Placement period of artificial retreats affects the number and demographic composition but not the body condition of skinks

Abstract

Monitoring is important in conservation management, essential for assessing population trends, making decisions and allocating resources. Artificial retreats can offer a reliable, low impact and efficient method for monitoring cryptic herpetofauna. Methods for monitoring artificial retreats vary between different conservation management programmes in New Zealand, however, and a deeper understanding of the consequences of these variations would encourage greater standardisation and enable more reliable comparisons to be made across temporal and spatial scales. We investigate how placement period of artificial retreats affects population size indices and the body condition of common skinks (Oligosoma polychroma) in a grassland in Fiordland, New Zealand. We made 3987 skink sightings during 8250 visual checks and captured 638 skinks in 1200 physical searches of 400 artificial retreats during the summer of 2010/11. We observed more skinks under artificial retreats with a placement period of 2 years (permanent retreats) than under adjacent retreats in place for <2 months (temporary retreats). Placement period influenced the demographic composition of skinks occupying the artificial retreats, with a greater proportion of subadults and juveniles seen beneath temporary retreats and a greater proportion of adults occupying permanent retreats. Physical condition of the skinks occupying the retreats was not related to placement period. Our findings suggest that permanent placement of retreats is unlikely to increase survival or reproductive rates by increasing the body condition of individuals occupying retreats, but may result in competitive exclusion of juveniles and subadults by resident adults. We recommend temporary placement of retreats for assessing monitoring trends in skink populations in environments with moderate to high population densities, and permanent placement in areas of low population densities, due to the potential detection advantage. Our results emphasise the need for placement period to be considered when interpreting results from monitoring programmes using artificial retreats.