Assessing and comparing population densities and indices of skinks under three predator management regimes

Introduced mammalian predators threaten populations of endemic New Zealand skinks. Their effects on skink populations have been not often quantified on the mainland and are known primarily from skink population increases on islands from which mammals have been eradicated. Estimating skink population density with capture–recapture trapping is time-consuming and costly. Counting skinks in artificial retreats in specific weather conditions may be a useful and relatively quick way to index population density, but needs calibration for different habitats and species. In 2007 and 2009, we estimated the population density of small terrestrial skinks (McCann’s skink Oligosoma maccanni , southern grass skink O.  aff. polychroma  clade 5 and cryptic skink O. inconspicuum), based on spatially explicit capture–recapture (SECR) in pitfall traps in three mammal-management treatments at Macraes Flat, Otago. The treatments were eradication of large predators and near-eradication of rodents inside a mammal-resistant fence, suppression of mammalian predator populations through continuous trapping (two locations within an extensive area), and no mammal management. We tested for relationships between the estimated population densities and dawn and late-morning counts of skinks in artificial retreats. Skink density (three species combined) ranged from c. 1200 per ha at the experimental control site to c. 4000 per ha at the fenced site. These treatment differences in skink density may be the combined effect of predator management and pre-existing differences due to habitat characteristics and farming practices. Skink counts done in late morning (2009) were related to estimated skink densities but did not differ significantly between treatments. Skink counts done at dawn (both years) were not related to densities. Counts, but not densities, were significantly higher at locations with a more northerly aspect. We recommend further investigation of the utility of skink counts in artificial retreats for monitoring skink density at this location, with careful control of ambient temperature during sampling, and of aspect, habitat and device placement.