Print PageMost oceanic islands have ecosystems that have evolved in the absence of large mammals. Feral pigs (Sus scrofa) and Philippine deer (Cervus mariannus) have sustained populations on the island of Guam for centuries, and resource managers have been concerned with controlling their populations for native forest rehabilitation. However, little has been done to look at the ecological roles that these introduced ungulates play in the island’s ecosystems. Do they affect forest composition? Do they act as dispersers in an environment that has lost its native dispersers (birds were effectively exterminated by the brown treesnake)? This study uses fenced and unfenced seedling plots to compare seedling survival across 6 species of limestone forest trees. Scats from both animals were also collected for a germination experiment to investigate seed dispersal. In addition, vegetation surveys compared forest composition across areas of differing ungulate-abundance. Relative ungulate abundance was determined using scat counts.
Results/Conclusions
Results strongly indicate that both deer and pigs influence forest composition, especially when they exist in high densities. Certain seedling species had higher survival rates in fenced (ungulate-free) compared to unfenced (ungulate-accessible) plots, indicating that ungulates selectively impacted seedling survival. A number of species germinated from collected pig scats, mostly common invasive vines, as well as two native and one invasive fruit tree. Vegetation surveys indicated deer and pig abundance were each associated with certain forest characteristics. High deer abundance correlated with low seedling and low vine abundance, and high pig abundance correlated to increased abundance of preferred fruit trees. This study suggests that deer influence forest composition by selective browsing on seedlings, while pigs influence forest composition by frugivory and seed dispersal. Conservation and management efforts should consider their respective ecological roles.
