Predator control allows critically endangered lizards to recover on mainland New Zealand

Two conservation tools have been developed over the last 10–15 years for species on the New Zealand mainland that are vulnerable to introduced mammalian predators: landscape-scale predator trapping networks, and eradication of predators within mammal-proof exclosures. We tested whether these tools would allow population growth of critically endangered grand skinks (Oligosoma grande) and Otago skinks (O. otagense) over three years. Skink populations were subjected to one of three predator treatments: (1) near-eradication inside a mammal-proof fence; (2) suppression by trapping within a 2100-ha area; and (3) unmanaged predator populations. Monitoring by non-invasive photo–resight methods, and data analysis using program MARK, showed that the greatest increase in abundance of both species occurred at the centre of the predator trapping treatment and inside a mammal-proof fence. For grand skinks, there was little or no change in population size at the trapping periphery. At the unmanaged sites, the grand skink population underwent a catastrophic decline whereas the Otago skink population was stable. A grouping analysis showed that the unmanaged grand skink treatment was clearly distinguished from the other predator treatments based on survival rate. Results suggest that: (1) predation by introduced mammals is a key driver in the decline of these skinks, and episodic predation events may be a component of this process; and (2) use of mammal-proof fences or intensive predator control over a large enough area should allow skink populations to recover.