Decline and homogenization of Pacific faunas: the land snails of American Samoa

Native Pacific island biotas are disappearing rapidly. Among these native b iotas the land snails are especially recognized not only for their high div ersity and high levels of endemism but also for being under severe threat, with many species already extinct. Many non-indigenous snail species are be ing introduced, leading to a homogenization of land snail faunas across the Pacific. Field survey work in American Samoa in 1998 recorded 19 of the 32 previously known native land snail species, 11 of the 12 alien species, an d three of the six cryptogenic (unknown origin) species previously known fr om American Samoa. Eight species were recorded from American Samoa For the first time: three described species that are presumed to be native, four no n-indigenous species; and one cryptogenic species. Two undescribed and pres umed endemic species were discovered, as was one unidentified and perhaps u ndescribed species. One species, previously thought to be extinct was redis covered, although in very small numbers. The known island by island distrib utions of 11 species (five native, five alien, one cryptogenic) were extend ed. Comparisons with surveys undertaken predominantly in the 1920s and 1930 s, 1975, and 1992 showed that: the majority of native species are declining ; some native species may be stable or even increasing, with one species ab undant; only seven of the non-indigenous and cryptogenic species are declin ing or probably declining, while six are increasing, and for others no tren d was assessed; some non-indigenous species are extremely abundant. Threats to the fauna include: alien predators, predominantly rats and predatory sn ails introduced in attempts to control the giant African snail; possible co mpetitors, including non-indigenous snail species; invasive plants and habi tat modification; development, including recreational, agricultural, and do mestic/urban development. A review should be undertaken of the IUCN and US Fish and Wildlife Service listings of endangered land snail species in Amer ican Samoa. (C) 2001 Elsevier Science Ltd. All rights reserved.