Degree of Threat: Very high - high
Comments: From the recovery plan of the Oahu tree snails of the genus Achatinella (USFWS, 1993):
Decline is the result of many factors acting over an extended period of time coupled by low reproductive rates and limited dispersal abilities. Habitat destruction was caused by removal of forests and introduction of invasive vegetation for pasture, agriculture, or housing. Forests not cleared for agriculture were invaded by feral cattle, horses, goats and pigs and their grazing reduced the forest understory and aided the invasion of exotic plants. Human activities such as hunting, hiking, military maneuvers, clearing for illegal marijuana patches, construction of helicopter landing sites, and building of roads and trails contribute to exotic vegetation spread. Logging has significantly altered native forests including reforestation with non-native species. Forest fires have had a catastrophic effect on small populations (kill snails, allow for spread of non-native plants, alteration of understory leading to moisture and humidity changes unsuitable for tree snails).
The carnivorous snail, Euglandina rosea, and European rat, Rattus rattus, are predators of native tree snails. Other predators are the predatory flatworm, Geoplana septemlineata, terrestrial snail, Oxychilus alliarus, Norway rat, Rattus norvegicus, and Polynesian rat Rattus exulans. Parasitism and disease have not been documented in Achatinella but may contribute to future decline.
Collecting historically was probably responsible for decline in ranges and abundance of some species and even today (despite listing on the U.S. Endangered Species List), removal of even two or three adults can remove a large percentage of the reproductive population in a bush or tree.
This species is threatened by introduced predators (including Euglandina, rats, and Oxychilus), by habitat destruction, and by overcollection. Early naturalist and hobbyists began a shell collecting frenzy in the mid-1800's which precipitated population declines and extinctions that have continued to this day. Shell collection (see picture to right) combined with the clearing of land by Polynesians and early settlers, left many lowland species and varieties extinct by the turn of the 20th century. Habitat destruction and degradation continue today, but the most insidious and intractable threat comes in the form of introduced predators (Hadfield et al. 1993).
In an attempt to control the giant African Snail, the Department of Agriculture introduced Euglandina rosea, a predatory snail, as a biocontrol agent in the mid 1950's. Euglandina has since become well established; consuming native snails at an alarming rate. Three species of rat (R. rattus, R. norvegicus, and R. exulans) have also been decimating native snail populations across the island.