Effects of the Argentine ant on arthropod fauna of Hawaiian high-elevation shrubland

Human-caused biological invasions by an alien species are a worldwide phenomenon. They are particularly significant on isolated oceanic islands and represent a serious threat to endemic biota. The Argentine ant (Iridomyrmex humilis) has become established in portions of the high-elevation shrubland of Haleakala National Park, Maui, Hawaii, over the past 25 yr. This ecosystem lacks native ants but possesses many locally endemic and rare anthropod species. Pitfall trapping and under-rock surveys were conducted to determine the effects of I. humilis on the local arthropod fauna. More than 180 taxa were sampled, mostly Arthropoda. Presence of the Argentine ant is associated with reduced populations of many native and non-native anthropod species, including important predator species and major pollinators of native plants. Effects of ant invasion were particularly severe at higher elevations of Haleakala volcano where endemic species normally exist at low densities. Some taxa, primarily alien species, were more abundant in the presence of ants. Invasion of the Argentine ant has locally reduced the abundance of many endemic species in the shrubland ecosystem. Although the spread of this ant species is slow, I. humilis appears to have the potential to invade a much larger area of Haleakala National Park than it now occupies. Active management of Argentine ant populations will be necessary if the endemic fauna is to be preserved.