Biases associated with population estimation using molecular tagging

Although capture-recapture techniques are often used to estimate population size, these approaches are difficult to implement for a wide variety of sp ecies. Highly polymorphic microsatellite markers are useful in individual i dentification, and these 'molecular tags' can be collected without having t o capture or trap the individual. However, several sources of error associa ted with molecular identification techniques, including failure to identify individuals with the same genotype for these markers as being different, a nd incorrect assignment of individual genotypes, could bias population esti mates. Simulations of populations sampled for the purpose of estimating pop ulation size were used to assess the extent of these potential biases. Popu lation estimates tended to be biased downward as the likelihood of individu als sharing the same genotype increased (as measured by the probability of identity (PI) of the multi-locus genotype); this bias increased with popula tion size. Populations of 1000 individuals were underestimated by greater t han or equal to 5% when the PI was as small as 1.4 x 10(-7). A similar-size d bias did not occur for populations of 50 individuals until the PI had inc reased to approximately 2.5 x 10(-5). Errors in genotype assignment resulte d in overestimates of population size; this problem increased with the numb er of samples and loci that were genotyped. Population estimates were often >200% the size of the simulated populations when the probability of making a genotyping error was 0.05/locus and 7-10 loci were used to identify indi viduals. This bias was substantially reduced by decreasing genotyping error rate to 0.005. If possible, only highly polymorphic loci that are critical for the identification of the individual should be used in molecular taggi ng, and considerable efforts should be made to minimize errors in genotype determination.