Invasive feral pigs impact native tree ferns and woody seedlings in Hawaiian forest
Invasive mammals can fundamentally alter native plant communities, especially on isolated islands where plants evolved without them. The globally invasive feral pig (Sus scrofa) can be particularly destructive to native plant communities. Tree ferns are an important understory component in many forests facilitating the establishment of a variety of species. However, the extent and effects of feral pig damage to tree ferns, and associated impacts on plant community regeneration, are largely unknown. We quantified the effect that feral pig damage has on tree fern growth, survival, and epiphytic woody seedling abundance over 1 year on 438 randomly selected tree ferns of three endemic species (Cibotium chamissoi, Cibotium glaucum, and Cibotium menziesii) in a Hawaiian montane wet forest with high tree fern and feral pig densities. Across all tree fern species, feral pigs damaged 13 % of individuals over 1 year. Compared with undamaged tree ferns, moderately- to heavily-damaged individuals had decreases of 4 to 27 % in trunk length increment and lost tenfold more fronds. Tree fern angle (standing, leaning, prone, or semi-prone) and woody seedling abundance co-varied with feral pig damage. Specifically, damaged tree ferns were more often prone or semi-prone and supported more seedlings, but also had annual mortality up to 34 % higher than undamaged tree ferns. Overall, feral pig damage had substantial negative effects on tree ferns by reducing growth and survival. Given the importance of tree ferns as regeneration sites for a variety of native plants, feral pig damage to tree ferns will likely alter future forest composition and structure. Specifically, feral pig damage to tree ferns reduces potential establishment sites for species that either regenerate preferentially as epiphytes or are currently restricted to epiphytic establishment due to ground rooting by feral pigs.