Print PageNonnative feral pigs (Sus scrofa) drastically alter native vegetation and terrestrial ecosystems by browsing, rooting, and trampling, and may also facilitate invasion by nonnative plants. Feral pigs are particularly problematic in geographically isolated areas such as islands. Hawaiian wet forests are thought to be particularly vulnerable to feral pig activity, as the native vegetation evolved in the absence of large mammals. Fencing and eradication of feral pigs has long been considered a crucial first step in conserving and restoring Hawaiian wet forests. However, the extent and rate of the recovery of native plant communities following pig removal is poorly understood. Moreover, the long-term effects of pig removal on invasive plant dynamics in these forests are largely unknown. We examined changes in understory vegetation in five paired fenced (pig-free) and unfenced (pig-present) forest sites with native overstories in Hawaii Volcanoes National Park on the Island of Hawaii. Ground cover, herbaceous species cover, tree fern density, and density, species richness, and rooting point (epiphytic vs. mineral soil) of woody plants were quantified in 1994 following fencing and eradication of pigs, and again in 2010 (15 yrs pig-free).
Results/Conclusions
Native and nonnative understory vegetation responded strongly to feral pig removal in this system. Density of native tree seedlings and shrubs rooted in mineral soil increased six-fold over 15 years of pig removal, whereas native regeneration was almost exclusively restricted to epiphytic establishment in pig-present sites. Stem density of small tree ferns (>0.5m stem length) increased significantly (51.2%) in pig-free, but not pig-present sites. Species richness did not vary with treatment or time, and new native woody plant establishment was limited to common species. Herbaceous cover was highest in pig-free sites, and decreased significantly (67.9%) over time in pig–present sites. Area of exposed soil was high in pig-present (32.0%) but almost nonexistent in pig-free sites (0.08%), whereas litter cover varied inversely (53.4% pig-present, 88.6% pig-free). Nonnative woody species, particularly Psidium cattleianum, responded strongly to release from pig disturbance with a 5-fold increase in density. These results demonstrate that while common native Hawaiian species recover rapidly in the understory following feral pig removal, subsequent control of nonnative, invasive plants and outplanting of rarer native species should be critical components of management for conservation and restoration in wet Hawaiian forests.
