Assessing the combined effects of selective predator control and habitat enhancement on endemic lizard persistence in a New Zealand urban landscape
Authors:   , McGee Kathryn 
Published: February 2025

<p><strong>Urbanisation presents significant challenges for native lizard populations by altering habitats and increasing exposure to introduced predators. In New Zealand, where native lizards evolved without mammalian predators, these threats are particularly severe, as species lack effective anti-predator defences. Conservation strategies such as predator control and habitat enhancement have the potential to support urban lizard populations. While the eradication of introduced mammalian predators on offshore islands has led to the recovery of several native skink and gecko species, uncertainties remain regarding the intensity of predator control required to achieve positive outcomes on the mainland. The Predator Free 2050 (PF2050) initiative aims to selectively eradicate rats, mustelids, and possums from New Zealand by 2050, but the effectiveness of this approach for lizard conservation remains unclear. Habitat enhancements, including plantings and rock piles, are widely recommended, yet there is limited understanding of native lizard habitat preferences and the medium to long-term effectiveness of these enhancements. This study investigates the habitat preferences and effects of predator control and habitat enhancement on three native lizard species: northern grass skink (Oligosoma polychroma), copper skink (Oligosoma aeneum), and Raukawa gecko (Woodworthia maculata).</strong></p><p>I conducted laboratory choice experiments to examine the habitat preferences of the three lizard species in experimental arenas containing vegetation and rock pile habitats, observing their location and activity at specified intervals. The results revealed species-specific and time-dependent habitat preferences. Raukawa geckos exhibited a strong preference for rock habitats over vegetation, particularly during daylight hours, but occasionally used vegetation at night. Copper skinks preferred rock habitats during daylight but showed no clear preference at twilight. Northern grass skinks displayed size-dependent habitat preferences, with smaller individuals favouring rock habitats and larger individuals exhibiting no clear preference. I utilised six years of lizard monitoring data to assess the outcomes of selective predator control and habitat enhancement on lizards within the Miramar Peninsula, the focus of Phase 1 of Predator Free Wellington (PFW). Before-after-control-impact (BACI) experiments were conducted to compare lizard populations inside and outside the peninsula before and after predator control began and to investigate the effects of habitat enhancement using rock piles and vegetation within the peninsula. A storm surge inundated one enhanced site, limiting the habitat enhancement analyses. Selective predator control increased capture rates of northern grass skinks, but overall capture rates were negatively correlated with mouse tracking rates. Raukawa gecko capture rates increased following predator control, and the species was not detected outside of the Miramar Peninsula. Copper skink capture rates increased in some areas but not across the entire peninsula, with the species appearing at new sites both inside and outside the peninsula in the period following PFW Phase 1.</p><p>The research highlights the positive impact of selective predator control on lizard capture rates, while emphasising the need to consider mice in eradication efforts for long-term conservation success. The study underscores the importance of maintaining diverse habitats to accommodate the varying ecological requirements of lizards and the necessity of tailored conservation strategies that consider taxon-specific habitat preferences in urban lizard conservation efforts.</p>