Divergent effects of native deer and alien wild pigs on forest understoreys

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, although this effect weakened in warmer-wetter or more human-dense environments. Conversely, deer tended to increase invasive plant abundance and richness, consistent with selective browsing on palatable native species combined with the resistance of many common invaders. In contrast, wild pigs typically suppressed invasive plant abundance and richness, while their effects on native seedlings were neutral and strongly context dependent, consistent with their rooting behaviour and broad diet. These contrasting outcomes persisted after accounting for climate, human pressure and forest structure, supporting an important role of mesoherbivores in structuring understorey communities. Collectively, our findings indicate that native and alien mesoherbivores exert divergent and environmentally contingent effects on forest regeneration and plant invasion. Recognizing species identity, functional traits and environmental context will be essential for anticipating mesoherbivore impacts and managing forest biodiversity under global change.