Accuracy and precision of skink counts from artificial retreats

Index counts are commonly used to detect spatial and temporal changes in the size of wildlife populations. For indices to be valid there must be a constant (usually linear) relationship between the index and population size. In a study conducted in the Eglinton Valley (Fiordland, South Island, New Zealand), single-day index counts of common skinks (Oligosoma polychroma) from artificial retreats were compared with capture–mark–recapture (CMR) estimates of population size (Nˆ) obtained by pitfall trapping. Generalised linear models revealed that skink counts from artificial retreats provided a reasonably accurate (P < 0.05) and highly precise (P < 0.001) index of population size, but only if sampling was conducted under optimal weather conditions. Density ranged from 3639 (2591–6827; 95% CI) to 9245 (6346–16431) skinks ha^–1, which was high compared with other common skink populations. We recommend: (1) long-term monitoring of common skinks in the Eglinton Valley, using the index method described herein; (2) calibration of index counts against population size estimates collected from other habitats and species.