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Some Animals Are More Equal than Others: Keystone Species and Trophic Cascades

Topic

  • Ecology
  • Communities
  • Ecosystems
  • Matter & Energy
  • Science Practices
  • Experimental Design

Resource Type

  • Videos
  • Short Films

Level

High School — GeneralHigh School — AP/IBCollege

Duration

00:19:29
Favorited By
103 Users
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Description

This film tells the story of the ecologists who first documented the role of keystone species in ecosystem regulation.

Some Animals Are More Equal than Others explores the work of ecologists Robert Paine and James Estes. Robert Paine’s experiments showed that removing starfish from tidal pools has a big impact on the population sizes of other species. James Estes and colleagues discovered 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 direct and indirect effects of starfish, sea otters, and other so-called keystone species describe a phenomenon known as a trophic cascade. These early studies were the inspiration for hundreds of subsequent investigations on how population sizes are regulated in a wide variety of ecosystems.

The “Abbreviated Film Guide” provides a short summary of the film, along with key concepts and connections to curriculum standards.

An audio descriptive version of the film is available via our media player.

Student Learning Targets

  • Use evidence to explain why some species play the role of keystone species in their ecosystems.
  • Explain the concept of a trophic cascade using examples from different ecosystems.

Key Terms

apex predator, food web, herbivore, keystone species, trophic cascade

Primary Literature

Dyer, Lee A., and Deborah K. Letourneau. “Trophic cascades in a complex terrestrial community.” Proceedings of the National Academy of Sciences 96, 9 (1999): 5072–5076. https://doi.org/10.1073/pnas.96.9.5072.

Estes, James A., Martin T. Tinker, Terrie M. Williams, and Daniel F. Doak. “Killer whale predation on sea otters linking oceanic and nearshore ecosystems.” Science 282, 5388 (1998): 473–476. https://doi.org/10.1126/science.282.5388.473. To access this article, set up a free AAAS account.   

Terms of Use

Please see the Terms of Use for information on how this resource can be used.

Accessibility Level (WCAG compliance)

PDF files partially meet criteria. Video files meet criteria.

Version History

Date Published 04.01.16
Date Updated 11.05.21

NGSS (2013)

HS-LS2.A, HS-LS2.B, HS-LS2.C, HS-LS4.C

AP Biology (2019)

ENE-1, ENE-4, SYI-1, SYI-2, SYI-3

IB Biology (2016)

4.1, 4.2, C.1, C.2, C.3, C.4, C.5

AP Environmental Science (2020)

Topic(s): 1.1, 1.11, 2.7

IB Environmental Systems and Societies (2017)

2.2, 2.3, 2.4, 3.3

Vision and Change (2009)

CC4, CC5

Materials

HD (M4V) 744 MB
HD (WMV) 605 MB
SD (M4V) 164 MB
SD (WMV) 165 MB
Transcript (PDF) 152 KB
Abbreviated Film Guide (PDF) 752 KB
Spanish dub (MP4) 313 MB
Transcript - Español (PDF) 313 KB
Abbreviated Film Guide - Español (PDF) 232 KB

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Trophic Cascades

Tim Guilfoyle describes how he uses the BioInteractive short film "Some Animals Are More Equal than Others" and a claim-evidence-reasoning activity to have his students examine Robert Paine's starfish exclusion experiment.
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