Rainforests on Haleakala's steep windward slopes receive as much as 120 inches of annual rainfall. These forests are stable and enduring when not ravaged by non-native pigs and goats. Ungulate rooting and grazing quickly breaks down the native plant cover triggering devastating erosion. Landscape scale erosion in tropical mountain slopes devastates both the native biodiversity and the island's precious groundwater reserve.
Yet, Haleakala upper elevations are now free of pigs and goats, and the park serves as a core area in the East Maui Watershed Partnership to protect this native rainshed and its groundwater from ungulate destruction.