Small-scale, intensive predator control benefits small- but not large-bodied skinks

Aotearoa | New Zealand’s endemic terrestrial fauna is threatened by invasive mammalian predators. Significant resources are invested controlling a subset of mammalian predators (primarily stoats, brushtail possums, and rats) to protect birds on the New Zealand mainland, including those invested in the national Predator Free 2050 (PF2050) strategy (now also including feral cats). Yet to date, there is scant evidence that predator trapping benefits mainland lizard (skink and gecko) populations, which generally require higher levels of predator suppression, including of mice. We conducted a 10-year study on three sympatric skink species to assess their responses to small-scale (< 10 ha), intensive predator control that included mouse suppression on the Port Hills, Canterbury. We found that: (1) annual counts of two small skink species (southern grass skink [Oligosoma chionochloescens] and McCann’s skink [Oligosoma maccanni]) increased significantly while the larger and rarer Canterbury spotted skink (Oligosoma lineoocellatum) declined to near-zero; and (2) proportionally more weasels (Mustela nivalis) were trapped than stoats (Mustela erminea) in all but the first year, with > 300 weasels trapped in total. Contrasting responses of small- versus large-bodied skink species may reflect differences in life history and refuge use. Small (≤ 15 ha), full mammal-exclusion (predator-proof) fences may be the only effective means for in-situ protection of mainland spotted skink populations. Interactions among multiple predator species, difficulty in suppressing mice, and lack of non-treatment sites make potential adverse effects of predator trapping (e.g. meso-predator release of mice, competitor release of weasels) difficult to detect. We suggest that, without adequate control tools for weasels and mice, PF2050 and other, similar control programmes will worsen ecological outcomes for indigenous lizards.