Lecture Notes: Trophic Cascades and the Reintroduction of Wolves to Yellowstone

Introduction to Trophic Cascades

• Definition: An ecological process that starts at the top of the food chain and affects the entire ecosystem down to the bottom.
• Significance: One of the major scientific discoveries of the past fifty years.

Case Study: Wolves in Yellowstone

• Historical Context:
  • Wolves were absent from Yellowstone National Park for 70 years.
  • Reintroduced in 1995.

Impact on Deer Population

• Initial Issue:
  • Deer populations had overgrown due to lack of predators.
  • Resulted in overgrazing and significant reduction of vegetation.
• Effect of Wolves:
  • Predation on deer was not the main effect.
  • Changed deer behavior, leading them to avoid certain areas like valleys and gorges.
  • Resulted in regeneration of vegetation in these areas.

Vegetation and Biodiversity Recovery

• Tree Growth:
  • Tree heights quintupled in six years in some areas.
  • Growth of aspen, willow, and cottonwood forests.
• Impact on Birds and Other Animals:
  • Increase in songbirds and migratory birds.
  • Beavers returned, creating habitats for otters, muskrats, ducks, fish, reptiles, and amphibians.

Predation and Scavenging Effects

• Coyotes:
  • Wolf predation reduced coyote numbers.
  • Increase in rabbits and mice populations.
• New Predator and Scavenger Dynamics:
  • More hawks, weasels, foxes, and badgers.
  • Ravens and bald eagles fed on wolf carrion.
  • Bears benefited from more food and berries due to regenerating shrubs.

Changes in Physical Geography

• Rivers:
  • Less meandering, reduced erosion, narrowed channels.
  • Formation of more pools and riffle sections, improving habitat quality.
• Vegetation’s Role:
  • Stabilized riverbanks and reduced soil erosion.

Conclusion

• Wolves, though few in number, had a profound transformative effect on both the ecosystem and physical geography of Yellowstone National Park.
• This case study illustrates the interconnectedness of species and the significant role of top predators in ecological balance.