Yellowstone Without Wolves

How did the absence of wolves impact the Yellowstone National Park ecosystem?

Wolves are the top, or apex, predator of their ecosystem because they have no natural predators in their food web. Removing a top predator like the wolf can create a chain reaction of effects on other species in the web, referred to as a trophic cascade. Wolf reintroduction into Yellowstone National Park offers a powerful case study for Colorado to learn from. Decades of scientific research paint a picture of the wolf’s impact on its ecosystem and people, both during its absence and once it returned. Let’s start by going back in history to figure out when wolves were thriving in Yellowstone, when and why they were eliminated, and the impact that their elimination had on the park ecosystem.


What’s the history of wolves in Yellowstone?

In 1872, the world’s first national park, Yellowstone, was created. However, while it protected approximately 3,472 square miles of land (2,221,766 acres), it did not protect the gray wolves that lived inside its boundaries. In fact, in the early years of the park any visitor could hunt and kill any wild game or predator they encountered. Wolves were especially vulnerable because they were seen as an undesirable predatory species.

At least 136 wolves were killed in the park between 1914 and 1926. By the 1940’s, wolf packs were seldom reported in the park. In fact, by the mid-1900’s wolves had been nearly eliminated not just from Yellowstone but from the lower 48 states entirely.

In the 1940’s a growing movement of conservationists, environmentalists, biologists, and park officials supported reintroduction of the wolves to Yellowstone. However, it wasn’t until 1991 that Congress directed the U.S. Fish and Wildlife Service to develop an environmental impact statement regarding wolf reintroduction. In 1995, the park moved forward with a plan to reintroduce wolves to the area.

Reintroduction was not without controversy. There were, and still are, many stakeholders involved in the Yellowstone area, each with unique concerns. However, in the twenty years since the reintroduction of wolves, scientists have noticed many positive changes to the park ecosystem.

[Excerpt from Wolves of Yellowstone Teacher Guide, p. 2]

One possible explanation for these changes is a trophic cascade. Scientists are trying to determine how much of an effect the reintroduction of Yellowstone’s top predator - the wolf - has had on the ecosystem.

Questions to Consider

  • How did the wolf disappear from Yellowstone?
  • When were wolves eliminated from Yellowstone?
  • When did efforts begin to reintroduce the wolf?
  • 2

    How does a trophic cascade help explain the effects of wolves being eliminated from Yellowstone?

    When the wolves of Yellowstone National Park were removed, the ecosystem started to change. What happened, and why? Watch the first part of this video to figure out how the absence of wolves impacted the ecosystem. It describes the phenomenon scientists call a trophic cascade. Scientists are still building on this research as they continue to investigate the effects of the wolf’s absence, and return, on the Yellowstone ecosystem.

    Make Sense of This Video

    1. Map out this ripple effect of change that happened through the ecosystem when the food web’s top-predator trophic level was eliminated.

    a. Watch the first part of the Video: Wolves of Yellowstone: A New Wild video clip from 0-2:36 minutes.
    b. Draw a bubble concept map that shows how removing wolves caused changes to other parts of the ecosystem. To start your map, write “Wolves removed” in the central bubble, and build out from there.
    c. Watch the video segment several times to make sure you catch and record all the connections described.

    1. Search your bubble concept map for examples of food chains. Draw a visual model of these food chains, including an explanation of how they were a part of this trophic cascade when wolves were eliminated. Consider labeling producers, primary consumers/herbivores, secondary consumers/carnivores or omnivores.
    2. How were producers (plants) indirectly affected by the elimination of wolves from the Yellowstone ecosystem?
    3. Give several examples of how changes to these plants affected different animal species.
    4. Illustrate in a visual map or write a paragraph describing how the elimination of wolves cascades through the ecosystem to affect water sources.
    5. Write a hypothesis stating what you think might happen to plant and elk populations when wolves return to Yellowstone. Defend your position. (“If__, then I think__ because__. Supporting evidence includes__.”)

    What does the data say?

    Let’s look at the plant and animal data scientists collected for the 20-year span from 1990-2010, noting that wolves were reintroduced in 1995. First read the background on the research (Excerpt from Wolves in Yellowstone p. 2), and then analyze the graphs of data.

    1. Analyze the data: Open and download this handout with the data from the scientific research.
    2. Reflect on wolves, their ecosystem, and trophic cascades: After analyzing the data, ponder your new insights.
      • Write 3 ah-ha’s that you have about wolves and their effects on the ecosystem.
      • Write 2 questions you now have related to your new understanding.
      • Write 1 personal connection you have made to wolves, their ecosystem, or trophic cascades.
    educator note
    • Instructional support can be found on p. 1-11 of Wolves of Yellowstone Teacher Guide.
    • The video Wolves of Yellowstone: A New Wild (5:30 minutes) is divided into two parts. Only show the first half (0-2:36 minutes) for this webpage activity. It talks about Yellowstone without wolves. The second half (2:36-end) describes the park after the wolves were reintroduced, which is the focus of an activity on the next webpage.
    • This student handout (Defining Trophic Cascade) was modified for the Quest. The original is here.
    let's take a look at what happened when wolves returned to Yellowstone