An invasive imported snake is exterminating the emblematic Ibiza lizard

Much more than an endemic species, the Ibiza wall lizard (Podarcis pityusensis),which has inhabited Ibiza and Formentera and the surrounding islets for millennia, is a living cultural and identity symbol, as well as a key piece of the island ecosystems. And yet, it is being erased from the territory. In fact, the situation it is in is so critical that the International Union for Conservation of Nature (IUCN) has placed it on its red list in the category ofendangered.The reason for this extreme situation is that an invasive snake, the horseshoe snake (Hemorrhois hippocrepis),which arrived hidden in ornamental olive trees imported from the Iberian Peninsula and North Africa in the 2000s, has already spread across 90% of Ibiza's territory, and is devouring most specimens of lizards. Now, moreover, and worryingly, the snake has made the leap to Formentera and many of the nearly 40 surrounding islets, where unique populations of this small lizard live.

A team of researchers from CREAF, led by Oriol Lapiedra and who have been monitoring this invasion in real time for six years, have documented for the first time how the horseshoe snake is capable of swimming across the sea for distances of half a kilometer or more, to reach the islets. “It is a new behavior, which has never been observed in this species of snake or in almost any other case worldwide,” indicates Lapiedra to ARA, lead author of the work, published in the journal Ecology.For this reason, “it was previously considered unlikely that a snake would actively colonize other territories by swimming,” adds the researcher. Therefore, when they began to find them on the islets, they attributed it to accidental introductions linked to boats or human transport. However, in recent years they have collected videos and photographs, as well as testimonies that corroborate that these snakes swim in the open sea and are capable of reaching them on their own.

“Presumably, what drives them is hunger”, considers Lapiedra. Once the horseshoe snake finishes off the populations of lizards, but also other small mammals such as mice, shrews or bats in the territory, it advances looking for food and it is probable that it reaches the coast and, as it floats, motivated by chemical stimuli such as smells or simply visual ones, it decides to move through the water until it reaches the islets. “We have been able to observe a pattern in the invasion: the more snakes there are for more years, the greater the impact they generate”, assures Lapiedra. In this regard, 10 years ago, the time that passed from when a snake reached a specific area and eradicated the lizard was about 8 years. In the last two years, this time has been greatly reduced and, in the case of the islets, they can take only months to eradicate the entire population of Balearic lizards.

An emblem and key piece of the ecosystem

This species of lizards already inhabited the islands before the arrival of humans. It is the only endemic Balearic terrestrial vertebrate. And since it had no competitors or predators, it presented very high densities and had lost all capacity to respond effectively to threats. It had also acquired a fundamental role in the ecosystem: it not only pollinates most of the flowers, but also controls pests by feeding on insects and dispersing seeds.

As they have evolved spread across the islands and islets independently, they belong to different evolutionary lineages and present unique colorations in each area as a result of adaptation to different habitats. In fact, it is one of the animal species with the most color variations in the world, from green to brown, blue, gray, or orange tones, which arouses great interest among the scientific community. For the moment, as researchers have been able to confirm, in 10 islets, such as Santa Eulària and s'Ora, they have already become extinct.

The horseshoe snake began to establish itself in specific regions of the island due to human action two decades ago, and between 2010 and 2015 it began an unstoppable expansion. Without competitors or predators, it found a great abundance of food. "It's as if they had arrived at an all-you-can-eat buffet," says Lapiedra. In fact, the CREAF team has produced a map of the snake's expansion that shows that in 2010 it was present in less than 5% of the Ibizan territory, in 2016 it was already found in 40% of the island and, finally, by 2025 it had expanded to 90%. When it conquers a new area, it can take less than three years to eradicate the entire population of lizards. As the food runs out, the horseshoe snake moves on. “They are so well-fed that we have managed to capture specimens of a meter and a half and two meters, 200% larger than those found on the Peninsula,” emphasizes the CREAF researcher.

In order to save the Pitiüses lizard, several initiatives are underway, with a lot of public support, to capture these snakes, which are not venomous, through the dispersion of traps. More than 3,500 have already been captured this way. In parallel, an ex situ breeding program has been established in collaboration with the Barcelona Zoo. Lapiedra and her team selected 17 healthy and genetically diverse specimens, 10 from Ibiza and 7 from Formentera, which were taken to the zoo. So far, 12 lizards have already been born from five different clutches. Given the success, 33 more new specimens are planned to be transferred to continue breeding. In addition, a small network of enclosures has begun to be installed in Ibiza to keep the lizards safe from snakes. Meanwhile, researchers will study the feasibility of reintroducing lizard populations with a low snake density.