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Isle Royale
ISLE ROYALE
LEARNING OBJECTIVES:
1. Examine exponential and logistical growth models and learn about carrying capacity.
2. Examine how predators affect the population of herbivores.
3. Look at the effect of environment on populations.
ASSIGNMENT:
During your lab section, work with a partner to complete the Isle Royale SimUText
module, including all of the simulations and questions (20 points)
● Following completion of the module, complete the Canvas quiz (20 points)
●
INTRODUCTION
If you were to travel on Route 61 to the farthest reaches of Minnesota and stand on the shore of
Lake Superior looking east, on a clear day you would see Isle Royale. This remote, forested
island sits isolated and uninhabited 15 miles off the northern shore of Lake Superior, just south
of the border between Canada and the USA. If you had been standing in a similar spot by the
lake in the early 1900s, you may have witnessed a small group of hardy, pioneering moose
swimming from the mainland across open water, eventually landing on the island. These
fortunate moose arrived to find a veritable paradise, devoid of predators and full of grass, shrubs,
and trees to eat. Over the next 30 years, the moose population exploded, reaching several
thousand individuals at its peak. The moose paradise didn't last for long, however.
Lake Superior rarely freezes. In the 1940s, however, conditions were cold and calm enough for
an ice bridge to form between the mainland and Isle Royale. A small pack of wolves found the
bridge and made the long trek across it to the island. Once on Isle Royale, the hungry wolves
found their own paradise—a huge population of moose. The moose had eaten most of the
available plant food, and many of them were severely undernourished. These slow-moving,
starving moose were easy prey for wolves.
The study of moose and wolves on Isle Royale began in 1958 and is one of the longest-running
studies of its kind. The island provides conditions for a unique natural experiment to study this
predator-prey system. Isle Royale is large enough to support a wolf population, but small enough
to allow scientists to keep track of almost every single wolf and moose on the island, making it a
practical place to study population ecology.
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Population ecologists are interested in how and why ecological populations grow and shrink in
the ways that they do. The basic ideas in population ecology are not complicated, but they do
involve a little math.
In this lab, you will perform experiments to study some interesting population dynamics using a
simulation model based on the Isle Royale community. Although it is greatly simplified
compared to "real life", the simulation captures the salient features of how wolves, moose, and
three plant species interact.
SECTION 1: THE MOOSE ARRIVE
MEET THE ISLE ROYALE COMMUNITY
The first exercise focuses on the population of moose that first swam to Isle Royale discovering
an island with no predators and lots of plants to eat.
Grasses
The Isle Royale model simplifies the vegetation community and treats all grass species as one. In
reality, many different species of grass exist on the island and colonize open areas. These open
areas often develop into forest with time.
True grasses are monocotyledons of the family Poacae (also known as Gramineae). Worldwide
there are up to 10,000 species of grass.
Grasses typically have hollow stems and leaves that arise from nodes, hugging the stem. Leaf
blades of many grass species are hardened to discourage herbivory. Grass stems grow from the
base of the stem rather than from the tips. This provides them with a competitive advantage over
other plants when under pressure from herbivores, because the growing parts of stems are
unlikely to be harmed when grazed.
Grasses reproduce by growing out from a central parent plant. Grasses can by bunch-type,
stolonigerous, or rhizomatous. Bunch-type grasses grow tillers (each with a shoot and roots;
tillers can be vegetative or reproductive) out from the central plant and have a bunch-like
appearance. Stolons are aerial shoots that give out roots to form new individual plants. Rhizomes
are underground, horizontal shoots that form new plants in a similar way to stolons. Grasses are
also classified depending on the metabolic pathway used to fix carbon during photosynthesis:
grasses are either C3 or C4. C4 plants have a competitive advantage in areas with drought, high
temperature, and low carbon dioxides concentration.
Grasses are important in providing food and shelter for many animal species. They also play a
vital ecological role in soil formation, soil atabilization, and nutrient dynamics. We, as humans,
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also greatly rely on grasses as food (e.g., cereals), structural materials (e.g., bamboo) and food
for domestic animals (e.g., hay for cattle).
Fir
Balsam fir, Abies balsamea, is a short- to medium-height evergreen conifer that can reach about
15m in height and 200 years in age. The species can live from sea level up to around 2000m
elevation and on a variety of organic and inorganic soils in relatively moist conditions. It occurs
across a wide geographic range from northern Alberta in Canada southward to Pennsylvania.
Balsam fir is one of the most aromatic and cold-hardy fir tree species. It is not very fire resistant
and is susceptible to attack by ambrosia beetles, balsam wooly adelgids, spruce budworm,
longhorn beetles, buprestid beetles and wood wasps.
The tree is used extensively by wildlife for food and shelter. Moose, white-tailed deer, red
squirrels and various bird species use balsam fir as a food resource and winter cover.
Balsam fir grows with a very narrow, spire-like crown, and hence is a popular Christmas tree. It
is also used by the timber industry for wood pulp and light frame construction. The bark blister
resin is an ingredient in Canada balsam, which is used in making microscope slides. The resin
was also used to make a chewing gum-like confection before the advent of chewing gum itself.
Sugar maple
The sugar maple (Acer saccharum) is one of the largest hardwoods, growing on approximately31
million acres of land but restricted to the cool, moist climates found throughout New England,
the Great Lake states, the mid-Atlantic U.S. and several Canadian provinces. The sugar maple’s
shade tolerance, for which it is famous, allows it to germinate and persist under a closed canopy
as an understory plant, responding to increased light with rapid growth when an opportunistic
gap becomes available in the canopy.
Serving as an immensely important species to the ecology of many forests in North America, the
sugar maple engages in hydraulic lift, drawing water from lower soil layers and exuding that
water into upper, drier layers, benefitting the tree itself and any other plants around it.
Unfortunately, as with many species, human environmental influences have contributed to the
decline of the sugar maple in many regions. The trees are highly susceptible to pollution, acid
rain and soil acidification, and the increased use of salt on roads for deicing has decimated the
tree’s presence in urban areas.
Despite the challenges, the sugar maple remains highly valued and perhaps the most famously so
as the principal source of maple sugar. The trees are tapped early in the spring for the first flow
of sap, which collected and boiled into syrup of further evaporated into sugar.
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Moose
The North American moose, Alces alces, is the largest member of the deer family (Cervidae).
The species can be found in North America, northern Europe (where it is known as the European
elk), and Asia. In North America moose are found in the forests of Canada, Alaska, New
England, and the upper Rockies.
Moose live in forested areas, adjacent to freshwater bodies, where there is winter snow cover.
They are primarily crepuscular (act9ve at dawn and dusk), have home territories between 5 and
10 square kilometers, and are good swimmers, able to swim at speeds of up the 6mpg. The word
“moose” derives from a Native American word meaning “twig eater”. Moose browse on your
shoots, buds and leaves of trees and shrubs, tree bark and water plants. Through their browsing
moose can have a dramatic effects on the composition of the vegetation community in their
habitat. As with other species in the deer family, moose can negatively impact the timber
industry through retarding the regeneration of pine, fir and spruce forests.
Moose mate each year in the early fall, at which time females attract males with pheromones and
vocalizations. Bulls (males) compete for females by fighting each other with their antlers and
hooves. Females breed from age two and give birth to one or two calves in the spring of early
summer after an eight-month gestation. Calves are weaned after six months and stay with their
mothers until they give birth the following year.
Moose populations tend to have high mortality in the zero- to one-year age group. Adult moose
reach their prime between 5 and 12 years, but begin to lose vitality when they are 10 years old.
Few bull moose survive beyond 15 years (due to the rigors of mating-related fighting) but
females generally live up to 15 to 20 years in the wild. Due to their large size, moose are not
highly susceptible to predation as healthy adults. Most moose are preyed upon as juveniles or as
weaker, older adults.
Moose populations have been greatly reduced by human hunting and habitat destruction in
certain parts of North America. In the eastern U.S. however, moose populations have been
expanding in recent years. While moose are not listed as threatened or endangered species
nationally, there is some concern regarding declining numbers in Michigan.
Grey wolf
They gray wolf, Canis lupus, also known as the timber wolf, tundra wolf or arctic wolf, is the
largest of the wild canid species and is found in North America, Europe, the Middle East and
parts of Asia.
Despite the name gray wolves vary in color from creamy white to sandy-red, gray, or black.
Wolves can live 16 years in the wild and 20 in captivity.
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Wolves are highly territorial with territories that can vary from 25 to 1000 square kilometers.
They gray wolf has the largest natural range of any terrestrial mammal apart from humans. When
prey availability is high wolf packs tend to have well-defined territories, reducing encounters
between rival packs. During periods of food shortages, however, wolf packs often come into
conflict.
Gray wolves are highly social forming packs of 2 to 20 individuals, depending on the
availability, abundance, and size of prey. Each pack has a strict social hierarchy based on
submission to dominant animals. Withing a pack, normally only the dominant male and female
(known as the “alpha male” and “alpha female”) reproduce to produce pups. Mating occurs
between January and April, and females give birth to pups in underground dens. Pups reach
sexual maturity at one year and may leave the pack at age two. They then often lead a solitary
lifestyle for some time before finding a mate and establishing a territory and pack of their own.
Mortality is highest in the young, particularly when food is scarce (when reproductive rates also
decline).
Gray wolves hunt prey that are generally larger than themselves, including moose, deer, elk,
caribou, bison and mountain sheep. They will eat smaller prey such as rabbit and beaver is larger
prey are scarce. Wolves hunt both during the day and at night. Pack animals tend to move in
single file and can travel over 30 km in one day. Most hunting pursuits aim to identify weaker
prey and result in a prolonged chase of the targeted animal. Prey are encountered by chance or
tracked down via scent. Once a prey animal is targeted, the pack generally stalks it before
attacking. Chases have been documented to vary from 100 m to beyond 5000 m, with success
occurring in about 30% of attempts. A wolf can eat over 15 kg of meat in one sitting. The pack
often remains within the vicinity of the kill for several days.
Historically, wolves have come into conflict with humans. There have been very few
documented accounts of wolves attacking people. However, with encroachment into their
habitat, wolves increasingly attack domestic livestock, particularly cattle and sheep. In the 20th
century, wolves were persecuted and eradicated from large areas of North America. More
recently, however, reintroductions (most famously at Yellowstone National Park) and
conservation strategies have allowed wolf populations to recover in a few areas in the Rocky
Mountains and Western Great Lakes. These remain contentious issues hotly debated by
conservation biologist, wildlife managers, farmers, and hunters.
In this lab section you will examine ecological succession. Plant succession is quite simple on
this virtual island. The grasses that cover the island in early succession are replaced by late
successional sugar maples and balsam fir trees.
After about 5 simulated years, the first moose swim over to the island from the mainland and
start munching voraciously on the plants.
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MODELLING POPULATION GROWTH
What determines how populations change in size over time?
Population ecologists address questions like this using mathematical models that incorporate
important aspects of the population's biology into equations. As you'll see, these models tend to
be very generalized and not particularly realistic, but they are important tools because they help
scientists develop broad theories, make predictions, and test hypotheses.
The simplest model of population growth ignores life's challenges and assumes that all necessary
resources for survival and reproduction are always present in excess. Under these conditions, the
population will grow exponentially: The larger it becomes, the faster it will grow. In
mathematical terms, the exponential growth model assumes that a population's size (N) increases
according to its per capita growth rate (r):
𝑑𝑁
= 𝑟𝑁
𝑑𝑡
The notation dN/dt represents "the change in population size with respect to time."
The following time-series graph depicts exponential population growth. When the population is
small, the shallow slope of the curve indicates a slow increase in size. As the population size
increases, the curve becomes steeper and steeper, indicating more rapid growth.
CARRYING CAPACITY
Population growth is typically limited by the availability of important resources such as food,
nutrients, or space. A population's carrying capacity is the maximum stable population size of the
species that the local environment can support over time. When a population is small, such as
during the early stages of colonization, it may grow exponentially (or nearly so), as you've seen
with moose on Isle Royale. As resources become scarce, population growth slows down and
eventually the population size levels off at the population's carrying capacity.
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Ecologists use the logistic growth model to include resource limitation in their projections of
population growth. This model is very similar to the exponential growth model: r is the per
capita growth rate, N is the population size, and t is time. A new term, K, represents the carrying
capacity:
𝑑𝑁
𝑁
= 𝑟𝑁(1 − )
𝑑𝑡
𝐾
As before, dN/dt represents the change in population size with respect to time.
The graph below depicts logistic growth. Notice how the curve initially resembles the
exponential growth graph but then levels off as N approaches K.
Graphs based on real population data rarely follow clean, smooth curves like those generated by
models. Random events and variability in the environment cause population sizes and carrying
capacities to fluctuate through time. Interactions with other species, such as predators, prey, or
competitors, can also cause population sizes to change in complex ways.
When population size fluctuates, ecologists often use an estimate of the average value around
which the size fluctuates as an approximation of carrying capacity.
SECTION 2: THE WOLVES ARRIVE
INTRODUCTION
One especially cold and harsh winter in the late 1940s, Lake Superior froze between the
mainland and Isle Royale. A small pack of wolves travelled across the ice from Canada and
reached the island. In this exercise, you will investigate how the presence of predators affects the
moose population through time.
The size of a population is just one of its characteristics, and wolf predation can affect moose in
ways besides reducing their numbers. One other potential effect involves the health of moose
that manage to avoid being eaten. Energy reserves, in the form of fat storage, are important
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health indicators for moose. The greater a moose's fat stores, the more likely it will survive the
winter and produce offspring.
SECTION 3: CHANGES IN CLIMATE
INTRODUCTION
The prospect of climate change due to increasing atmospheric greenhouse gases is cause for
concern. Recent evidence suggests that temperatures around the world are rising. In particular,
the average yearly temperature in northern temperate regions is expected to increase
significantly.
This change will lead to longer, warmer spring and summer seasons in places like Isle Royale.
The duration of the growing season for plants will therefore be extended, resulting in more plant
food for moose living on the island. In the Great Lakes, the growing season is expected to
increase in length by 4-9 weeks by 2100. How might a longer growing season affect the moose
and wolf populations on Isle Royale? Would the number of moose and wolves both increase with
higher and higher temperatures, and longer and longer growing seasons?
One method ecologists make predictions about the impacts of global warming is by testing
different scenarios using simulation models similar to the one you've been using in this lab. Even
though simulations are simplifications of real-world dynamics, they can be useful tools for
exploring how things might change in the future. In this exercise, you will use the Isle Royale
simulation to investigate how changes in average yearly temperature due to global warming may
affect the plant-moose-wolf system on the island.
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SECTION 4: AN EXTENDED EXPLORATION: THE T-TEST
INTRODUCTION
In Section 2 you compared fat storage for moose with wolves absent to fat storage for moose
with wolves present. In this section, you will determine whether or not any difference in fat
stores that you observed is meaningful.
Unless you examine every single moose in both populations, there is no way to know for sure
whether the difference in mean fat stores is real and not an accidental result. But you can make a
good guess using statistics. In fact, inferential statistics were invented to allow us to better
uncover the truth and answer these sorts of questions. In this section, you will perform a simple
statistical test, called a t-test, to decide whether or not the wolves' presence had a significant
effect on moose fat stores.
If we were to be very thorough and formal in this t-test lesson, we would include a lengthy
discussion of such concepts as random variables, sampling distributions, standard errors, and
alpha levels. These are important, but to keep this section short, we will focus on the big picture
of what it means to try to answer a question with inferential statistics.
The core ideas underlying the t-test are as follows:
Start with a statistical question: Are the mean moose fat stores different when wolves are present
versus absent?
The null hypothesis is a negative answer: there is no
real difference. Under the null hypothesis, any
difference between your sample means arises from
chance. This scenario is illustrated on the top half of
the figure on the right.
The alternative hypothesis is that the means of the
two groups are different—that wolf presence is
associated with a meaningful and significant effect
on moose fat stores. This is represented on the
bottom half of the figure on the right. In order to
know which hypothesis your data support, the null
or the alternative, you examine the difference in
means, relative to the variability you observed.
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