INVASIVE RAT PREDATION RECORDED IN 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 ~1000 yr B.P., and the second after Western contact in 1778 C.E. Such extinctions are often attributed to invasive species, particularly rats: Rattus exulans, introduced by Polynesians, and R. rattus and R. norvegicus, 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.

We bulk-sampled the northwest pit of Makauwahi Cave at stratigraphic intervals of 100 cm. Samples with ³30 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 repair frequency would increase after the arrival of rats on the island. Examination of >3500 specimens showed that the average percentage of scarred Orobophana doubled from pre-human (2.6%) to post-colonization periods (6.3%), while Leptachatina scar frequencies varied little (1.3-1.5%). Both species decrease in abundance soon after Polynesian colonization. Native species capable of producing scar traces (eg. land crab Geograpsus sp.) were extinct or in decline during the Polynesian, which leaves rats as the only known co-occurring predator that could have produced the scar traces. These results may indicate that rats played a lesser role than predicted in the extinction of Leptachatina, possibly because the snails lived in a microhabitat less accessible to rats. However, the change in scarring frequencies of Orobophana supports our hypothesis that increased predation pressure by introduced rats was an important factor in the extinction of this species.