Conventional distance sampling versus strip transects and abundance indices for estimating abundance of greater gliders (Petauroides volans) and eastern ringtail possums (Pseudocheirus peregrinus)

Abstract

<b>Context:</b> Accurate estimates of abundance are extremely useful for wildlife management and conservation. Estimates generated from distance sampling are typically considered superior to strip transects and abundance indices, as the latter do not account for probability of detection, thereby risking significant error.<br/>

<b>Aims:</b> To compare density estimates generated from conventional distance sampling (CDS) of arboreal marsupials with strip transect density estimates and abundance indices.<br/>

<b>Methods:</b> Off-track CDS and strip transects were used to estimate densities of <i>P. volans</i> and <i>P. peregrinus</i> across ∼ 2.6 km² of remnant eucalypt forest at Mt Duval in north-eastern New South Wales.<br/>

<b>Key Results:</b> CDS density estimates for <i>P. volans</i> (1.36 ha⁻¹, 95% confidence interval (CI) of 1.07-1.72 ha⁻¹) and P. peregrinus (0.28 ha⁻¹, 95% CI 0.22-0.35 ha⁻¹) were consistent with densities reported in other studies conducted in open eucalypt forests. A strip transect width of 40m for <i>P. volans</i> resulted in a collective set of values for density (1.35 ha⁻¹), error (s.e. ∓ 0.14), precision (cv 0.10) and 95% CI (1.07–1.62 ha⁻¹) closest to those associated with the CDS-generated density estimate (1.36 ha⁻¹, s.e. ∓ 0.15, cv 0.10, 95% CI 1.07–1.72 ha⁻¹). Strip widths of 10 to 40m resulted in density estimates for <i>P. peregrinus</i> closest to those generated through CDS, but much less precise.<br/>

<b>Conclusions:</b> Although a 40-m wide strip transect provided a robust density estimate for <i>P. volans</i> at Mt Duval, this is unlikely to be consistent across different study areas. Strip transects provided less precise density estimates, or underestimated <i>P. peregrinus</i> density at Mt Duval, when compared with CDS density estimates. CDS should be favoured over strip transects or abundance indices for estimating <i>P. volans</i> and <i>P. peregrinus</i> abundance, because it is capable of providing more meaningful and robust abundance estimates by accounting for the probability of detection from the transect line across different habitats.<br/>

<b>Implications:</b> Researchers, conservation managers and decision makers should be aware that common methods for assessing arboreal marsupial abundance have serious potential weaknesses. Thus, it would be prudent to invest in studies that address imperfect detection to improve the quality of monitoring data.