This page last revised 04 September 2006 -- S.M.Gon III
TNC Action Sites
Maps & Figures
Habitat-modifying weeds such as the alien melastome Miconia calvescens represent a major threat to native biodiversity.
Understory obliteration by feral pigs in a montane wet forest community.
Over human history in the islands, several major groups of alien species have emerged as the most damaging to native ecosystems and species:
o Ungulates – lacking any large native herbivorous mammals, the Hawaiian flora is not adapted to ungulate browsing or trampling. Feral pigs, goats, sheep, deer, and cattle were responsible for destruction of lowland ecosystems, and continue to degrade remaining native ecological systems.
o Invasive weeds – through a history of increasing introduction of alien plants, there are now more species of naturalized alien vascular plants in the wilds of Hawai‘i than there are native species. Perhaps 200 of these are extremely aggressive, habitat-modifying weeds.
o Predators – small mammals such as rats, mongoose, and feral cats prey on native birds. Rats are implicated as wholesale vegetation modifiers via selective seed predation. Predatory invertebrates such as ants and other social hymenoptera have greatly disrupted invertebrate communities at all elevations, particularly in the lowlands.
For these reasons, conservation in Hawai‘i depends more than anything on keeping remaining, relatively uninvaded native areas intact, stemming the establishment of new invasive species, and devising practical strategies to limit the impact of widely-established species.
|A summary of threats across conservation areas in the Hawaiian ecoregion reveals that alien species such as ungulates and weeds are prominent and ubiquitous, with other threats active in specific locations.Threat assessment followed Conservation Area Planning (CAP) guidelines.||
Threat Ranks: VH = Very High, H = High, M = Medium, L = Low.
Wildfire in Hawai'i is invariably followed by invasion by fire-adapted alien grasses and shrubs, creating a cycle of destruction for native species and ecosystems.
Goats and sheep are a prevailing threat to drier native areas in Hawai‘i.
Development remains a threat on O'ahu, where 80% of the state's population resides.
Threat Summary (continued from above)
Fire: Wildfire is an uncommon natural occurrence in Hawai‘i, where ground-strike lightning is rare and wet plant communities cover large areas. Fire-adapted aliens (especially grasses and short-lived shrubs) are established in lower, leeward slopes and some subalpine areas. Normally ignited by people, these weeds fuel major wildfires that can carry into native forests. Native forests are destroyed and replaced with fire-adapted weeds in a trend that increases the range and intensity of these fires.
Small Mammals: There are no native small mammals (e.g., rodents, cats, dogs, rabbits, mongooses) in Hawai‘i, and the ecological effects of either herbivorous, omnivorous, or predatory small mammals is to reduce populations of native species, sometimes to extinction. Rodents are implicated in damage to lowland forests via seed predation, as well as on both ground-nesting seabirds and forest birds. Feral cats and dogs impose similar impacts. Rodents seem particularly damaging in the Wai'anae conservation area of O‘ahu, where they affect endangered tree snails, rare native plants, and an endangered forest bird, the ‘elepaio (Chasiempis sandwichensis gayi).Pathogens (including invertebrate pests): Diseases and pests can play an important role in reduction of viability of native species, and indirectly, the natural communities and ecological systems comprised of these species. Pathogens and pests related to declines in native species include mosquitoes and mosquito-borne diseases (avian malaria and pox), ants (various species), Erythrina gall wasp (Quadrastichus erythrinae), two-spotted leafhopper (Sophonia rufofascia), slugs (various species), and black twig borer (Xylosandrus compactus). Often the role of pathogens is tied to other threats. For example, avian diseases affecting native forest bird concentrations are spread by mosquitos, and spread of mosquitos into forest bird habitat is tied to wallows of feral pigs that create mosquito breeding sites where none otherwise exist.
Development : Widespread conversion and development of the lowlands of the Hawaiian ecoregion took place from prehistoric times to the first half of the twentieth century. Following statehood, strong conservation zoning has largely limited development of natural areas and forest reserves, but incremental conversion of lowland native areas continues on the most densely populated island (O‘ahu), as well as the largest island (Hawai‘i), particularly in the Windward Mauna Loa and Kona conservation areas.
(continued next column)
Military training activities: Live-fire training, large-scale troop
movements, and heavy equipment operation are serious threats to native species
at U.S. Army training facilities in the
Typical recreational uses of native ecosystems include hiking, camping,
and off-road vehicle touring. Restrictions on damaging activities in
the state conservation district somewhat limits the impacts of recreational use,
although indirect effects of recreational activities such as hiking,
e.g., spread of invasive weeds, has been documented.
Overharvesting: Most minor forest and stream “commodities” (plant materials for lei making, flower arrangements, and herbal use; stream fishes and invertebrates for food) can be harvested for home and cultural use on a sustainable basis. These activities are not sustainable at a large, commercial scale in most situations, and are already restricted by permit systems.
Logging: Although logging and other high-intensity harvesting is no longer widespread in Hawai‘i (most loggable areas were cleared in the last century), these and other clearing practices are important concerns in some conservation areas on Hawai‘i Island. Commercial logging of native koa (Acacia koa), ‘ōhi‘a (Metrosideros spp.), sandalwood (Santalum spp.), and hāpu‘u tree ferns (Cibotium spp.) are approaching the limits of available resources, and the forest products industry supports planting programs to restore former forest lands.
Grazing: Clearing of forest for production of cattle has a 200 year history in Hawai‘i. Cattle, being large ungulates, have the same damaging effects on native vegetation as other ungulates, and the devastating effects of cattle in Hawai'i have been well documented. Today, there remain large private ranches, several of which occur within native ecological systems. Ranching-related loss of native ecosystems is active in the Kona conservation area in particular.