Large Carnivore Trophic Cascades in Western North America

by
Robert L. Beschta & William J. Ripple
Forest Ecosystems and Society
Oregon State University, Corvallis, OR 97331

Listed below are PowerPoint presentations covering several aspects of large carnivore trophic cascades (i.e., indirect species interactions that originate with predators and spread downward through food webs). These presentations summarize approximately two decades of field research, starting in Yellowstone National Park and subsequently expanded to other national parks in the western United States and Canada.* These presentations are intended for general audiences and/or classroom use, and a PDF of each presentation can be accessed by clicking on its respective hotlink. Supporting publications are identified at the end of each presentation.

The first three presentations emphasize photographic comparisons, over time, to illustrate shifting relationships between large carnivores, ungulates, and woody plants in Yellowstone National Park. These presentations should be viewed sequentially.

1 - The Removal of Wolves and the Reign of Elk– Following the extirpation of wolves, and cougars in the 1920s, increased elk browsing had devastating effects on woody plant communities in northern Yellowstone National Park.

2 - The Return of Wolves – The reintroduction of wolves in 1995-96 completed the park’s large carnivore guild and was followed by decreased elk browsing in various portions of northern Yellowstone National Park, thus initiating a recovery of woody plant communities in those areas.

3 - Bison Limit a Trophic Cascade – Bison numbers in northern Yellowstone have increased dramatically since 1995 and the herbivory and trampling associated with this large herbivore is having a major impact on many northern Yellowstone plant communities.

Presentations 4-9 are intended to represent “stand-alone” topics related to large carnivore trophic cascades. Presentations 4 and 5 are specific to Yellowstone National Park whereas presentation 6 focuses on Zion National Park. Presentations 7, 8, and 9 summarize results from across several national parks.

4 - Bears, Berries, and Trophic Cascades in Yellowstone National Park – Berry production, normally an important source of food for bears each fall, rapidly declined when elk numbers increased in the late 1900s but has seen some recovery following the return of wolves.

5 - Can Wolves Change Streams? – Riparian plant community recovery in northern Yellowstone, following the return of wolves, has allowed for increased shading of some streams and stabilizing of channel banks.

6 - Maintaining Biodiversity with Apex Predators – In Zion National Park, cougars are a major factor for maintaining an abundance of hydrophytic plants, wildflowers, amphibians, lizards, butterflies, and fish.

7 - Trophic Cascades without Large Predators – Following the loss of large predators in five western national parks, increased browsing led to decimated woody plant communities.

8 - Large Carnivore Loss, Diminished Forests, and Altered Rivers – Studies from Yellowstone, Olympic, and Zion National Parks indicate that the long-term effects of large predator loss can extend to rivers.

9 - Human Shielding – Increased ungulate browsing can occur in areas where the presence of humans limits predator activity, thereby protecting ungulate prey from predation, a situation representing “human shielding.”

*The research summarized in the above presentations is based on studies conducted in several western U.S. national parks (i.e., Olympic, Wind Cave, Yosemite, Yellowstone, and Zion), as well as Jasper National Park in Canada. National parks offered major advantages for evaluating the long-term trophic effects of large predator presence, removal, and reintroduction:

• Large mammalian predators were historically present in national parks.
• Some large predators were extirpated/displaced from national parks approximately a century ago.
• Long-lived woody browse species were useful for assessing long-term changes in plant communities following the removal or reintroduction of a large predator.
• Historical park service reports and photos, illustrating plant community conditions before and after large predator removal, were often available.
• Parks typically occupied large areas and occurred across a spectrum of ecoregions.
• The potentially confounding effects of various land uses (e.g., grazing, roads, logging) were usually absent.
• Rivers and streams were free-flowing, i.e., undammed, thus helping to maintain riparian plant communities.
• Areas protected from browsing were frequently present (e.g., fenced exclosures, refugium), thus providing important spatial controls for comparative purposes.