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Activity: Yellowstone Community Dynamics
General Introduction:
The movie “Strange Days on Planet Earth: Predators” depicts the dynamic relationships within
the Yellowstone National Park ecosystem. The Yellowstone community consists of interactions
between populations of elk, wolves, aspen, willows, songbirds, beavers and many other species.
Species can interact in a number of ways. There can be competition between and within species
for food or territory, predator/prey interactions, mutualisms and parasitism. In order to discuss
the movie in this context, several terms need to be introduced.
A dominant species is the species (typically an animal) within a community whose population is
the most abundant or which has the highest biomass (total mass of all individuals in a
population). Dominant species may control which other species are present in a community, due
to species interactions like competition. Traits that may make a species dominant in one
environment may not be competitive in another environment.
A keystone species is a species that has a very strong impact on the other species in its
ecosystem relative to its abundance (which is typically low). Keystone species are typically not
the dominant species in an ecosystem, yet despite their low population numbers, they play key
ecological roles that control community structure.
A foundation species, also known as an “ecosystem engineer” is a species that plays a major
role in shaping ecosystems by directly altering its physical environment, typically creating and
enhancing a habitat in a way that benefit other species.
We also need to look at what factors affect the size and growth of a population. There are two
main factors that determine how much a population can grow: biotic potential and
environmental resistance. A species biotic potential defines its reproductive capacity, which
can lead to population growth. It contains such life history traits as how many offspring an
individual can produce, how often it can reproduce, how old it has to be before reproducing and
how many of its offspring survive to reproduce themselves. If a population has plenty of food
and space and no predators or illness, they will probably be reproducing at their maximum
biotic potential and the rate of population growth will increase.
Carrying capacity is the maximum population of a given species that a particular habitat can
sustain indefinitely without degrading the habitat. Species reproducing at their maximum biotic
potential can exceed the carrying capacity of their environment. Environmental resistance is
all the factors that act to limit the growth of a population. These can include predation,
competition for limited resources (e.g., food, breeding sites), diseases and also abiotic (nonliving) factors such as severe storms and droughts. Environmental resistance usually acts to
keep a population under the carrying capacity, but factors that change the dynamics within a
community can also affect this.
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Activity: Yellowstone Community Dynamics
Watch the movie “Strange Days on Planet Earth: Predators”. Discuss the following questions
with your group.
Questions
1. Which organism(s) is/are the dominant species in this community? How do they affect the
community?
2. Which organism(s) is/are the keystone species in this community? What affect do they have
when present? When absent?
3. Which organism(s) is/are the foundation species (ecosystem engineers) in this community?
How did they change the community?
4. What would happen if the elk were removed entirely from the ecosystem or if their numbers
were greatly reduced due to disease?
5. Describe the population ecology of the elk prior to the reintroduction of wolves in terms of
biotic potential and environmental resistance.
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Functional groups
Functional groups are non-phylogenetic, aggregated units of species sharing an important
ecological characteristic and playing an equivalent role in the community. They show either
similar responses to the environment or similar effects on major ecosystem processes.
6. Make a list of the different functional groups or roles that would be a part of the Yellowstone
ecosystem.
Food Webs
A key finding in research on food webs is that they typically consist of many weak links, rather
than a few strong links. Two species are strongly linked if they interact a lot, such as a predator
that consistently eats one prey type. Most food webs used to be thought to be dominated by
bottom-up effects, where changes in the lowest trophic levels have the strongest impact. The
Yellowstone case study is an example of top-down control.
Examine the food web in the Atlantic Ocean of the coast of the U.S.
7. What changes are occurring within this food web? Are the changes under top-down or
bottom up control?
8. If this trend continues, what do you predict will happen to populations of scallop and clam?
9. Make a sketch of a food web based on the Yellowstone case study. Indicate where links are
weak or strong.
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8. Why do you think most food webs are made of weak links? What would be the benefit of
weak over strong links?
Food chain in Peter Lake
Peter Lake is the site of an ecological experiment on food webs. The lake is currently populated
by fathead minnows and pumpkin seed (small predatory fish) which feed on water fleas and
other tiny animals (zooplankton). The zooplankton feed on the algae.
Minnows (small predatory fish)
Water fleas (zooplankton)
Algae (phytoplankton)
9. What are the possible consequences of increasing the nutrient load of Peter Lake due to runoff
from nearby farms?
10. How can the food chain be changed to avoid the problems discussed in question #1?
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