The skinks of North Brother Island: abundance, habitat use and species interactions

Abstract

North Brother Island, Cook Strait, New Zealand, is a small (4 hectare), exposed and resource-limited ecosystem. Nevertheless, it supports five species of reptile: Brother's Island tuatara (Sphenodon guntheri), Duvaucel's gecko (Hoplodactylus duvaucelii), the common gecko (H. maculatus), the regionally rare spotted skink (Oligosoma lineo-ocellatum) and the common skink (O. nigriplantare polychroma). I studied the distribution and abundance of the two skink species in relation to three habitats and frequencies of tail loss across the reptile community on North Brother Island, in April and November 1999. This was to further research already undertaken on the island's other reptile species. Adult spotted skink densities were higher in shrubland (3,700/ha in April and 3,100/ha in November) than tussock (2900/ha in April and 1600/ha in November). Densities were lowest in iceplant habitat (2900/ha in April and 1200/ha in November). The combined adult and juvenile density for common skinks was 2,900/ha in April, but captures were too low in November to produce an estimate. Adult spotted skink survivorship over winter (54%) varied with sex, habitat and density. The percentage of tail loss in the North Brother Island reptile community increased with decreasing size of reptile species. The skink populations exhibited 90-93% tail loss, whereas the tuatara population exhibited 45%-60% tail loss. This trend suggests that predation intensity is highest in skinks. However, in the Stephens Island reptile community no such relationship occurred. Skinks exhibited 70.5-79.5% tail loss and tuatara exhibited 81.5% tail loss, suggesting that other factors may regulate tail loss. Factors most likely to explain these patterns of tail loss in skinks are competition from other skinks and inefficient predation by larger reptiles. In tuatara I also hypothesize that competition increases tail loss but also that most tail loss will occur in young tuatara by inefficient predation. Older tuatara may have lower risks of predation, so I hypothesize that tail loss may become inhibited, especially in a resource-limited environment such as North Brother Island. In summary skink densities were high, being similar to those found on other islands in the Cook Strait region. This is surprising considering the island's small size and exposure. Possibly, as a result of these high densities and sparse vegetation cover, tail loss is high in the skinks. The high densities of spotted skink on North Brother Island provide the possibility to form new populations by translocation.