Bellemain, E., Swenson, J.E., Tallmom, D.A., Brunberg, S., Taberlet, P. 2005. Estimating population size of elusive animals using DNA from hunter-collected feces: comparing four methods for brown bears. Conservation Biology, 19(1), 150-161

Noninvasive genetic methods can be used to estimate animal abundances and offer several advantages over conventional methods. However few attempts have been made to evaluate the accuracy and precision of the estimates. We compared four methods of estimating population size based on fecal sampling. Two methods used rarefaction indices and two were based on capture-mark-recapture (CMR) estimators, one combining genetic and field data. Volunteer hunters and others collected a total of 1904 fecal samples over 2 consecutive years in a large area containing a well-studied population of brown bears (Ursus arctos). On our 49,000-km_ study area in south-central Sweden, population size estimates ranged from 378 to 572 bears in 2001 and 273 to 433 bears in 2002, depending on the method of estimation used. The estimates from the best model in program MARK appeared to be the most accurate, based on the minimum population size estimate from radiomarked bears in a subsection of our sampling area. In addition, MARK models included heterogeneity and temporal variation in detection probabilities, which appeared to be present in our samples. However, all methods incorrectly suggested a biased sex ratio, probably because of sex differences in detection probabilities and low overall detection probabilities. The population size of elusive animals can be estimated reliably over large areas with noninvasive genetic methods, but we stress the importance of an adequate and well-distributed sampling effort. In case of biased sampling, calibration with independent estimates may be necessary. We recommend this noninvasive genetic approach, using the MARK models, to be used in the future in areas where sufficient numbers of volunteers can be mobilized.

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