Predator Prey Relationships - National Wolfwatcher Coalition
The forgotten prey of an iconic predator: a review of interactions between Studying the Winter Nutritional Status of Moose and its Relationship to Moose Survival, result of human activities that adversely altered the caribou's environment. Role play moose and wolves to re-enact the predator/prey relationship between the two species found in Denali National Park. 2. Demonstrate the relationship. This research project is the longest continuous study of any predator-prey system replace hateful myths and form the basis for a wiser relationship with wolves. of interest for so long because they offer some very important, general lessons.
After Handout 2 is completed, choose a different group to share their answers with the class. There can be questions and further points of discussion, but the minute time frame should be observed.
About The Project: Overview
Optional resources for student groups may include a map of Isle Royale to point out locations where the data on the graph was collected. After Handout 3 is completed, choose a different group to share their answers. After the last data set on Handout 3 has been discussed, allocate 30 minutes to a final synthesis and an explanation of any follow-up assignments. Thus the total time allocated to this case will be two minute class periods.
To complete the final synthesis, student groups should prepare one of the following follow up exercises. Choose one exercise or let students choose, depending on the time available or instructional goals. Write a position paper defending one of four possible conclusions based on the data presented: Reject the primary productivity hypothesis and accept the trophic cascade hypothesis.
Accept the primary productivity hypothesis and reject the trophic cascade hypothesis. Find and evaluate a second article on trophic interactions from the primary literature. Be sure to include the following parameters: What specific hypotheses were evaluated?
Can you identify the predictions of each hypothesis? Are they mutually exclusive? What assumptions are made by the authors? Can you identify any caveats to the design or the data presented in the article? Do the authors suggest alternative explanations that would also explain their findings?
Can you identify alternative hypotheses? Many of the questions raised in this case could become the basis for problem-based learning exercises. Data relevant to several of the assumptions in this paper have been published elsewhere.
Students should be able to find additional information on the relationship between canopy dynamics, environmental conditions, and tree growth rates: A second case study could be analyzed example: In the wolf population crashed when humans inadvertently introduced a disease, canine-parvovirus.
Inthe moose population collapsed during the most severe winter on record and an unexpected outbreak of moose ticks. In the late s, intense levels of inbreeding among wolves were mitigated when a wolf immigrated from Canada.
In response, the wolf population generally increased throughout the early 21st century, despite declining moose abundance. For a more detailed interpretation of this graph, click here.
How are wolf and moose abundances related to one another? Each symbol on this graph represents the density of moose read from the horizontal axis and the density of wolves read from the vertical axis for a particular year.
- Predator Prey Relationships
- The Population Biology of Isle Royale Wolves and Moose: An Overview
The open circles represent years prior toand the black circles are years after The year was an important tipping point in wolf-moose dynamics, triggered by canine parvovirus Wilmers et al. Wolf and moose densities are the total number of wolves and moose on Isle Royale, divided by the size of Isle Royale, km2. Expressing abundance in terms of density allows us to compare Isle Royale wolf-moose dynamics with those observed in other parts of the world.
This graph tells a great deal about how wolf and moose populations are interconnected. If, for example, wolf abundance was determined primarily by food availability and if total moose abundance indicated food availability, then wolf and moose abundances would be positively related. This kind of relationship is what ecologists refer to as the bottom-up control of a food chain. By contrast, if moose abundance was determined primarily by wolf predation, and if wolf abundance was a good indication of predation pressure, then wolf and moose abundance would be negatively related.
This kind of relationship is what ecologists would refer to as the top-down control of a food chain. This graph shows that wolf and moose abundances are neither positively nor negatively related. In other words, fluctuations of wolves and moose on Isle Royale are not explained by either simple top-down or bottom-up explanations.
The true explanation is quite a bit more complex. How much food does each wolf get? The per capita kill rate or sometimes just called kill rate is a special statistic indicating how much food the average wolf in a population gets. The kill rate is calculated by observing how frequently wolves kill moose.
More specifically, the kill rate is the number of moose killed, divided by the number of wolves in the population making those kills, divided by the time we spent observing those kill events. Consequently, the units on kill rate are kills per wolf per month. In this graph, the symbols represent the average monthly kill rate for each winter between and The most remarkable observation is just how variable kill rate is. The highest kill rate we ever observed was in That kill rate was more than twice the average kill rate.
The lowest kill rate we ever observed was in That kill rate was less than a third of a typical kill rate. Imagine an entire winter where you got twice the normal amount of food, or less than a third.
Ecologists have long known that predator kill rates are variable. The challenge has been to understand why. One of the earliest theories, going back nearly a century, is that kill rate should be greatest during years when prey are most abundant, mainly because abundant prey should be easier to find when they are abundant. A more recent idea is that kill rate might also tend to be lower during years when predators are more abundant. One reason is that when predators are more abundant, they might spend more time on activities like defending their territories from other predators.
These two ideas can be combined by supposing that kill rate should be greatest during years when the ratio of moose per wolf in the population is greatest. The graph above suggests that these processes are important on Isle Royale. R2 is a statistic that can range from zero to one. A value of one would indicate that fluctuations from one year to the next in the number of moose per wolf i.
A value of zero would indicate that moose per wolf explains none of that fluctuation. Our challenge is to better understand what accounts for the other half of the fluctuations. For a more technical treatment of these ideas, see Vucetich et al. How does food supply kill rate affect the wolf population?
In the previous section we learned a bit about what causes the kill rate to fluctuate from year-to-year. For example, look at the symbol in the lower left portion of the graph. That was the yearwhen kill rates were the lowest ever and the wolf population crashed from 30 to 14 wolves. That population crash is indicated by how low that observation is, with respect to the vertical axis.
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. This observation is far to the left side of the graph and a little high.Wolves of Isle Royale
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.
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.
The Living Edens: Activities for the Classroom - The Wolf and the Moose
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.
Intermountain Journal of Sciences
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.