Ripple+and+Beschta+2006
Linking Cougar Decline to Ecosystem Changes in Zion National Park
Authors and Publication Details
William J. Ripple, Robert L. Beschta
College of Forestry, Oregon State University
Article reviewed and accepted in July 2006
Keywords
Cougar, Trophic Cascades, Mule Deer, Cottonwood, Stream Biodiversity
Abstract
This study hypothesizes that increased human visitation in Zion Canyon leads to a catastrophic regime shift through trophic cascades and environmental changes. Cougars' decline (Puma concolor) led to increased mule deer (Odocoileus hemionus) density which resulted in decreased cottonwood (Populus fremontii) recruitment and biodiversity loss in streams.
1. Introduction
Importance of large carnivores in ecosystems is debated; few examples highlight their role.
Historical context: Aldo Leopold's arguments on the impacts of predator removal on ecosystems through the Kaibab Plateau study in Arizona.
The significance of understanding long-term impacts due to declines in large carnivores is emphasized.
Trophic Cascade
Defined as top predator presence affecting consumers, which then alters plant species composition.
Ongoing debates on the relative strengths of trophic versus bottom-up controls in ecosystems.
Objective of Study
To investigate the link between predator loss (cougars) and ecosystem changes using comparative sites within Zion National Park.
2. History of Zion Canyon
Early settlement led to ecological changes and deer population decline due to hunting.
By the 1940s, deer populations increased due to the absence of cougars, leading to ecosystem imbalances and vegetation degradation.
Human activities in the park were identified as stressors leading to decreased cougar densities and increased deer populations.
3. Study Areas
Zion Canyon: High human visitation, low cougar density, abundant mule deer.
North Creek: Limited human activity, stable cougar populations.
Comparison of ecosystem dynamics between the two study areas.
4. Methods
Historical data used for estimating deer populations.
Measurement of Fremont cottonwood tree diameters and sampling in various reaches of both areas to determine tree recruitment and ecosystem health.
Surveys conducted for channel dimensions, streambank health, and biodiversity indicators.
5. Results
Increased deer population in Zion Canyon linked to decreased diversification of riparian vegetation and cottonwood recruitment.
Significant correlation between cottonwood tree rings and their diameters indicating growth patterns.
Notable eroding banks in higher human-use areas versus stable banks in controlled areas (North Creek).
Biodiversity Findings
Biodiversity indicators including various plant and animal species revealed lower abundance in Zion Canyon, attributable to high deer browsing and habitat loss.
Specific plant species correlated with better ecosystem health in areas with stable cougar populations.
6. Discussion
Findings illustrate connections between predator presence and ecosystem health.
Reinforcement of earlier theories (Leopold) regarding the cascading effects of carnivore removal leading to increased herbivory and habitat degradation.
The necessity of assessing human influences on large mammal dynamics that affect ecological structure and function.
Implications for Management
Understanding how predator dynamics impact riparian and upland ecosystems emphasizes the need for management strategies that consider predator-prey relationships to enhance biodiversity and ecosystem resilience.
Conclusion
Cougars play a critical role in the ecological balance of Zion National Park and their decline has broad implications for plant communities, stream health, and overall biodiversity. Further studies are needed to determine the potential recovery of ecosystems with increased cougar populations.
Acknowledgments
Contributions from several colleagues and funding support from the National Park Service.
References
Cited various studies and observations relevant to predation and ecosystem dynamics, notably Aldo Leopold's early work on the subject.