Survey methods in the spotlight: a comparison of two survey methods for estimating the population density of an arboreal marsupial

Accurate estimates of population density are needed to guide conservation priorities for threatened fauna but can be difficult to produce for species where detection is challenging such as nocturnal arboreal marsupials. The accuracy of population estimates can depend on whether the survey methods’ assumptions are satisfied but attempts to verify these are infrequent. We compared two methods used to estimate the abundance of arboreal marsupials; conventional distance sampling, which is frequently used to survey these species, and quadrat sampling, which has previously been trialled as an alternative method. For each method we assessed the accuracy of density estimates and their ability to meet assumptions of perfect detection using a wild population of western ringtail possums ( Pseudocheirus occidentalis ) as a case study species and a ‘population’ of artificial reflective eyes that we deployed in the same habitat. We found that both methods underestimated the known population of artificial eyes, and that quadrat sampling performed significantly better in terms of detection efficiency, but its estimates were less precise. Quadrat sampling’s improved detection efficiency potentially explained our finding that the density estimate of wild possums based on quadrat sampling (6.25 possums ha −1 (95% Confidence Interval 3.29–10.20)) was substantially higher than the estimate derived from distance sampling (2.40 possums ha −1 (95% CI 1.54–3.62)). Our results highlight neither method achieved perfect detection and that resulting population estimates may not accurately represent true animal abundance. Nonetheless, both methods could be used as a measure of relative abundance to assess population changes over time. Given that the methodological assumptions of these methods are rarely assessed, our findings emphasise the importance of testing these assumptions when reliable estimates of population abundance are required.