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1 April 2011 Andean bear density and abundance estimates — How reliable and useful are they?
David L. Garshelis
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Few attempts have been made to estimate numbers and densities of Andean bears (Tremarctos ornatus). It is understandable that the many challenges involved in these efforts have made it difficult to produce rigorous estimates. A crude estimate of ∼20,000 Andean bears was derived by extrapolating the lowest observed density of American black bears (Ursus americanus) across the range of Andean bears. A second estimate, based on rangewide genetic diversity, produced a wide range of values; however, the low end of the confidence interval roughly matched the population estimate based on minimum black bear density. A mark–recapture analysis of 3 camera-trapped bears in Bolivia also yielded a similar density (4.4–6 bears/100 km2), but overlapping home ranges of 2 radiocollared bears at that same site suggested a higher density (≥12 bears/100 km2). Neither of these estimates can be considered reliable or representative of the wider population because of the small sample sizes. Moreover, the effective sampling area for the camera-trapping study was uncertain. A DNA hair-trapping mark–recapture study in Ecuador sampled a greater number of bears (n  =  25) within a larger study area, but a male-biased sex ratio suggested that closure was violated, precluding a simple estimate of density based on the area of the trapping grid. Also, low capture rates in what was perceived (from incidence of bear sign) as prime bear habitat might be indicative of a sampling bias. These issues are not simply incorporated into confidence intervals (CIs): CIs only include uncertainty due to sampling error, not biased sampling or an ambiguous sampling area. Whereas these (low density) estimates may provide guidance for conservation, their greatest usefulness may be in providing directions for improvement of future studies of Andean bears, as well as bears in Asia, which also lack rigorous population estimates.

David L. Garshelis "Andean bear density and abundance estimates — How reliable and useful are they?," Ursus 22(1), 47-64, (1 April 2011).
Received: 14 November 2010; Accepted: 1 February 2011; Published: 1 April 2011
biased estimates
DNA hair-sampling
effective trapping area
genetic diversity
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