Evaluation of Walked Line Transect Counts for Estimating Macropod Density

Techniques for estimating wildlife density require evaluation before they can be applied with confidence. Walked line transects may be useful for study and management of macropods (Macropus spp.), but they have seldom been evaluated. I used field experiments to assess assumption violation and estimator performance. Experiments were based on 11 trials with known macropod density at 4 sites in southeastern Australia. In 8 trials macropods were habituated to humans and did not react to surveyors. In 3 trials macropods were wild and reacted to surveyors with evasive behavior. Negative density-dependent bias in the relationship between estimated density and true density for tame populations was attributed to failure in counting all animals on or close to the line at high densities (counting saturation). Bias was small and considered to have little practical consequence to line transect surveys of macropods. The limited number of trials on wild populations suggested a bias of similar form but greater (P < 0.05) magnitude to that for tame populations. Therefore the transect assumption concerning no animal movement prior to detection by a surveyor was violated. A consequence of density-dependent bias is that the apparent performance of estimators will change with density. When bias is due to reactive movement, the investigator may need to develop and apply correction factors, use different estimators at different density levels, or use faster means of traversing the transect (vehicle or aircraft) to achieve accuracy across all densities. J. WILDL. MANAGE. 58(2):348-356