DEAD SNAILS DO TELL TALES: INVASIVE RAT PREDATION ON FOSSIL ENDEMIC SNAILS AT MAKAUWAHI CAVE, KAUA'I

The extinction of Kaua’i’s endemic terrestrial snails occurred in two discrete waves: the first followed Polynesian colonization around 400 C.E., and the second after Western contact in 1778. Such extinctions are attributed in large part to the ecological impacts of invasive species. Infamous among these invasives are rats Rattus exulans, introduced by Polynesians, and R. rattus and R. norvegicus, which were both introduced during Western arrival. This study addresses the role of invasive rat predation in the extinction of two ground-dwelling snail species, Orobophana juddii and Leptachatina cf. fossilis,which are well represented in the Makauwahi Cave record.

To assess the role of rat predation in the extinction of these two species, we bulk-sampled the northwest pit of Makauwahi Cave at intervals of 0.1 m. Samples with a minimum of 30 individuals of a species were used to estimate the frequency of shells with repair scars, which are diagnostic indicators of failed shell-crushing predation. We predicted that the frequency of repair would sharply increase after the arrival of rats on the island. Examination of more than 3000 specimens showed that the average percentage of scarred O. juddii more than doubled from pre-human (0.030%) to post-colonization (0.073%), while Leptachatina showed no statistically significant increase in scarring frequencies, which ranged from 0.017% to 0.021%. Both species also show a decrease in relative abundance soon after Polynesian colonization. Native species capable of producing scar traces (such as the land crab Geograpsus sp.) were extinct or in decline during the time interval in which scarring frequencies were increasing, which leaves rats as the only co-occurring predator that could have produced the scar traces.

These results indicate that rats may have played a lesser role than predicted in the extinction of Leptachatina, possibly resulting from low preference by selectively feeding rats for this species. However, for O. juddii, the change in scarring frequencies we found supports our hypothesis that increased predation pressure by introduced rats was an important factor in the extinction of this species.