Double-observer distance sampling improves the accuracy of density estimates for a threatened arboreal mammal
Jemma K. Cripps
A B C , Jenny L. Nelson A , Michael P. Scroggie A B , Louise K. Durkin A , David S. L. Ramsey A and Linda F. Lumsden A
+ Author Affiliations
- Author Affiliations
A Arthur Rylah Institute for Environmental Research, Department of Environment, Land, Water and Planning, 123 Brown Street, Heidelberg, Vic. 3084, Australia.
B School of BioSciences, The University of Melbourne, Parkville, Vic. 3010, Australia.
C Corresponding author. Email: Jemma.Cripps@delwp.vic.gov.au
Wildlife Research 48(8) 756-768 https://doi.org/10.1071/WR19136
Submitted: 8 August 2019 Accepted: 27 June 2021 Published: 18 October 2021
Abstract
Context: Determining population size or density is often fundamental for wildlife conservation. For nocturnal species, indices are commonly used in place of abundance estimates, with spotlighting indices (e.g. sighting rate per km) being prevalent. Distance sampling is a collection of techniques that provide estimates of wildlife abundance from line-transect data, by correcting raw counts for imperfect detection. These methods have rarely been used to assess the abundance of nocturnal arboreal mammals.
Aims: To develop and evaluate a method for estimating the abundance and density of nocturnal arboreal mammals using double-observer distance sampling, and to apply the approach to a survey of the southern greater glider (Petauroides volans) in the Strathbogie Ranges, Victoria, Australia.
Methods: Two observers, 15–20 min apart, surveyed 25 randomly located 500 m transects, and recorded greater gliders using spotlights and binoculars. Densities and abundances were derived from the line-transect data by using mark–recapture distance sampling (MRDS) models and were compared with conventional distance sampling analysis (CDS).
Key results: Using the double-observer approach, we estimated an overall density of 0.96 gliders ha−1 (95% CI 0.60–1.50), giving a population estimate of 24 575 greater gliders across the Strathbogie Ranges (25 865 ha, 95% CI 15 620–38 661). The corresponding estimates for the study area derived using CDS applied to either both observers’ observations or to the first observer’s observations only, were 87% and 53% respectively, of the MRDS estimate. The analysis confirmed that the probability of detection of gliders along the transect line was less than one, justifying the use of the double-observer method to obtain accurate estimates of abundance.
Conclusions: The low detectability of greater gliders means that uncorrected spotlight counts will underestimate abundance, as will CDS. The double-observer method corrects for the negative bias associated with raw counts, enabling more accurate estimation of abundance for survey, monitoring and management purposes.
Implications: We recommend that double-observer distance sampling is adopted as a standard technique for estimating the abundance of greater gliders. The double-observer method potentially has wider relevance for assessing population size of other arboreal mammals, providing the assumptions of the approach can be met.
Keywords: arboreal mammal, distance sampling, greater glider, mark–recapture, population estimation.
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