Population abundance, size structure and sex-ratio in an insular lizard

Estimating population size and understanding its variation is a fundamental, yet complicated, aim of many ecological studies. We considered the problem of estimating spring and autumn population abundance, size-dependent population structure and sex-ratio of the endemic Balearic Lizard, Podarcis lilfordi from a three occasions capture-recapture study. We used a Bayesian formulation of individual covariate models to incorporate individual sex, size and trap-response. We first considered a set of simulated data with a medium-to-low probability of recapture and individual recapture heterogeneity to evaluate potential problems in model fitting and selection. Results from simulated data indicated a low performance in parameter estimation and model selection when probability of detection was low (0.15-0.30). We found a negative permanent trap response and a positive effect of size on detection probability in the spring survey but not in the autumn one. The estimated mean densities varied from about 800 to 1000lizardsha-1, a high value when comparing with mainland lizard populations. The observed increase in abundance was probably due to a drop in territorial behaviour and the immigration of females into the area sampled. As a consequence, sex-ratio changed from nearly even in June (mean posterior, 95%CRI; 0.928, 0.676-1.167) to a female-skewed population in October (0.612, 0.478-0.772). © 2013 Elsevier B.V.The research was funded by the project BFU-2009-09359 from the Minister of Economy and Innovation of the Spanish Government. We thank Lucia Bonnet for facilitating the access to the study area. Thanks to IB port and “ExcursionBoat” Colonia St.Jordi, for their help with the logistics. The Conselleria de Medi Ambient of the Balearic Government for the permission to carry the study. Funds were partially provided by the Regional Government of Balearic Islands and FEDER funding. ST was funded by a PhD grant from the MUSE – Museo delle Scienze (Trento) in collaboration with the University of PaviaPeer Reviewe