Trends and weather events in long-term densities of an insular lizard

Lizard populations have been traditionally considered unusually stable. However, there are very few examples to verify this hypothesis on long-term studied populations. Over almost four decades, we have obtained long time series of the abundance of Lilford's Wall Lizard, Podarcis lilfordi, from all known populations of Menorca (Balearic Islands, Spain). Lizard densities were estimated from line transects data using N-mixture models. Irregular time series were then checked for the presence or absence of a monotonic trend and for density dependence of each population. With multiple regression models we studied the relationship between weather events and the annual abundance of lizards. Density dependence was detected only in two populations: Addaia Petita and Binicodrell Gros. The analysis with EGSS and OUSS models, as well as the results with a Bayesian analysis, showed that most populations seem stable over the years, with a positive and statistically significant trend in five of the 17 populations under study. Only on Sanitja Island the population of lizards exhibits a significantly negative trend. Weather events have a very variable influence on each of these populations. In two small islets, Mel and Ses Mones, the annual number of cloudy days have a negative effect on the abundance of lizards. In Bledas Island, with an exposed geographical situation, we observe a significantly negative effect of all three weather events considered: storms, cloudy days and strong winds. Surprisingly, in Aire and Addaia Gran islands, the annual number of days with storms had a significantly positive effect, probably because the marine subsidies contributed to trophic availability of these two islands by the storms. This work is a first attempt to establish the real abundance of this species, its long-term trend and the influence of weather events on densities of a large set of populations. The relevance of weather events is strongly different on each population and depends on environmental conditions of each population, especially lizard abundance itself, the island surface, geographical location and degree of exposure to weather events.