Twenty years on: changes in lizard encounter rates following eradication of rats from Kāpiti Island

Introductions of mammalian predators have led to extinctions or declines of many species on islands; hence eradications of these mammals have played a major role in biodiversity conservation. However, eradications are costly and sometimes controversial. It is therefore important to conduct carefully designed sampling programmes that allow benefits to native species to be quantified. We report the results of sampling conducted in 1994–1996 and 2014–2015 to estimate changes in relative abundance of lizards on Kāpiti Island over 20 years following the eradication of Norway rats (Rattus norvegicus) and kiore (Rattus exulans) in 1996. Sampling was conducted in five different habitats over the island, and in each habitat involved repeatedly sampling 4–5 pitfall stations (five pitfall traps each) and conducting spotlighting and daytime searches along 2–3 transect lines. We used generalised linear mixed modelling to estimate proportional changes in lizard encounter rates while accounting for effects of month, weather variables, and changes in vegetation density. Pitfall capture rates for northern grass skinks (Oligosoma polychroma), brown skinks (Oligosoma zelandicum), and copper skinks (Oligosoma aeneum) were estimated to increase 2- to 28-fold in habitats where they were detected in 1994–1996, and these species were also found in 2014–2015 in habitats where they were not detected in 1994–1996. Spotlighting encounter rates for geckos (predominantly Raukawa geckos Woodworthia maculata) were estimated to increase 3.7-fold between the two time periods. There were sparse observations of ornate skinks (Oligosoma ornatum), forest geckos (Mokopirirakau granulatus) and Wellington green geckos (Naultinus punctatus), whereas goldstripe geckos (Woodworthia chrysosiretica), which were discovered on the island in 2013, were not detected in the areas sampled. Most lizards continue to be found in habitats with low, dense vegetation, a pattern that may be at least partially attributable to predation pressure from the abundant weka (Gallirallus australis) on the island.