Effects of the introduction of an herbivore on an endangered lizard

Abstract Sanitja Island, the northernmost land of the Balearic Islands, is occupied by a population of Lilford’s Wall Lizard, Podarcis lilfordi , under extreme environmental conditions. It is an islet of less than 10 hectares, with low plant diversity and a complete absence of shrubs. During long-term monitoring, we studied lizard abundance over a period of approximately 40 years. Throughout several years, a group of domestic goats were introduced into the islet, leading to a collapse of the lizard population, with important collateral effects on their trophic ecology. On Sanitja Island, we observed the only case of a population of P. lilfordi in which the trend of the time series of abundance was significantly negative. The analysis of the time series indicated a lack of effect of the abiotic factors. The only factor that had a detectable effect on the population was the presence of goats. The introduction of goats caused the impoverishment of trophic resources for lizards and the change to a more diverse diet, with significantly lower consumption of plant matter. The removal of goats resulted in a partial recovery of lizard density, the recovery of a more specialized diet and the restoration of foraging behaviour on the dominant vascular plant, the sea fennel Crithmum maritimum . This foraging behaviour allows the pollination syndrome between the sea fennel and P. lilfordi. This study shows the dramatic effect of the introduction of an herbivorous competitor in an insular lizard population.