Lecture Notes: Trophic Cascades and the Yellowstone Case Study

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: A major scientific discovery in the past half-century.

Case Study: Yellowstone National Park

• Context: Reintroduction of wolves in 1995 after a 70-year absence.

Effects on Deer Population

• Initial Condition: Deer numbers were very high due to lack of predators, leading to overgrazing.
• Post-Wolf Reintroduction:
  • Direct Effect: Some deer were killed by wolves.
  • Indirect Effect: Deer changed behavior, avoiding certain areas, leading to vegetation recovery.

Vegetation Recovery

• Areas Impacted: Valleys and gorges where deer avoided.
• Resulting Changes:
  • Tree height in some areas increased fivefold within six years.
  • Forests of aspen, willow, and cottonwood emerged.

Increase in Biodiversity

• Birds:
  • Increase in songbirds and migratory birds.
• Beavers:
  • Population increased due to availability of trees.
  • Role as ecosystem engineers.
• Other Animals:
  • Increase in otters, muskrats, ducks, fish, reptiles, and amphibians due to beaver dams.
  • Rise in rabbits and mice due to decrease in coyotes, leading to more predators like hawks, weasels, foxes, and badgers.
  • Ravens and bald eagles fed on carrion.
  • Bears' population increased due to more berries and feeding on carrion.

Impact on River Geography

• River Behavior:
  • Less meandering and erosion.
  • Narrower channels and more pools.
• Mechanism:
  • Stabilization of river banks by regenerating forests reduced bank collapses.
  • Vegetation recovery reduced soil erosion.

Conclusion

• Wolves, though small in number, had a transformative impact not only on Yellowstone's ecosystem but also on its physical geography.