ArticleComparison of Two Sampling Methods to Estimatethe Abundance ofLucanus cervuswith Applicationof n-Mixture ModelsFrancesca Della Rocca1,*, Pietro Milanesi2, Francesca Magna1, Livio Mola3, Tea Bezzicheri1,Claudio Deiaco4and Francesco Bracco1,51Departement of Earth and Environmetal Sciences, University of Pavia, Via Ferrata 9, 27100 Pavia, Italy;[email protected] (F.M.); [email protected] (T.B.);[email protected] (F.B.)2Swiss Ornithological Institute, Seerose 1, 6204 Sempach, Switzerland; [email protected]3Via Madonna del Boschetto, 16, 25030 Castel Mella, Italy; [email protected]4Via San Rocco 3, 24060 Predore, Italy; [email protected]5Botanical Garden, University of Pavia, Via S. Epifanio 14, 27100 Pavia, Italy*Correspondence: [email protected]Received: 28 August 2020; Accepted: 6 October 2020; Published: 12 October 2020Abstract:Monitoring programs should be based on the measurement of two main pillars for evaluatingthe conservation status of a species: population size and geographical distribution. To date, the onlyway reported in the literature to obtain detailed information onL. cervuspopulation size is to use thecapture-mark-recapture method. This is an expensive and time-consuming technique that impliesphysical capture and handling of individuals, which could affect their survival. Therefore, in this studywe tested and compared two non-invasive sampling approaches, namely evening walk transects anddiurnal tree trunk surveys, to derive accurate abundance estimates by means of N-mixture models in aBayesian framework. In our study, both methods showed relatively high detection probability (≥56%).However, tree surveys performed better than walk transects (≈80%), especially with the progressionof the sampling season. Tree surveys proved to be more effective than walk transects in providingdata for an accurate population density estimate (much smaller 95% Bayesian Confidence Intervals).In light of a cost and benefit assessment, the tree survey is undoubtedly more convenient, as well asmore effective, as it is more time consuming but less expensive than a walk transect (one operator for2–3 h vs. two operators for 30 min each). Moreover, it needs fewer expert operators because of thegreater proximity to the species, increasing the probability of correctly identifying it, i.e., reducing typeI error (false positive or overestimation of counts). For the first time, we applied N-mixture modelsfor estimating population abundance ofL. cervus. Overcoming all the limits imposed by the use of thecapture-mark-recapture method, in this study we performed a further step forward in the planningof monitoring aimed at the conservation ofL. cervusand the evaluation of its demographic trend.