Estimating density of terrestrial reptiles in forest habitats: The importance of considering availability in distance sampling protocols

Reptile populations are relevant components of biodiversity in both temperate and tropical forests. However, in forest habitats reptiles are secretive and the complex structure of the environment makes it difficult to assess with confidence their abundance and density. In general, capture-mark-recapture (CMR) or distance sampling (DS) are used to estimate demographic parameters of reptiles in these complex habitats. CMR may be expensive in terms of time, materials and sampling effort, while DS is strongly biased when animals lying on the transect line are overlooked. In this study, we applied a combination of CMR and hierarchical distance sampling (HDS) to estimate the density of the Common Wall Lizard (Podarcis muralis), a widespread Mediterranean terrestrial reptile. We randomly placed linear transects in a deciduous woodland (i.e. Castanea sativa), in a coniferous plantation forest (Pinus nigra) and in a dry prairie habitat, and we applied a hierarchical mark-recapture distance sampling (HMRDS) protocol. Density estimates were similar between the deciduous woodland (47 individuals/ha) and the dry prairie habitat (44 individuals/ha), while markedly lower in the coniferous plantation forest (13 individuals/ha). HMRDS data, analyzed in a Bayesian framework, showed the importance of correcting for the assumption of complete detection on the transect line, in all the three habitat types. Therefore, our approach should be useful when assessing the density of small and cryptic terrestrial animals, not only in forest but also in habitat with an apparently less complex vegetation structure.