A Baseline Study of the Kapiti Island Lizard Fauna, Prior to Rat Eradication

Abstract

Lizard populations were surveyed in six areas on Kapiti Island over the summers of 1994-5 and 1995-6. Methods used were pit traps, artificial retreat sites, Longworth live capture traps, spotlighting and day searching. Six lizard species were recorded, a seventh (the green gecko Naultinus elegans punctatus) is also known from Kapiti Island. Pit traps, spotlighting and day searching were found to be the most appropriate methods for survey and monitoring. The common skink (Oligosoma nigriplantare polychroma) was found to be abundant in coastal and ridge grassland, and also occurred in kanuka forest. The brown skink (O. zelandicum) was moderately abundant in coastal grassland and occurred in coastal forest. The copper skink (Cyclodina aenea) was rarely found in coastal and ridge grassland. The ornate skink (C ornata), the largest skink recorded, was very rare and appeared to be limited to screes in coastal forest. The common gecko (Hoplodactylus maculatus) was moderately abundant in coastal grassland and rare in kanuka forest and ridge grassland. The forest gecko (H. granulatus) was recorded once, in high forest. No lizards were found in kohekohe forest. The relationship between vegetation and lizard assemblages and abundance was investigated between and within the six habitat types. Within the grassland habitats variation of shrub and tussock cover occurred. Brown skinks in coastal grassland were more common at sites with extensive cover, so this species is likely to become more abundant as succession continues. No strong relationships were found between capture rate of skinks in pit traps and six weather variables. However capture rate tended to increase with increasing maximum temperature, sunshine and decreasing midday relative humidity. The relationship between skink capture rates from pit traps, vegetation and weather data was modelled, so that expected capture rates can be predicted for given conditions. Comparisons of aspects of the populations of common geckos and common skinks on Mana and Kapiti Islands were made. Common geckos were much more abundant on Mana Island, and about half observed were foraging at ground level. Terrestrial activity was virtually absent on Kapiti Island. Common skinks grew to a greater maximum size on Mana Island, and size frequency distributions differed significantly between the two islands. The differences observed between populations of both species are regarded as being due to predation by rats (Rattus exulans and R. norvegicus) on Kapiti Island. The impact of rodents on the Kapiti Island lizard fauna is assessed. Predictions are made as to how the lizard fauna will respond to rat eradication. Recommendations are made for continued monitoring of lizards on Kapiti Island, and other management options.