No longer a pipe dream: monitoring a cryptic, endangered skink population (Oligosoma otagense) using passive eDNA detection devices

Using a sequential adaptive experimental design, we successfully demonstrate that a passive eDNA detection field tool, consisting of pear-baited alkathene pipes equipped with Dacron filters for eDNA capture, can effectively monitor occupancy by a cryptic Otago skink (Oligosoma otagense) population. Detection rates using our eDNA tool were approximately 30–50% higher than those recorded by field teams historically in the Awa Nohoaka Conservation Area. Furthermore, our findings show that the Awa Nohoaka Otago skink population likely occupies less habitat in 2024 than it did between 2005–2007, equivalent to a range retraction of 38% (95% CI: 14‒70%). This decline aligns with expectations given that 50 animals were removed from the wider site for the establishment of a captive population between 2009 and 2014. Additionally, the site has been exposed to ongoing predation by unmanaged invasive predators since 2017 and was only subject to limited predator control before this time. Our findings suggest that our eDNA field technique could be useful for periodic long-term monitoring of other cryptic or sparse lizard populations, particularly those inhabiting rock outcrops due to the ease of inferring probable movement routes. While eDNA methodologies remain expensive, they offer advantages in terms of minimising observer heterogeneity, increasing detectability, and improving field team safety and field scheduling certainty. To become cost-competitive with single species visual occupancy monitoring, the 2024 costs of eDNA field tools and laboratory analyses would need to decrease by c. 25%. However, given that eDNA assays can detect thousands of species simultaneously, the technique is likely to be cost-effective for occupancy scenarios requiring simultaneous monitoring of two or more hard-to-detect species, or where non-economic factors are prioritised.