Some Animals Are More Equal than Others: Keystone Species and Trophic Cascades
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.
apex predator, food web, herbivore, keystone species, trophic cascade
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.
Accessibility Level (WCAG compliance)
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)