4.2 Apex Predators & Trophic Cascades

Trophic Cascade

A trophic cascade occurs when the REMOVAL or ADDITION of an apex predator causes ripple effects throughout the entire food web. Top-down regulation: Apex predators control prey populations from the top. Remove them → prey species multiply → overgraze producers → ecosystem collapses.

Wolves Removed — Negative Effects

Wolves REMOVED → Chain of Negative Effects: 1. Elk populations skyrocket (no predation pressure) 2. Elk overgraze willows, aspens, grasses along riverbanks 3. Beavers decline (no willow branches to build dams) 4. Songbirds lose nesting habitat in shrubs/trees 5. Riverbanks erode (no plant roots holding soil) 6. Biodiversity crashes across the entire ecosystem.

Wolves Reintroduced — Positive Effects

Wolves REINTRODUCED (1995) → Chain of Positive Effects: 1. Wolves control elk population numbers directly (predation) 2. Wolves change elk BEHAVIOR — elk avoid open valleys (fear of predation) → 'landscape of fear' 3. Vegetation recovers in valleys and riverbanks 4. Beavers return, build dams, create wetland habitats 5. Rivers stabilize (plant roots prevent bank erosion) 6. Biodiversity rebounds — birds, deer, squirrels, fish increase.

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