Overview
This lecture explains trophic cascades using the example of wolf reintroduction to Yellowstone National Park and its wide-ranging ecological and physical impacts.
Trophic Cascades: Definition and Example
- A trophic cascade is an ecological process starting at the top of the food chain, impacting all levels below.
- Yellowstone National Park saw this after wolves were reintroduced in 1995 following a 70-year absence.
Ecological Impacts of Wolf Reintroduction
- With no wolves, deer numbers increased, overgrazing vegetation and reducing plant life.
- Wolves reduced deer populations directly by predation and indirectly by changing deer behavior.
- Deer began avoiding valleys and gorges, allowing vegetation in these areas to regenerate rapidly.
- Tree heights increased up to fivefold in six years, leading to the return of aspen, willow, and cottonwood.
Effects on Other Species
- Regenerating forests led to an increase in songbirds, migratory birds, and beavers.
- Beavers, as ecosystem engineers, built dams that created habitats for other animals like otters, muskrats, ducks, fish, reptiles, and amphibians.
- Wolves reduced coyote numbers, which allowed rabbit and mouse populations to rise, benefiting hawks, weasels, foxes, and badgers.
- More carrion from wolf kills attracted ravens, bald eagles, and bears.
- Bear populations grew due to increased food sources, including both carrion and berries.
Physical Changes to the Landscape
- Regenerating vegetation stabilized riverbanks, reducing erosion and causing rivers to meander less and form more wildlife-friendly habitats.
- Vegetation recovery stabilized valley sides and reduced soil erosion overall.
- Wolf-induced changes extended beyond biology to alter Yellowstone’s physical geography.
Key Terms & Definitions
- Trophic Cascade — an ecological process where changes at the top of the food chain influence many lower levels.
- Ecosystem Engineer — a species that creates, modifies, or maintains habitats for other organisms.
Action Items / Next Steps
- Review the Yellowstone case as a classic example of trophic cascades.
- Understand how apex predators can influence entire ecosystems and physical landscapes.