Acacia koa for up to 25 years. Scarification is needed before "A. koa" seeds will germinate. Koa is endemic to the islands of Hawaii, Molokai, Maui, Lānai, Oahu, and Kauai, where it grows at elevations of . It requires of annual rainfall. Acidic to neutral soils (pH of 4-7.4) that are either an Inceptisol derived from volcanic ash or a well-drained histosol are preferred. Its ability to fix nitrogen allows it to grow in very young volcanic soils. Koa and ōhia lehua ("Metrosideros polymorpha") dominate the canopy of mixed mesic forests. It is also common in wet forests. The koa's trunk was
ʻAnianiau Its call is a pair of notes, "tew-weet", while its song is a trill of "wee-see, wee-see, wee-see". Anianiau are found in mesic and wet forests at elevations above . The highest densities occur above . Dominant tree species in its habitat include koa ("Acacia koa"), ōhia lehua ("Metrosideros polymorpha"), ōlapa ("Cheirodendron trigynum"), and lapalapa ("C. platyphyllum"). The anianiau mainly feeds on nectar from the flowers of plants such as ōhia lehua ("Metrosideros polymorpha"), ōhelo ("Vaccinium" spp.), and alani ("Melicope" spp.). It will also take arthropods from trees, shrubs, or vines. The anianiau breeding season ranges from February to June.