Trophic Cascades: What Happened When Wolves Were

Trophic Cascades: What Happened When Wolves

Were Reintroduced to Yellowstone Park?

Lesson Question

How did the reintroduction of wolves into Yellowstone Park affect the

other animals and plants in the ecosystem?

Lesson Tasks

Students analyze data to determine the effect of wolves on Yellowstone’s elk population, on the plants that elk

graze on, and on the animals that compete with elk for food. They write a report describing how the

reintroduction of wolves has created a trophic cascade—not just a few direct changes in one food chain, but a

series of indirect changes throughout the food web.

Standards

• HS-LS2-2 Ecosystems: Interactions, Energy, and Dynamics

NGSS Science and Engineering Practices

• Constructing Explanations and Designing Solutions

• Engaging in Argument from Evidence

• Evaluate the claims, evidence, and reasoning behind currently accepted explanations or solutions to determine the merits

of arguments.

NGSS Disciplinary Core Ideas

• LS2.C: Ecosystem Dynamics, Functioning and Resilience

• ETS1.B: Developing Possible Solutions

Crosscutting Concepts

• Stability and Change, Patterns

Connections to Nature of Science

• Scientific knowledge is open to revision in light of new evidence.

• Most scientific knowledge is quite durable, but is, in principle, subject to change based on new evidence and/or

reinterpretation of existing evidence.

Trophic Cascades: What Happened When Wolves Were Reintroduced to Yellowstone Park?

TABLE OF CONTENTS

OVERVIEW ........................................................... 3

• Content Objectives

• Data Skill Objectives

• Instructional Sequence

• Lesson Background

[Estimated time: 15 minutes]

THE HOOK ............................................................ 6

TRANSITION TO BACKGROUND ............................ 6

[Estimated time: 30 minutes]

BACKGROUND ..................................................... 6

• Background 1: Meet the Researchers

• Background 2: Food Webs and Trophic Cascades

• Background 3: Predator-Prey Dynamics

• Background 4: The Role of an Apex Predator in an Ecosystem

TRANSITION TO DATA ORIENTATION .................. 10

[Estimated time: 30 minutes]

DATA ORIENTATION ......................................... 11

• Data Orientation 1: The Table and the Graph

TRANSITION TO INVESTIGATION ....................... 12

[Estimated time: 90 minutes]

INVESTIGATION ................................................ 13

• INVESTIGATION 1: Wolves-Elk Trophic

Relationship

• INVESTIGATION 2: Wolves-Elk-Aspen Trophic

Relationship

• INVESTIGATION 3: Wolves-Elk-Berries Trophic

Relationship

• INVESTIGATION 4: Summarizing Your Results

• INVESTIGATION 5: Interactions Within Trophic Cascades

• INVESTIGATION 6: Summarizing the Results

TRANSITION TO WRITING TASK ........................ 20

[Estimated time: 30 minutes]

WRITING TASK ................................................... 21

• Purpose

• Big Ideas

• Facilitation Suggestions

ASSESSMENT .................................................... 23

• Writing Product Assessment

• Key Question Notes

Trophic Cascades: What Happened When Wolves Were Reintroduced to Yellowstone Park?

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OVERVIEW

Content Objectives

Students will understand

Trophic cascades can occur when a top predator is removed or introduced into a food web; the

resulting changes in the relationships among species are not simply linear, but can be web-shaped.

Trophic cascades can and do have effects on other biotic and abiotic elements of the ecosystem.

A population’s abundance fluctuates depending on its interactions with other species and with the

environment.

Skill Objectives

Analytical Thinking

• Assessing evidence in relation to claims

• Generating claims based on data

• Integrating information across sources

• Reasoning with data

Data Communication

• Communicating ideas from data

Data Visualization/Interpretation

• Reading measurements from graphs or tables

• Identifying and describing data patterns

Math/Statistics

• Calculating statistics from a sample

• Evaluating and interpreting relationships between two variables

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Instructional Sequence

Share this plan of action with students so they know what to expect from the lesson.

HOOK We will start by watching a video about what happened when wolves were reintroduced to

Yellowstone National Park.

BACKGROUND Then we’ll go over background information you’ll need to understand the data you’ll

look at—about food webs, trophic cascades, and population dynamics.

DATA ORIENTATION Next you’ll practice with tools for manipulating data on Yellowstone

populations.

INVESTIGATION Then, working on your own or in pairs, you’ll analyze data on trophic relationships

among wolves, elk, aspen, berries, and bears. You’ll build and analyze graphs and take notes on the

patterns you see.

WRITING Finally, you’ll write a report describing the trophic cascade you found—the way wolves

influenced, in turn, elk, aspen, berries and bears. You’ll present data to support your description.

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Lesson Background for Teachers

This lesson is about the interactions among species within an ecosystem, and how these connections affect the

abundance of those species. The lesson has four basic ideas:

• In an ecosystem, the interactions among organisms and the environment are dynamic and complex.

• Within a food web, there are food chains that describe trophic relationships from one organism to the next.

• Ecological relationships determine the population dynamics of the species.

• A trophic cascade can indirectly affect organisms that are not part of a food chain.

In the lesson, students investigate a set of trophic relationships in the Yellowstone food web. First, they trace the

dynamic relationships among three species pairs—wolves and elk, elk and aspen, and elk and berry bushes— and

construct a food chain consisting of wolves-elk-plants. Encourage students as they work to think about the

population dynamics among these species.

Students then examine the cascading effect, beyond a simple food chain, of wolves on other organisms in the

ecosystem—in this case, bears. This is where a trophic cascade becomes visible: Because elk and bears compete

for some of the same plants, students are able to trace how the wolf population indirectly affects bears.

In their final writing, students describe a mini-food web with these organisms (wolf, elk, aspen trees, berry bushes

and fruit, and bears), and write about the relationships they found among these species, using graphical data as

evidence.

Three final notes: First, it may be helpful to tell students how the data was collected by scientists; this information

can be found in “Background 1: Meet the Researchers” and at the beginning of each investigation.

Second, concepts maps in the form of food webs are used extensively in the lesson. One of the most useful roles of

concept maps in science education is as a means of assessing student understanding of scientific concepts. We

encourage you to use them in this way.

Finally, the video shown in “The Hook” is used to set the context of the study. It was created to educate the public

about what happened when apex predators—wolves—were reintroduced to Yellowstone Park, and has had

millions of viewers on the Web, along with some criticism that some of its claims are exaggerated or

oversimplified. You may wish to point out that the video does not present actual data, and that the lesson gives

students a chance to see if some of the claims are in fact true. If students are interested, there is more data

available on both the same and different aspects of this trophic cascade, beyond what is included in the lesson..

Student Background Knowledge

Before beginning the lesson, students should know the following

• Food webs show the relationships among organisms in an ecosystem.

• Trophic cascades are caused by the addition or removal of an organism in a food web. They can affect

organisms not just at various trophic levels (that is, in a single food chain), but throughout the food web.

Trophic Cascades: What Happened When Wolves Were Reintroduced to Yellowstone Park?

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The Hook (Estimated time: 10 minutes)

How Wolves Change Their Ecosystem.

Purpose

● To engage students by showing a video about

the effects of the reintroduction of wolves to

Yellowstone National Park

Big Ideas

• The reintroduction of wolves to the park had a

wide and positive influence in the park’s

ecosystem.

Facilitation Suggestions

• After the video, ask students to write

their answers to the questions on the Hook

screen—“What problem led to wolves being

reintroduced to Yellowstone?” and “What is an interesting change that occurred in Yellowstone? How did

the reintroduction of wolves lead to this change?” Ask some students to share and compare their responses,

and be alert for misconceptions.

• Tell students that the video summarizes the findings of scientists, but does not include data.

Their task is to examine real scientific data for evidence that some of the events described in the video are

backed up by evidence. (The film was made by husband-and-wife filmmakers who say their mission is to

“create inspiring video stories that evolve human consciousness by addressing the roots of our many

sustainability crises.” It has had tens of millions of views on the Web. .https://sustainablehuman.org/).

TRANSITION TO BACKGROUND

Inform students that there are important concepts that they need to know in order to

understand and interpret the data and results correctly.

Background [Estimated time: 35 minutes]

Project the background slides to the class, and have students actively read and discuss the content and questions,

so they develop background knowledge needed in the investigation.

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Background 1: Meet the Researchers

Purpose

• To make the connection between the data

presented and the scientists who

produced it.

Big Ideas

• Science is a collaborative endeavor and is

based on past findings.

Facilitation Suggestions

• Introduce the researchers. This panel is

an opportunity to connect the students

with the researchers, where they

work, and what their interests are.

• Emphasize that research is a collaborative effort by many people and institutions, and is always

based on past investigations. The exact methods scientists use to study a phenomenon are different, but if

the results are the same—if they are “corroborated”—then it strengthens the evidence that they are seeing

something real.

• Inform students that they can find more information about the researchers by going to the links

provided under their pictures, where it says, "show data source."

TRANSITION POINT

In order to understand the ecosystem that the investigators study, Yellowstone National Park, we need to find out the

general structure and function of ecological networks such as food webs and trophic cascades.

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Background 2: Food Webs and Trophic Cascades

Purpose

• To understand that ecosystems are composed

of several interacting elements. These

relationships can be depicted as a web

• To learn that subtracting or adding an

organism to a food web causes ripple effects

throughout the food web. This is called a

trophic cascade

Big Ideas

• Changes on one element of the food web

can have repercussions on other

components of the food web.

Facilitation Suggestions

• Go through the cartoon of the kelp forest food web slowly, as it is important that students understand

the structure and mechanics of a food web. Point out the trophic levels and ask students what they think

the arrows mean.

• Make sure students know the definition of a food web: a depiction of the several interactions among the

components of an ecosystem. Remember that the example provided is a cartoon simplification of a kelp

forest food web.

• Review the concept that a food web can have several food chains, and explain how a food chain

describes how one organism eats another and in turn this eats the next, going from a top predator to

herbivores to plants. Notice that the arrows depict who feeds whom.

• Pause for student input, before students answer the question in step one. Give them time to think, and

invite participation. Make sure students explain their answers and encourage them to use words like

predation, competition, and trophic.

• Assist students in understanding step two. This is crucial, since they are supposed to define

trophic cascades and make the connection between trophic cascades and food webs. It may be

challenging to have students verbalize the relationship between trophic cascades and food webs: A trophic

cascade can start at any trophic level within the food web and its effects may follow one or more food

chains.

• In step three, students explore the indirect effects of adding or removing a component of the food

web and how this affects specific interactions among the varioius organisms. Make sure students become

familiar with these concepts, since they will be encountered in the lesson.

•

TRANSITION POINT

Let us look more closely at the interactions between two trophic level organisms in a food web. We will examine closely the population

dynamics of predation.

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Background 3: Predator-Prey Dynamics

Purpose

• To emphasize the close relationship between the population cycles of two interacting species (predator-

prey).

Big Ideas

• The oscillation observed in the

abundance of predator and prey

species is due to a time delay in their

responses to each other's abundance.

• Population dynamics between species

is determined by the type of ecological

relationship (predation, competition,

symbiosis, parasitism, etc.) as well as

abiotic elements from the environment

(harsh winter, wet spring).

Facilitation Suggestions

• After reading the text, invite students to describe the pattern in the graph. Encourage them to tell a

story over time and to use scientific vocabulary. Ask students: Why are the peaks and valleys of the two

populations out of phase?

• Note that in nature, these clean-looking cycles are not the norm because there are other issues that may

affect the relationship—for example, disease, crowding, competing predators, or food shortage.

• Point out that while the diagram shows predator-prey cycles over 80 years, in the lesson students will

look at a much more narrow window, only one cycle. They will have much less data from which to

extrapolate about predator-prey dynamics.

• At this point, students should be able to define what “predator-prey dynamics” means. To make

sure students are on the right track, you may spot check by asking a few students to share their responses

TRANSITION POINT

Before we go on to see the data, we will apply some of the concepts learned so far in the Background sections to the

Yellowstone ecosystem.

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Background 4: The Role of an Apex Predator in an

Ecosystem

Purpose

• To apply the concepts learned by

hypothesizing about the structure of a

trophic cascade, using the animals in the

picture

Big Ideas

• A trophic cascade involving wolves has

repercussions on other components of

the food web.

• Animals in the same trophic cascade

often compete for the same food

Facilitation Suggestions

• This slide previews what students are

going to do in the lesson. Remind them

what they saw in the video, and ask them

to make a hypothesis using the concepts

learned in the Background sections.

• Invite students to read the text on the

left-hand panel and examine the food

web in the picture. Read the questions

and direct students to work with a partner in constructing their hypothesis.

• Point out the complexity of the food web. Notice that there is more than one top predator, and that

they will compete for the same prey.

TRANSITION POINT

Remind students of the lesson task—to determine the effect of wolves on other Yellowstone organisms and write a report

describing how the reintroduction of wolves has created a trophic cascade—and inform students that in the next section,

the Data Orientation, they will be learning to use the necessary tools to analyze the data collected by the investigators.

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DATA ORIENTATION Estimated Time: 15 minutes

We suggest that you walk through the Data Orientation as a class, projecting your screen and guiding students as

they practice manipulating the data. As students complete each exercise, show them how to use CODAP to

construct a graph, and point out how the data in the table and the graph are related.

Data Orientation 1: The Table and the Graph

Purpose

• To familiarize the students with the data in the table and the source of the data

• To have students practice simple data manipulations they’ll use in the investigation

• To learn how to use CODAP features and tools

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Facilitation Suggestions

• Go slowly through the steps in the left-hand panel so that students are able to perform the operations by

themselves—for example, dragging attributes from the table to the y-axis and x-axis on the graph.

• Encourage students to view the “Show Me” videos that guide them in each operation.

• Encourage students to explore the connections between the data points represented in the

graph and the table. Click on the data points on the graph to see corresponding data in the table; click on

rows in the table to see corresponding data points on the graph. Point out the connections between the

table and graph. Ask students what each point on the graph means.

• Inform students that they can go back to any of the previous sections (Background, Orientation) by

clicking on the top green panel. Show students that they can always undo or reset their work by clicking on

these icons

Start Over

Undo

TRANSITION TO INVESTIGATION

Tell students that now that they know how to work with CODAP, they will continue to use these tools to work with the data on their own

to answer the question, “How did the reintroduction of wolves into Yellowstone National Park affect the other animals and plants in the

ecosystem?”

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INVESTIGATION 1: Wolves-Elk Trophic

Relationship?

Estimated Time: 15 minutes

Purpose

• To investigate the population dynamics between wolves and elk

Big Ideas

• The wolf-elk trophic relationship shows a classic predator-prey population dynamic.

Facilitation Suggestions

• In the first step, students propose a hypothesis about the trophic relationship between wolves and

elk. Encourage them to use what they have learned in the Background section.

• Help students construct three graphs using the data in the table—wolf and elk population trends

separately, and then both together—and analyze each. The questions are designed to help students be

detailed in their examination.

• Remind students to save the Elk-Wolf graph in their notebook by taking a snapshot with the camera

tool located in the graph’s right-hand menu.

• Point out that the numbers of elk and wolves are very different. You may want to use the small

exercise in the step four "Hint" to explain how the predator-prey ratio concept works.

• In step five, make sure that students use the concept of population dynamics correctly in the

explanation of their results. Invite a few students to verbalize their answers.

• You may wish to use the last question for a small discussion; this may be useful in getting all students on

the same page before going on to Investigation 2.

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INVESTIGATION 2: Wolves-Elk-Aspen Trophic

Relationship Estimated Time: 30 minutes

Purpose

• To investigate the trophic relationships among wolves, elk, and aspen trees

• To construct a three-level trophic structure or food chain.

Big Idea

• The trophic cascade initiated by the reintroduction of wolves causes the decrease of its prey, the elk; in turn,

there is an increase in one of the prey's food sources, aspen trees.

Facilitation Suggestions

• In the first step, students predict how the reintroduction of wolves will affect the elk's food—the aspen

population—and why. Encourage them to use words and terms such as predation, trophic levels, and

trophic cascade.

• Note: Since CODAP graphs display only two populations at a time, students will have to create and analyze

multiple graphs on one screen in order to understand relationships among three organisms. This may be

challenging, as the prompts ask them to consider each graph one by one. It would be helpful to project

the graphs and go through the analysis with students. It may be helpful to have students work in

pairs before writing their responses individually.

• Remind students to save their graphs (elk-aspen and wolves-aspen) in their notebook by taking

a snapshot of each with the camera icon.

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• You may wish to note that the data show only eight years of spring aspen height. Ask students whether

they think it is valid to extrapolate from the available data. For example, “Do you think the aspen height

trend will keep increasing/decreasing? Why?”

• Point out that the nature of real data is that often the trends are not smooth. Other elements

may come into play to upset the trend; for example, disease, migration, environmental events that

positively or negatively affect the organism’s health.

• Invite students to read aloud the paragraph in step five. Here students put together their thoughts

and write about the dynamics of the interactions among the wolves, elk, and aspen trees. You may have

students talk in pairs, then direct them to answer the prompt in the box using the information from the

graphs.

INVESTIGATION 3: Wolves-Elk-Berries Trophic

Relationship

Estimated Time: 15 minutes

Purpose

• To investigate whether the reintroduction of wolves has had an effect on the elk's food, the berry bushes

Big Idea

• A trophic cascade can be traced from the wolves to the elk and in turn to the primary producers, berry

bushes.

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Facilitation Suggestions

• In the first step, students predict what effect the reintroduction of wolves might have on the availability of

berry bushes.

• Point out that in this investigation we are not using elk number but elk density, a different way to

measure the elk. You may wish to discuss the importance of corroboration in science, and the fact

that different ways of measuring a phenomenon should point to the same outcome.

• As in the prior investigation, students are asked here to answer questions by putting together the

information from two different graphs. Tell students to go step by step and study each graph

alone, then in combination with the others. For example, after establishing the relationship between

elk and berry bushes (step three), go over the graph showing the density of elk in the presence/absence of

wolves. Then put these two results together by answering the questions on step four. This section may feel

repetitive in the sense that we are looking at predator-prey/primary consumer- primary producer/plants as

in investigation 2. However, we are looking at the berry bushes because of the competition of elk and bear

for the berry bushes that will be studied in investigation 5.

• Tell students to make sure to save their graphs in their notebook by taking snapshots.

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INVESTIGATION 4: Summarizing Your Results

Estimated Time: 15 minutes

Purpose

• To stop and think about the results collected so far and how they fit into the idea of a trophic cascade

• To construct a three-level trophic cascade using the results to this point.

Big Idea

• The reintroduction of wolves causes a decrease in the elk population, allowing the elks’ food—aspen trees

and berry bushes—to thrive.

Facilitation Suggestions

• Make time for students to briefly discuss, in pairs or table groups, the trophic relationships among

the creatures in the table.

• Ask students to construct a trophic cascade using all the animals in the table. You may want

to invite students to draw a trophic cascade and to discuss as a class what animals go in what trophic levels

and how are they ecologically related.

TRANSITION POINT

Inform the students that the next investigation is going to be wider in scope, looking at other animals that may be influenced indirectly by

the reintroduction of wolves into Yellowstone Park.

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INVESTIGATION 5: Interactions Within Trophic Cascades

Estimated Time: 15 minutes

Purpose

• To investigate whether the reintroduction of wolves to Yellowstone Park has had an indirect effect on other

animals and plants in the ecosystem

Big Idea

• The presence of wolves in Yellowstone Park favored bears because it decreased the competition between elk

and bears for food resources.

Facilitation Suggestions

• In the first step, students predict how the reintroduction of wolves will affect the competitive relationship

between elk and bears.

• In the data analysis, students are asked to establish whether there is a correlation between elk numbers and

fruit in the bears’ diet. Go over the significance of the slope—and particularly whether it is

positive or negative. Ask students to put the relationship into words.

• Remind students to save both graphs in their notebook by taking snapshots of them.

• Note that because we do not have direct data on the number of bears, we are using a proxy variable—the

amount of fruit in the bears’ diet. Our assumption is that if there is more fruit in their diet (by scat

measures), this suggests that bears are eating more, will thrive, and thus there will be an increase in their

number.

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• Before going to step three, remind students that the number of elk in Yellowstone Park depend on

whether there are wolves around. Ask students what they would expect the percentage of fruit in bears’ diet

to be when wolves are present versus when they are absent. Make sure students explain their answers and

frame their responses using the graph and results from previous investigations.

• Students can find whether their answers were correct by looking at the amount of fruit eaten by bears when

wolves were present versus when they were absent. Ask students to put these results together and

decide: Is there enough evidence to say that there is competition between elk and bear? That

wolves have an effect on this relationship? If students think there is not enough evidence, ask them

what additional experiments or observations would help to establish such relationships.

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INVESTIGATION 6: Summarizing the Results

Estimated Time: 15 minutes

Purpose

• To integrate the ideas and concepts learned in the lesson by completing the concept map depicting the

animals in the Yellowstone trophic cascade

Big Idea

• The reintroduction of wolves to Yellowstone Park directly affected the abundance of elk. This in turn has

implications for the elk’s food—the aspen and berries—as well as for animals, such as bears, competing for

this food.

Facilitation Suggestions

• Direct a class discussion aimed at filling out the concept map. Students should be able to

establish trophic relationships by filling in the boxes on the left-hand side panel. Encourage students to

verbalize their ideas and to give the reason for their ideas .

• To encourage participation, you may wish to ask, “What is the trophic relationship between the wolf

and the elk? If elk were not there, what would happen to the berries, the aspen trees, the bears?”

TRANSITION TO WRITING

Inform students that this last exercise will help them get ready to write an essay about the trophic cascade initiated by the reintroduction

of wolves to Yellowstone National Park.

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WRITING TASK: How Wolves Have Affected the Yellowstone

National Park Ecosystem? Estimated Time: 30 minutes

Purpose

• To produce a piece of writing using evidence from the

investigations to answer the question, “How did the reintroduction

of wolves into Yellowstone Park affect the other animals and

plants in the ecosystem?”

• To use the learned concepts and results in order to write a memo

which makes an argument supported by evidence?

Big Idea

• Effectively communicating scientific findings is as important as

conducting the investigations themselves.

• Presenting data, explaining what the data mean, and making a concise story of the result in writing is the

concluding piece of any scientific investigation.

Facilitation Suggestions

• Re-engage students with the main question they are addressing—“How did the reintroduction of

wolves into Yellowstone Park affect the other animals and plants in the ecosystem?”

• Remind students to use their notes and to draw their evidence from the snapshots and the answers

they saved.

• Students may have to be reminded that each paragraph is a coherent piece of writing composed of a

topic sentence, a body that develops the idea using evidence, and a conclusion.

• Preview the “Writing” page and the outline that students will follow in constructing their essay..

1. Intro Paragraph— You will introduce your audience to the reason wolves were reintroduced to

Yellowstone National Park and describe what is meant by a “trophic cascade.”.

2. Claim Paragraph— Briefly summarize the trophic cascade you constructed from the data

investigations—the relationships among wolves, elk, aspen, berries, and bears.

3. Evidence and Reasoning Paragraphs— You’ll write three short paragraphs presenting data that on

(a) wolf-elk food chain effects, (b) wolf-elk-plants food chain effects, and (c) ecosystem effects. In each

paragraph, you will

o present a data snapshot (saved in your Notebook);

o describe the data in words, and say how they support your claim;

o discuss the population dynamics operating between species.

4. Conclusion Paragraph— You will answer the question, “Do you think the trophic cascade involving

wolves as top predators in Yellowstone National Park has positively affected the ecosystem?” Also,

students judge whether the results are similar to those presented by the How Wolves Change Rivers

video we watched at the beginning of the lesson.

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Sample Writing Product

How Wolves Have Affected the Yellowstone National Park Ecosystem

Wolves were reintroduced in Yellowstone National Park with the hope of re-establishing the ecosystem's

biodiversity. The hypothesis was that, when all the wolves were hunted down, the food web was disrupted and

the ecosystem deteriorated. In the absence of wolves, too many elk in the park had negative implications. For

example, uncontrolled browsing of many plants by elk caused the disappearance of other animals because their

habitats and food supply were destroyed by the elk. As I learned in the Background section, a trophic cascade is

an event that has consequences throughout a food web. It is caused by adding or taking away a component in a

food web—in this case, the top predator, the wolves. A food web describes trophic relationships among

components of an ecosystem; and within a food web we can trace many food chains.

Overall, the investigation shows that the reintroduction of wolves was positive for the Yellowstone Park

ecosystem. These effects are consistent with a trophic cascade, where reintroduction of a predator has had

cascading or chain effects on other animals and plants. The reintroduction of wolves into Yellowstone National

Park had a negative impact on elk. The wolves are the top predators and they prey on the elk; these two animals

have a predator-prey relationship. By controlling the elk population, other organisms in the ecosystem, like

aspen trees and berry bushes, recovered. The elk consume plants—the berry bushes and the aspen trees. The elk

and the bears compete for the berries, and the wolves indirectly favor the bears by eating the elk. This allows

more berries to be produced, which are available for the bears to eat before hibernating.

The population dynamics between wolves and elk shows an out-of-phase undulating relationship

between these two species. As the prey, elk, are readily available, the predator's population, wolves, increases

because there is plenty of food to eat. My results in investigation 1 (snapshot) show that in 1995 there were

16,791 elk—plenty of food for the 21 wolves. After 1995, the number of wolves increased, from 21 up to 185 in

2004 and, as a response, the population of elk decreased. But eventually the wolf population also started to

decrease, perhaps because there were fewer elk to eat. There are no data after 2017, but if we extrapolate the

trend, we may see that the reciprocal relationship continues. There is a second piece of evidence that shows the

same wolf-elk relationship trend (Investigation 3 snapshot). In the presence of wolves, the elk density was only

0-1 elk per square kilometer compared to the high elk density of 16 per square kilometer when the wolves were

absent. Taking these two pieces of data together, my results suggest that the reintroduction of wolves reduced

the number of elk because wolves killed them.

One interesting consequence of the wolves eating the elk is that the plants that the elk eat increased. The

two plants under study are aspen trees and berry bushes. My results show that between 2003 and 2011, as the

number of wolves increased, the number of elk decreased, and the height of aspen trees increased (Investigation

2 snapshot). After 2011, the number of wolves reached a plateau; however, the aspen trees kept growing. This

result can be explained by the observation that elk can reach leaves only to a height of about 100 centimeters. If

the tree gets taller than that, then it is safe from elk and keeps growing. A similar trend is observed between the

wolves, elk, and berry bushes. There are not only more berry bushes, but they get taller and produce more

berries (Investigation 3 snapshot). The relationship between these species, herbivores and plants, also shows a

reciprocal relationship.

Trophic Cascades: What Happened When Wolves Were Reintroduced to Yellowstone Park?

23 Zoom In! Science Teacher Guide

As I learned in the Background section, in a food web there are many direct and indirect interactions among

its components; trophic cascades usually are not linear. In this study, we identified one indirect effect of the

reintroduction of wolves to Yellowstone National ParkP: Wolves and bears interact via the elk and berry bushes.

There is a strong negative correlation (r

=0.731, snapshot) between elk numbers and bear fruit diet. As the

number of elk increased, the percentage of fruit in the bears’ diet decreased. Bears eat as much as they can,

especially during late summer and fall before they go into hibernation (Investigation 5 snapshot). My results in

Investigation 5 show that when there were no wolves (but a lot of elk), the bears had a total of only 10% fruit in

their diet during August and September. As discussed above, this is because the elk ate the berry bushes, and the

inference is that the bears had no fruit left to eat. After the reintroduction of wolves (resulting in much fewer

elk), the percentage of fruit in the bears’ diet increased to 28%, with August being the month when the bears ate

lots of fruit (15%).

In conclusion, my results are consistent with the idea in the video that the reintroduction of wolves to

Yellowstone National Park positively influenced other components of the food web, and made the ecosystem

healthier. Even though we have studied a small part of the components that make up the Yellowstone Park

ecosystem, we can extrapolate these results and hypothesize that the positive effects of the reintroduction of

wolves to the park may have a wider reach. For example, more birds will be able to nest in the aspen trees and

small mammals and birds will come back to the park to eat the berries.

ASSESSMENT

Writing Product Assessment

Look for the following when evaluating students’ writing tasks.

INTRO PARAGRAPH:

Begins by stating why wolves were reintroduced to Yellowstone National Park

Defines what is a trophic cascade

CLAIM PARAGRAPH

Describes the trophic cascade constructed in the lesson

Identifies and describes the trophic relationships among wolves, elk, and plants

Identifies and describes the competitive relationship between elk and bears.

EVIDENCE AND REASONING PARAGRAPHS

❑

There should be three paragraphs—on wolves-elk, on wolves-elk-plants, and on the indirect effects of

the trophic cascade

❑

Each paragraph should present evidence and explain how it supports the claim. For example, by

including

o a summary sentence on the trophic relationships;

o a description of the results, citing data and including picture from the notes;

CONCLUSION PARAGRAPH

States whether the reintroduction of wolves was positive to the ecosystem of Yellowstone National

Park

Judges, based on the evidence, whether the information shown in the video is credible.

Trophic Cascades: What Happened When Wolves Were Reintroduced to Yellowstone Park?

24 Zoom In! Science Teacher Guide

Key Question Notes

We suggest that teachers use these Key Notes to find out whether students understand the concepts and whether

students are keeping pace with the rest of the class.

Key Question 1. In your own words, write a definition for trophic cascade?

Where: Background 2: Food Webs and Trophic Cascades

Understanding: Students should be able to apply the concepts of food webs and food chains from the

Background section to define a trophic cascade.

Answer: A trophic cascade is an event that happens when an organism is added or taken away from a

food web.

Key Question 2. Look at the three graphs you constructed side by side and write a paragraph about what

happened to the trophic cascade wolf-elk-aspen after the wolves were reintroduced to Yellowstone National Park

in 1995

Where: Investigation 2. Wolf-Elk-Aspen Relationship

Understanding: Based on the results obtained to this point, students should be able to extend the

concept of a trophic relationship from two to three organisms.

Answer: In the first trophic relationship, the wolves’ population increases and, as a result of

predation, elk population decreases. Since elk browse on aspen, the presence of wolves would allow

the aspen population to increase. The trees survive and become taller.

Key Question 3. Do you think there is enough evidence for food competition between elk and bears?

Where: Investigation 5: Interactions Within Trophic Cascades

Understanding: In the answer to this question, students should be able to choose appropriate data,

and they may require help interpreting the least square line.

Answer: Yes, the competitive interaction between bears and elk is affected by wolves. There is a strong

negative correlation (r2= 0.731) between fruit in the bear diet and number of elk, suggesting that the

elk eat the berries and thus the bear goes hungry. However, when wolves are present the percentage

of fruit in the bear diet increases from 6% to 15%.