REPRODUCTION OF A RARE NEW-ZEALAND REPTILE, THE TUATARA SPHENODON-PUNCTATUS, ON RAT-FREE AND RAT-INHABITED ISLANDS

We compared evidence for successful reproduction of tuatara (Sphenodon punctatus punctatus) in the presence or absence of introduced Pacific rats (kiore, Rattus exulans) for islands in the Mercury Group and Hen and Chickens Group, New Zealand. Tuatara were captured more frequently on the two kiore-free islands than on the six kiore-inhabited islands, suggesting reduced density in the presence of kiore. On the two kiore-free islands, 22.2-30.8% of all tuatara captured were juveniles or small adults (< 200 mm snout-vent length). In contrast, juvenile tuatara were not seen on five of the six kiore-inhabited islands and small adults were absent from three. These data support an earlier study suggesting that kiore inhibit recruitment of tuatara. Both sexes of adult tuatara were present on kiore-inhabited islands, and plasma concentrations of sex hormones suggest that at least some adults remain reproductively active on most islands. Throughout their range, tuatara have become extinct this century on four islands inhabited by kiore, they are on the verge of extinction on two kiore-inhabited islands sampled in this study and two kiore-inhabited islands not sampled, and they are absent from at least six other kiore-inhabited islands on which on biogeographic grounds they would be expected to be present. Although the data remain circumstantial, the most plausible explanation of these observations is that kiore can cause extinction of tuatara through competition for food and/or predation on eggs and juveniles. In the absence of kiore eradication the four worst-case populations of tuatara would probably be extinct within 10-20 years and populations on three other islands would probably be relictual or extinct within 30-50 years. Kiore eradiction is being carried out on most of these islands, in part to help ensure tuatara survival. Well-monitored eradication programs will enable an experimental test of the hypothesis that tuatara densities and recruitment rates are enhanced in the absence of kiore.