The O`ahu Uplands IBA encompasses most of the remaining native forest on O`ahu. The area is divided between the Ko`olau Mountain Range in the east, which contains 32,828 hectares, and the Wai`anae Mountain Range in the west, which contains 6,875 hectares. The IBA extends from 100 meters (330 feet) elevation in the southern Ko`olaus, to 960 meters (3150 feet) at the summit of Konahuanui in the Ko`olaus and 1220 meters (4003 feet) at the summit of Ka`ala in the Wai`anaes. Some of the area has been disturbed by human development and agriculture, and a variety of alien plants have invaded many areas. Disturbance has been generally more severe at lower elevations and in valleys, but much of the remote central and northern Ko`olau Range consists of largely intact native forest and shrubland. The area is split by several major roads. The terrain is rugged, with steep, narrow ridges separated by deep stream valleys, precipitous cliffs that rise 2000 feet in some areas, and numerous waterfalls. The area ranges from dry on the leeward (western) side of the Wai`anae range, to extremely wet on the summit ridge of the Ko`olaus, where annual rainfall exceeds seven meters (275 inches). Summit areas are often shrouded for days at a time in a dense cloud layer produced by adiabatic cooling of moist air swept upward over the mountains by northeasterly trade winds. Habitats include dry forest and shrubland, mesic forest and woodlands, dense montane rainforest, montane bogs, and wet moorlands with stunted vegetation on windswept ridges. Ownership and use of the land is complex and includes portions of several State Forest Reserves, two State Natural Area Reserves, several State Parks and Recreation Areas, the O`ahu Forest National Wildlife Refuge, Honouliuli Preserve managed by The Nature Conservancy, several large parcels owned or leased to the U.S. military, and various private lands.
The O`ahu Uplands IBA contains globally significant populations of several bird species endemic to the Hawaiian Islands. Virtually the entire population of the O`ahu `Amakihi (Hemignathus flavus) occurs within the O`ahu Uplands IBA. `Amakihi are fairly common in the central and southern Ko`olaus and the southern Wai`anaes, but inexplicably rare or absent in extensive tracts of native forest in the northern Ko`olaus and Wai`anaes. `Amakihi have recently increased in some areas, and can even be found in suburban gardens in the foothills behind Honolulu. The O`ahu `Elepaio (Chasiempis sandwichensis ibidis) occurs entirely within the O`ahu Uplands IBA. This subspecies occurs only on O`ahu and is listed as endangered under the U.S. Endangered Species Act, and may soon be split into a separate species. It has a highly fragmented range, with six core populations and numerous small population remnants. `Apapane (Himatione sanguinea) are fairly common on the mountain ridges of O`ahu, especially in the Ko`olaus, but have a globally restricted distribution limited to the main Hawaiian Islands. Very small numbers of `I`iwi (Vestiaria coccinea) persist in both the Wai`anae and Ko`olau Ranges. Their numbers are difficult to determine because they are observed infrequently and may move in search of nectar resources. The O`ahu Creeper (Paroreomyza maculata) has not been seen since 1985 and may be extinct, but if it still survives it is somewhere in the O`ahu Uplands. The Pueo or Hawaiian Short-eared Owl (Asio flammeus sandwichensis), a subspecies endemic to the Hawaiian islands, occurs in small numbers in forested areas and adjacent grasslands. Seabird nesting colonies exist in several areas of the O`ahu Uplands. White-tailed Tropicbirds (Phaethon lepturus) nest on cliffs in several valleys, and federally threatened Newell?s Shearwaters (Puffinus auricularis newelli) have been heard recently on the slopes above Kalihi Valley.
O`ahu is the most developed of the Hawaiian Islands, supports the majority of the human population, and has experienced some of the most severe threats to its native flora and fauna. Most native habitats in the lowlands have been lost to urbanization and agriculture. Invasive alien plants already dominate large areas of O`ahu, and several species continue to spread. Strawberry guava, common guava, christmasberry, Clidemia hirta, Eucalyptus, rose apple, Lantana camara, and Florida blackberry (Rubus argutus) are among the worst invasive alien plants. Feral pigs degrade forests in many areas by rooting in the understory, spreading seeds of invasive plants, and creating breeding sites for mosquitoes that carry avian diseases. Browsing by feral goats damages forest in some drier areas. Alien predators are abundant throughout O`ahu, including rats (Rattus rattus, R. exulans, and R. norvegicus), small Indian mongoose (Herpestes auropunctatus), feral cats, and feral dogs. Predation on nests by black rats is an especially serious problem for the endangered O`ahu `Elepaio. Mongoose, feral cats, and feral dogs prey on ground-nesting species like Pueo and seabirds and limit them to remote areas with steep terrain. Avian malaria and avian pox virus, diseases carried by alien mosquitoes, have devastated native forest bird populations on all islands. These diseases are less prevalent in colder environments at higher elevations, but none of the mountains on O`ahu are high enough to provide a thermal refuge from disease. Most native bird species sensitive to disease have already been lost on O`ahu, but the remaining species still suffer some mortality. Fires started by arson and during military training have destroyed native forests and promoted the spread of fire-tolerant alien plants that are not favored by native birds, particularly in the drier Wai`anae Mountains.
Ownership of lands comprising the O`ahu Uplands IBA is complex and includes all or portions of a large number of parcels, including the following: Hau`ula, Kaipapau, Kuaokala, Mokuleia, Ewa, Makua-Kea`au, Wai`anae Kai, Waiahole, Nanakuli, Honolulu Watershed, Kuli`ou`ou, Round Top, and Pupukea-Paumalu State Forest Reserves; Pahole and Ka`ala State Natural Area Reserves; Kahana and Sacred Falls State Parks; Keaiwa Heiau State Recreation Area; the O`ahu Forest National Wildlife Refuge; Honouliuli Preserve managed by The Nature Conservancy; Kawailoa and Kahuku Army Training Areas, Schofield Army Barracks East Range, West Range, and South Range, Makua Military Reservation, Lualualei Naval Magazine, several large private parcels leased to the U.S. military, and a variety of other private parcels.
The O`ahu Uplands IBA contains a variety of forested habitats, and distribution of these habitats is complex. Rainfall generated by prevailing northeasterly tradewinds causes the eastern slopes of the Ko`olau and Wai`anae Mountains to be generally wetter than the leeward western slopes of each range. The Wai`anae Mountains are partially in the rain shadow of the Ko`olaus that lie to the east and are thus generally drier. Rainfall increases with elevation due to adiabatic cooling and condensation that occurs as air is forced upward over the mountain slopes. Annual rainfall ranges from about one meter (39 inches) in the leeward Wai`anaes and southern Ko`olaus, to over seven meters (275 inches) along the spine of the northern Ko`olaus. Rainfall gradients are steep in some areas, causing abrupt transitions from dry to wet habitats. In leeward areas the lower elevations contain dry forest and shrubland, which grades into mesic forest at middle elevations, and eventually montane rainforest. The wetter windward slopes support mesic or wet forest in the lowlands, which transitions rapidly to rainforest at higher elevations. The windswept, rain-soaked summits of the Ko`olau Mountains are covered in wet moorlands and shrublands. Steep cliffs occur over much of the windward Ko`olaus and leeward Wai`anaes, a result of massive slumping. These cliffs support wet or dry shrubland and provide a refuge for many rare plant species from browsing by alien ungulates. The summit of Mt. Ka`ala in the northern Wai`anaes supports an unusual montane bog with stunted vegetation that fills a long-extinct volcanic crater. Habitat in some areas has been substantially modified by human development and agriculture, and invasive alien plants are common in many areas. Habitat modification has been greatest in the lowlands and in valleys, but many higher slopes and ridges support largely intact native plant communities.
Land use within the O`ahu Uplands IBA is varied. Most of the land is zoned for conservation and managed for conservation purposes, but a variety of other activities takes place in some parcels. Public hunting of non-native ungulates, primarily feral pigs and goats, is permitted in most state forest reserves. Public hiking trails are located in many state forest reserves, and trails, picnic areas, and other public facilities are present in the state parks and recreation areas. Several of the state forest reserves and some private parcels are important watershed areas that provide water for household use and agriculture across the island. Several areas owned or leased to the U.S. military are used for military training. In most areas this training is limited to ground troop maneuvers and aerial fly-overs, but the O`ahu Uplands IBA includes land that is directly down-range from live firing ranges in Makua Military Reservation and Schofield Barracks West Range. Several areas are zoned for agriculture and have been used in part for agriculture purposes in the past. Some parcels are used for multiple purposes, such as the Poamoho section of the Ewa Forest Reserve and the Pupukea-Paumalu, which are leased to the U.S. Army for training.