Chapter 52
Ecological Subregions of the United States



Hawaiian Islands

One Section has been delineated in this Province:

This Section includes all islands of the State of Hawaii. The area of this Section is about 6,500 mi2 (16,800 km2).

Section M423A--Hawaiian Islands

Geomorphology. Most land areas of this Section were formed from shield volcanoes. Predominant landforms range from currently forming to deeply incised canyons; alluvial fans, coastal plains, and coral atolls. Elevation ranges from sea level to 13,796 ft (0 to 4,194 m).

Lithology and Stratigraphy. Volcanic flows of various ages are evident. The northwest islands and some coastal areas based on coralline limestone and sands.

Soil Taxa. Inceptisols, Histosols, Oxisols, Mollisols, and Ultisols occur in combination with isohyperthermic, isothermic, or isomesic soil temperature regimes and ustic, udic, or aridic soil moisture regimes.

Potential Natural Vegetation. K\"uchler classified potential vegetation as sclerophyllous forest, shrubland and grassland; guava mixed forest; ohia lehua forest; lama-manele forest; koa forest; koa-mamani parkland; grassland; microphyllous shrubland; and barren.

Over 100 distinct natural communities occur, including tropical coastal vegetation, lowland wet forests, montane wet forests and bogs, montane moist forests and parkland, subalpine vegetation, alpine vegetation, lowland grasslands and shrublands, lowland dry and mesic forests, montane grasslands and shrublands, and montane dry forests. About 90 percent of native plant taxa are endemic. O'hia lehua and koa are the two most common large trees and are found in a variety of ecosystems. Naturalized vegetation and fire dynamics have resulted in conversions to mixed exotic forests, shrublands, and grasslands.

Fauna. There are native sea birds, waterfowl, water birds, forest birds, raptors, honeycreepers, and numerous exotic birds. Over half of all native birds are endemic. One native terrestrial mammal (bat) is present; introduced species include mongoose, pig, deer, rat, mouse, sheep, and goat. No native terrestrial amphibians or reptiles occur. There are several hundred species of endemic tree snails. Cave and anchialine pool fauna are present.

This steep, highly dissected landscape of the Hawaiian Islands is volcanic in origin and is dominated by tropical vegetation.}

Climate. Precipitation ranges from 5 to 460 in (130 to 11,680 mm). Rainfall is highly seasonal in dry areas, and is less seasonal in wet areas. Relative humidity ranges from 50 percent (average daily minimum, dry coastal area) to 90 percent and above (average daily maximum, wetter areas). Some snow occurs on highest peaks. Temperature ranges from 48 to 77 oF (9 to 25 oC). The growing season is mostly 365 days, decreasing to about 300 days at highest elevations.

Surface Water Characteristics. Fast moving perennial streams are common in wetter parts of mountain areas. Few to no perennial streams occur elsewhere because of seasonal rainfall and a permeable substrate.

Disturbance Regimes. Lava flows and ash and cinder falls periodically occur on the Island of Hawaii. Fire rarely occurred before human settlement; now it is frequent in dry areas and significant in wet areas. Hurricanes were infrequent and mild before 1950, but three severe hurricanes have damaged Kauai since then. Drought tends to be localized. Nearly all streams on wetter slopes of Maui, Big Island, and Oahu are diverted by ditch and tunnel systems or urban channelization. Exotic animals have caused severe grazing and dispersal of exotic plants. Native plants suffer from severe competition and transformation of soil, water, and fire regimes.

Land Use. Settlement, agriculture, grazing, urbanization, and re-forestation have transformed most coastal and lowland areas and many upland areas. Conversion of upland and forest areas continues. Coastal and agricultural lands are being further urbanized. Land use changes plus impacts of exotic species are contributing to continuing extinctions. About half of all native bird species estimated to have existed in Hawaii are extinct; over a third of those remaining are listed as threatened or endangered. Nearly a quarter of native plants are proposed or listed as threatened or endangered.

Cultural Ecology. Substantial human influence began about the early fourth century with the introduction of animal and plant species by the settlers who were the ancestors of today's native Hawaiians. European contact, beginning in 1778 A.D., brought more major changes, continuing through present time. Contempory attitudes and beliefs are varied, and cultural diversity is very high.

Compiled by Pacific Southwest Region.