Empirical evaluation of non-invasive capture-mark-recapture estimation of population size based on a single sampling session

1. Non-invasive genetic data analysed with capture-mark-recapture (CMR) models can be used to estimate population size, particularly for elusive and endangered species. Data generated from non-invasive genetic sampling are different, however, from conventional CMR data because individuals can be contacted several times within a single sampling session. Two methods have been proposed recently to accommodate this type of data, but no study has attempted to compare their estimates and evaluate their reliability compared with independent estimates of population size. 2. We investigated the reliability and accuracy of estimating the abundance of lesser horseshoe bats Rhinolophus hipposideros by genotyping DNA from droppings collected non-invasively at three colonies over 2 consecutive years. The number of times that each individual was 'contacted' (i.e. the number of droppings per individual) was used to estimate population size with two different published methods: a maximum likelihood and a Bayesian estimator. 3. Among the 586 samples extracted, 534 provided a complete genotype at six to eight microsatellite loci, which enabled a reliable discrimination of 165 individuals. Statistical estimates of colony sizes often included independent estimates obtained from visual counts, validating the method. Discrepancies appeared when capture heterogeneity was not taken into account while it occurred. 4. Synthesis and applications. We have taken a first step towards improving methods of estimating numbers of bats by demonstrating that genetic data produced from bat faecal DNA are of high quality and can provide accurate estimates of population size even when samples are taken during only one sampling session. Such protocols provide valuable management tools for elusive and rare species in general. The method is relatively easy and cost-efficient because only one sampling session is required.