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Who Set the Moose Loose?

Trophic Interactions in the Greater Yellowstone Ecosystem


Kristina Hannam
Department of Biology
SUNY Geneseo


This “clicker case” focuses on the food web of the riparian bird communities of the Greater Yellowstone ecosystemand how community structure and productivity may be influenced by top-down mechanisms, resulting in a trophic cascade. As students examine the food web and non-feeding interactions among the community members, they uncover the effects of herbivore densities on songbird populations and gain an appreciation for species interactions and impacts in a biological community. The case is presented in class using PowerPoint slides (~2.6MB) that are punctuated by multiple-choice questions which students answer using clickers, though the case could be adapted for use without these technologies. Designed for an introductory biology course taken primarily by freshmen and sophomores to fulfill a general education requirement, it could also be used in an introductory course for biology majors. 


  • Understand how to construct a food web and how different feeding relationships are represented.
  • Examine data and infer what it suggests about species interactions.
  • Distinguish between top-down and bottom-up control of ecosystem productivity.
  • Understand the importance of non-feeding relationships among species on species abundance.
  • Understand the role of top predators in trophic cascades.
  • Recognize human influence on community structure and energy flow.


Food web; trophic cascade; riparian; species interactions; moose; songbird; birds; willow tree; hawk; coyote; predator; biodiversity; Yellowstone; Grand Tetons

Topical Areas


Educational Level

High school, Undergraduate lower division


PDF, PowerPoint

Type / Methods

Clicker, Interrupted



Subject Headings

Biology (General)  |   Ecology  |   Environmental Science  |  

Date Posted


Teaching Notes

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Supplemental Materials

The supplemental material below may be used in conjunction with this case study.

  Click-and-Learn: Exploring Trophic Cascades hhmi/

Answer Key

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The following video(s) are recommended for use in association with this case study.

  • Some Animals are More Equal than Others hhmi/
    This short film opens with two questions: "So what determines how many species live in a given place? Or how many individuals of the species can live somewhere?" The research that provided answers to these questions was set in motion by key experiments by ecologists Robert Paine and James Estes. The film discusses Paine's starfish exclusion experiments on the coast of Washington state as well as Estes' and colleague John Palmisano's discovery that the kelp forest ecosystems of the North Pacific are regulated by the presence or absence of sea otters, which feed on sea urchins that consume kelp. These early studies were the inspiration for hundreds of investigations on other keystone species and trophic cascades, as well as ongoing studies into the regulation of population sizes and species numbers. Running time: 19:29 min.Produced by HHMI BioInteractive.
  • Film Guide for "Some Animals are More Equal than Others" hhmi/
    Film guide as well as instructor materials and a student quiz that complement the film "Some Animals Are More Equal Than Others: Trophic Cascades and Keystone Species." Produced by HHMI BioInteractive.