Lecture on Trophic Cascades and the Yellowstone Wolves

Introduction to Trophic Cascades

Definition: A trophic cascade is an ecological process that originates at the top of the food chain and affects the entire ecosystem down to the bottom.
Significance: They illustrate the interconnectedness of ecosystem components.

Historical Context:
- Wolves were absent for 70 years before being reintroduced in 1995.
- Absence led to overpopulation of deer which heavily grazed vegetation.

Deer Population Control:
- Wolves began to prey on deer, directly reducing their numbers.
- More importantly, altered deer behavior—avoided certain areas.

Regeneration:
- Vegetation in valleys and gorges began to recover as deer avoided these areas.
- Tree growth increased significantly (e.g., height quintupled in some areas within six years).

Birds and Small Mammals:
- Increased presence of songbirds, migratory birds due to new habitats.
- Beavers increased in number, subsequently aiding habitat creation.
Predatory and Scavenger Species:
- Coyotes were killed, leading to more rabbits and mice.
- This change benefited hawks, weasels, foxes, and badgers.
- Ravens, bald eagles, and bears fed on carrion left by wolves; bear populations rose with more available food.
- Bears also hunted deer calves, reinforcing the wolves' impact.

River Behavior:
- Rivers became more stable (less meandering, formed more pools and riffle sections).
- The root systems of regenerated forests stabilized river banks and valley sides, reducing soil erosion.
Overall Transformation:
- The small number of wolves led to significant changes in both the ecosystem and physical geography of Yellowstone.

Ecological Engineers:
- Wolves act as ecosystem engineers, initiating a cascade of ecological and physical changes.
- Their impact demonstrates the critical role of top predators in maintaining ecological balance.