The population of the Ibiza wall lizard (Podarcis pityusensis) the last remaining endemic vertebrate on the island of Ibiza in the western Mediterrane
The population of the Ibiza wall lizard (Podarcis pityusensis) the last remaining endemic vertebrate on the island of Ibiza in the western Mediterranean Sea, has become prey over the last 18 years by an invasive snake that was brought to the islands inside the trunks of olive trees. The horseshoe whip snake (Hemorrhois hippocrepis) became widely established while two other stowaway snake species, the Montpellier snake (Malpolon monspessulanus) and the ladder snake (Zamenis scalaris).
Of the three, the horseshoe whip snake thrived while the Montpellier snake seems to have disappeared and the ladder snake maintains a steady population. It is the horseshoe whip snake that is causing population declines in the Ibiza wall snake, as studies have shown that upward of 56 percent to the horseshoe whip snake’s diet is the the Ibiza wall lizard.
The researchers point out that 30 of the islets that can be found around the main island of Ibiza has isolated populations of P. pityusensis (partitioned among 22 subspecies) but these populations are also potentially susceptible to predation by Hemorrhois hippocrepis by virtue of its capability to not only stowaway, but also a strong capability for swimming. It has been documented that the snake can swim between 10 and 1,000 meters of the Ibiza coast.
“Although we lack pre-invasion data on lizard numbers or densities, we nonetheless know that prior to snake invasion the lizard ranged extensively throughout the island and was quite common,” the researchers wrote in their paper. “Furthermore, snakes have not yet colonized the entire island, and this allows us to compare current lizard numbers between areas with and without established snake populations.”
The researchers mapped snake sightings from 2010 to 2018 to determine the snake’s range, adding current year ranges to previous year ranges and found the snake consistently maintained itself in the previous ranges. Lizards were likewise assessed in snake-present and snake-absent areas by halving Ibiza island into two areas: northeastern half of the island with established snake populations and the southwestern half of the island that has no snake presence.
The researchers determined that the invasive reptile occupies 49.31 percent of the island, and 43.04 percent of the lizards’ entire range area. Based on the data, the researchers believe the snakes will cover the entire island of Ibiza by 2027-2028. This very fast blanketing of Ibiza island is driving a very fast decline of P. pityusensis to the point of extirpation within the invaded range.
“Given the results reported herein, so long as snakes are thriving on Ibiza, the spread of H. hippocrepis entails a serious threat not only to the main Ibizan population of P. pityusensis, but also to Formentera, which is 7 km from Ibiza and transportation of goods is continuous between both islands, and the 22 additional subspecies that inhabit 38 islets surrounding Ibiza and Formentera,” the researchers wrote in their paper.
The complete paper, Collapse of the endemic lizard Podarcis pityusensis on the island of Ibiza mediated by an invasive snake” can be read on the Current Zoology website.