Endemic terrestrial tree snails of the Hawai'ian Islands, like those of other oceanic islands and even some continental areas, are extremely sensitive to disturbance because of their low population numbers and small geographic ranges. Like many other plants and animals of oceanic islands, they have evolved no defenses against introduced predators and competitors. The range of Achatinella mustelina, a tree-snail species found only in a short mountain range on the island of O'ahu, typifies this problem. Mark-recapture studies at two field locations reveal that the snails exhibit slow growth andlate maturity (3–5 years). Fecundity is estimated at about 7 offspring per adult per year. The young are born live at about 4.6 mm. Population growth typically depends on considerable longevity (> 10 years). Demographic effects of the depredations by alien predators, rats and a North American predatory snail, Euglandina rosea, were documented in two long-term study sites. The predatory snail eats all sizes of A. mustelina and can rapidly drive populations to extinction (less than one year). Rats tend to select larger snails as prey and may leave an area before destroying all of the prey snails present; while reproductive output is temporarily destroyed, populations may survive. Actions necessary to conserve Hawai;ian tree snails, or indeed any group of relatively sedentary invertebrates with small species ranges, must include predator abatement, but also preservation and restoration of sufficiently arge and complex forest habitats that the invertebrates may find refuge from alien predators.