Limitations of the Double-Observer Method for Estimating Population Size: A Case Study on the Southern Greater Glider (Petauroides volans)

Monitoring the size of wildlife populations is crucial for the effective implementation of conservation management strategies, and a variety of methods have been developed for this purpose. One such approach is the double-observer method, which has recently gained prominence in monitoring programs for the southern greater glider (Petauroides volans), an iconic nocturnal arboreal marsupial native to Australia. While this method has been successfully applied at lower population densities, its reliability and applicability at higher-density sites has not been evaluated. This case study represents the first instance of an investigation and discussion on the application of the double-observer method in greater glider monitoring at higher-density sites. We found that in higher-density areas, the proximity of individuals makes it more difficult to reliably distinguish unique (individual) animals between observers, and the increased number of observations per transect extends the time required for data recording. Transects with more animal observations showed significantly longer delays between observers (z = 5.062, p < 0.001). Additionally, discrepancies in the number of animal observations between observers significantly altered the intended 15–20 min interval (z = 2.71, p = 0.007). Deviations from the standard 15–20 min interval between observers were common, occurring at 44 of the 66 sites, where actual time-lags ranged from 0 to 64 min. Consequently, longer intervals increased the potential for animal movement, while shorter intervals risked observer independence. These factors, combined with our experience applying the double-observer method across sites with markedly different greater glider densities, suggest that the critical non-movement assumption may be violated more frequently than previously recognised. We discuss the limitations of applying the double-observer method to survey high-density populations and recommend prioritising research on greater glider movement patterns and alternative survey techniques to improve the accuracy and reliability of monitoring programs at higher-density sites.