An evaluation of distance sampling to estimate badger (Meles meles) abundance

Abstract

Distance sampling is a common and increasingly used method of estimating animal abundance in conservation and management programmes. The precision of a distance sampling survey can be estimated from the field data, but accuracy can only be evaluated by comparison with the true population size. For wild mammal populations, such opportunities are rare. The high-density badger Meles meles population at Woodchester Park in Gloucestershire, U.K. has been routinely and intensively live-trapped for the past 30 years. The estimates of abundance based on mark–recapture analyses of the live-trapping data provided a reliable baseline against which to evaluate distance sampling for a badger population. A distance sampling survey was carried out in autumn 2002 by using spotlighting along line transects. Owing to the impractical nature of spotlighting in closed habitats, such as woodland, surveys were conducted in open habitat only. A radio-tracking study provided behavioural data that were used to estimate the proportion of the population available for sampling in open habitats. This was incorporated as a multiplier into the DISTANCE analysis. The estimate of abundance from distance sampling was 76 badgers (% CV=42.2), using a multiplier based on radio-tracking data from autumn only. When the multiplier was derived from radio-tracking data from across the whole year, based on a larger sample of tracked badgers, the distance sampling abundance estimate was 77 badgers (% CV=25.9). These compared favourably to the mark–recapture estimate of 68 badgers (% C.V=7.4). The lack of precision in the distance sampling density estimate was largely owing to the inclusion of the multiplier. Distance sampling combined with data on habitat use can produce reliable estimates of badger abundance in areas of medium to high density, particularly in landscapes of primarily open habitat.