The reintroduction of grey wolves (Canis lupus) to Yellowstone National Park in 1995 is one of conservation's most studied interventions, demonstrating how apex predator welfare and ecosystem health are interconnected through trophic cascades that affect every species in the system.
Wolves reintroduced to Yellowstone faced immediate welfare challenges: some were killed by resident wolves defending territories, and mange spread through packs causing significant morbidity in the 2000s. Wolves killed outside park boundaries by livestock protection measures die from shooting, trapping, or poison. Pack cohesion disruption — through killing of alpha individuals — causes stress and temporary reproductive failure in surviving pack members. Within the park, wolves that develop mange suffer prolonged welfare decline unless treated through darting programs. The restoration of wolves created measurable welfare improvements for elk by reducing over-browsing stress on riparian vegetation, improving habitat quality for multiple species.