Status of the largest extant population of the critically endangered Aeolian lizard Podarcis raffonei (Capo Grosso, Vulcano island)

Abstract

The Aeolian wall lizard Podarcis raffonei is an island endemic that survives only on three tiny islets, and on the Capo Grosso peninsula of the Vulcano island, thus is among the European vertebrates with the smallest range and one of the most threatened by extinction. This species is declining due to competition and hybridization with the non-native lizard Podarcis siculus, but a regular monitoring program is lacking. Here we assessed the size and status of the Capo Grosso population of P. raffonei on Vulcano. In September 2015 we captured 30 individuals showing the typical brown phenotype of P. raffonei, while one single male showed a green phenotype, apparently intermediate between P. raffonei and the non-native Podarcis siculus. In May 2017, only 47% of 131 individuals showed the typical brown phenotype (P. raffonei-like) and 53% showed the green phenotype (P. siculus-like). Based on N-mixture models and removal sampling the estimated size of the Capo Grosso population was of 800–1300 individuals in 2017, being similar to 2015; available data suggest that the total range of the species could be as small as 2 ha. The frequency of individuals with the typical P. raffonei phenotype dramatically dropped between two samplings with a parallel increase of individuals displaying the green phenotype. Observation on outdoor captive-bred individuals demonstrates plasticity for colouration in P. raffonei individuals from Capo Grosso, with several individuals showing the typical brown pattern in September 2017 and a green pattern in March 2021. Non-exclusive hypotheses, including hybridization with P. siculus and plasticity in colour pattern of P. raffonei, are discussed to explain the phenotypic shifts of the P. raffonei population of Capo Grosso. While genomic evidence is required to reach conclusions and investigate eventual hybridization, it is urgent to undertake a programme for the monitoring and management of this lizard.