Abstract Mesoherbivores are expanding globally through both native population irruptions and alien introductions, yet their broad-scale ecological impacts remain poorly resolved. We assessed how native white-tailed deer (Odocoileus virginianus) and alien wild pigs (Sus scrofa) influence forest understoreys across the eastern United States by integrating forest inventory and analysis plots, large-scale camera-trap monitoring and environmental data. Deer generally reduced native seedling abundance