteers since