Chapter 1. Isle Royale Wolf and Moose Population Dynamics

1.1 Test New Tab

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1.2 Background

When studying populations of other animals, it is often necessary to study the relationships between two different populations that interact closely with one another, such as in predator/prey relationships. While predator/prey relationships can also be studied at the community level, they are important in population ecology because these relationships often cause a cycling of population sizes between high and low populations.

While we discussed exponential growth in the human population, continuous exponential growth rarely occurs in nature. Most populations exhibit a logistical growth pattern, which starts out slowly, enters an exponential growth phase, and then slows again as limiting factors take effect. If an environment had unlimited resources and no natural disasters, a population could grow at its biotic potential. This is its maximum reproductive rate provided by its fertility rate and survival rate. However, this potential is not realized in the presence of limiting factors.

There are two basic types of limiting factors. Density-independent limiting factors reduce the population size regardless of its density such as tornadoes, hurricanes, fires, tsunamis, volcanoes, and weather. Density-dependent limiting factors, such as food supply, available space, water, nutrients, and disease affect the population size the most when the density is high. Predation is also considered a density-dependent limiting factor because the density of the predators affects the density of the prey, and vice versa.

Isle Royale is a small island located in Lake Superior in waters belonging to the state of Michigan. The island is located 15 miles from shore and is approximately 40 miles long and 5 miles wide, and it covers about 200 square miles. As a national park, the island is protected, and camping and scientific research are limited by a system of permits. Subsequently, the animals and plants of the island are left to interact in a relatively undisturbed environment.

The island is a superb study site for predator – prey relationships because of its relatively undisturbed nature, location and size. Because the island is far from shore, animal migration has little to no effect on population size. The island’s area is large enough to support healthy population sizes, yet small enough to be manageable.

1.3 Objectives

1.4 Skills

Note: Only instructors see this section; it won't be shown to students.

• Understand elements of research design and how they impact scientific findings/conclusions (e.g., identify strengths and weaknesses in research that are related to bias, sample size, randomization, and experimental control)
• Read and interpret graphical representations of data
• Justify inferences, predictions, and conclusions based on quantitative data

• Compare/contrast, 4
• Predict, 6
• Experiment, 4
• Construct, 5
• Evaluate, 6
• Analyze, 4

• Competition
• Other predator/prey relationships
• Research tools
• Climate change
• Disease
• Research papers

1.5 Make Hypotheses

In this exercise you will conduct research on Isle Royale, to decipher the relationships between the populations of moose (Alces alces) and wolves (Canis lupus). You will also take into consideration other organisms in the food chain including moose ticks (Dermacentor albipictus) and trees, particularly balsam fir (Abies balsamea). Finally, you will consider how a warming climate has been having indirect effects on the moose population, and may subsequently affect the wolf population in the future.

Your research question concerns how the population sizes of wolves (predators) and moose (prey) affect one another. A prediction that states that two variables have no effect on one another is called a null hypothesis. Predictions that indicate a relationship between two variables are called alternative hypotheses.

1.6 Collect Data, Part 1

Now, let’s collect some data to see if your prediction is correct! You will begin your research by viewing aerial photographs to conduct population counts of the wolves and moose. Aerial survey is used to count wolves by flying circular patterns to monitor the entire island. While flying, you will look for wolf tracks that will lead you to the packs.

Moose are 50 times more plentiful on the island than wolves with an average population size ranging between 500-2,500 individuals over the last 50 years. Wolves are far less numerous on the island, with population sizes ranging from 10-50 individuals. Wolves usually exist in packs, displaying a clumped distribution, which makes them easier to count than the moose population, which is spread out in a random distribution pattern. Moose are good fighters so usually it requires a pack of wolves to take one down. Sometimes wolves will break off from the pack and live as loners, often scavenging food.

First, count the wolf population in Isle Royale. Click on the thumbnail map at right to launch the full map view. You'll need to navigate around the island, counting all the wolves you see. Remember: Wolves tend to travel in packs!

1.7 Collect Data, Part 2

Due to the large population size of the moose, it is not feasible to count the entire population. Instead, the 200 sq. mile island is broken into square kilometer plots and about 20% of the island is counted, plot-by-plot. Since moose are randomly distributed, this data can be extrapolated to the entire island. This means that the tabulated number is used to estimate the entire population on the island.

Click on each of the four plots in the thumbnail map at right and count the moose in each plot, then enter your results below.

1.8 Assessment

Your graph of wolf and moose populations by year on Isle Royale is shown at right. Now use this graph, along with the knowledge of population dynamics learned in this activity, to answer the following questions.