Natural History and impacts of an invasive snake: the horseshoe whip snake, Hemorrhois hippocrepis (Linnaeus, 1758), on Ibiza

The island of Ibiza, located in the Balearic Islands (Western Mediterranean), has remained free of introduced snakes for millennia, unlike the majority of Mediterranean islands. But in 2003, with the Mediterranean garden fad, the entrance of old olive trees to the island became common, and three snake species appeared on Ibiza, traveling as stowaways inside the trunks of the olive trees. The most successful invader was the horseshoe whip snake (Hemorrhois hippocrepis). This PhD dissertation studies the natural history of the invasive population and compares it to the source population’s, in the southern Iberian Peninsula, in order to acquire knowledge that helps fighting against the invasion. It also analyzes the impact on the native fauna, particularly on the endemic Ibiza wall lizard (Podarcis pityusensis), the only native terrestrial vertebrate of the island. The 1st chapter focuses on the snake’s trophic ecology, finding a critical result: more than half of the diet (55.4 %) consists of lizards, whereas on the source population, lizards constitute a 24.2 % of the snake’s menu. We also observed that the length and weight was the highest recorded for the snake species throughout its range. In sight of the results, in the 2nd chapter we analyzed the historic spread of the snake’s population (period 2009-2018) and how the presence of H. hippocrepis affects the abundance of P. pityusensis, through censuses on the island and the islets inhabited by the lizard’s subspecies. The results were conclusive: the invasive population occupied the northeastern half of Ibiza by 2018, where none of the 45 censuses in snake areas had lizards except for one, while the opposite happened in snake-free areas (all 42 censuses found lizards, except for two). In addition, one of the islets’ lizard populations, P. pityusensis hortae, has vanished some months after a snake was spotted swimming 20 meters away, and therefore, the subspecies has gone extinct. The rate at which the invasive population is spreading allows us to predict that the lizard will be extinct from the island of Ibiza before 2030. In order to understand the causes of invasion success of H. hippocrepis, after checking that its main prey is a low-energetic animal, we studied its reproduction ecology (3rd chapter), under the hypotheses of an increased fertility and productivity in the invasive population. However, the findings pointed otherwise: despite a widening of the reproductive season and an earlier sexual maturity for females, the number of eggs and body condition of neonates was the same than for the source population, and the reproduction frequency reduced to 50 % of the adult females, i.e., females are reproducing every two years on Ibiza. All these findings led us to the Enemy Release Hypothesis (ERH) supporting the success of the invasive population. The ERH states that the natural enemies of a species (predators, parasites and competitors) are no longer present in the invaded range, facilitating the invasion process. In order to test the ERH, we studied the predation pressure on the snakes (4th chapter) in an indirect way: by comparing tail breakage for both populations. In fact, the results supported the ERH, consistent with the fact that only two of the seven predators for the snake are present on the island, being able to predate only on small individuals. The study of parasites in the 5th chapter showed that, despite the higher prevalence on Ibiza, all of the parasites were larval innocuous forms, conversely to what was found in the source population, which harbored a higher number of parasite species and some adult and harmful forms. In addition, competitors of the invasive snake are inexistent on the island. Thus, the ERH is likely explaining the invasion success of the horseshoe whip snake. Finally, in the light of the results from these studies, we propose in the 6th chapter an upgrade in the IUCN Red List category of the lizard from Near Threatened to Endangered. In addition, and given that the absence of natural enemies is behind the snake’s invasive success, we propose some management proposals, such as controlling the entrance of olive trees and reinforcing the existing trapping campaigns, in order to artificially act as the missing predators for the snake and protect the lizard.