Estimating animal abundance by distance sampling techniques

Reliable and repeatable methods for estimating animal abundance are fundamental to the effective study and management of New Zealand species. New Zealand scientists and conservation managers currently rely on measures of conspicuousness rather than density for indexing the sizes of many New Zealand animal populations. Distance sampling offers a group of methods which estimate the absolute density of a population from the measured distances from the observer to each observed animal. Implicit to the theory is the idea that a large proportion of the population will go undetected as long as those individuals closest to the observer are detected with certainty. Distance sampling was assessed in three situations: by computer simulation and in studies of island populations of tuatara (Sphenodon punctatus) and North Island saddleback (Philesturnus carunculatus rufusater). The technique provided estimates of density which proved to be accurate estimates of absolute density. These estimates were comparable with mark-recapture measures but at greatly reduced effort and less disturbance to the animals and environment. Point counts, when used for estimating absolute density, demonstrated excessive bias for estimating density of saddleback. In this study, point counts proved unable to track the 'true' difference in density between habitats. Conservation managers and scientists are recommended to adopt methods which will improve their accountability and knowledge of population density and species abundance. Distance sampling techniques should be included as a suite of methods which with further investigation will help advance New Zealand managers and scientists toward this.