Ibiza’s iconic wall lizards are being wiped out by an invasive snake that’s already on 90% of the island

There once used to be thousands of endemic lizards from the Pitiusas – a name for the Spanish islands of Ibiza and Formentera and their satellite islets. The species (Podarcis pityusensis), more commonly known as the Ibiza wall lizard, is unique and a a cherished icon of its privileged environment in the Mediterranean. But now an invasive predator, the horseshoe whip snake (Hemorrhois hippocrepis) is eating the lizards, especially in Ibiza, where the snakes have already made their way across almost 90% of the territory; only 30% of the island still has wall lizards left. The islets are not safe either, as the snakes have managed to swim and colonize those, too. Scientists estimate that in two years, these snakes will have a presence across the entire island of Ibiza.

The first of these invasive snakes were detected in 2003, in a plant nursery where they arrived incognito in the roots or trunks of ornamental olive trees. The danger was underestimated; they left the nursery and began to spread across the island. In 2024, 3,072 snakes were captured on Ibiza in the traps distributed by the Balearic authorities. The numbers have been multiplying since 2016, when traps began to be laid. In 2023 2,007 were caught, and in 2022, 2,710. The largest of these snakes are bigger than those on the peninsula: upwards of 180 cm (nearly six feet). “That’s because they have a lot of food. As far as they’re concerned, this is an all-you-can-eat buffet,” explains Oriol Lapiedra, head of the research group at the Center for Ecological Research and Forestry Applications (CREAF), who has been studying their colonization for the past five years.

“They advance as if they were on the front of a battle zone,” says Lapiedra. “You have to imagine the snakes as if they were an actual wave, devouring what they find.” As time goes on, this front becomes stronger, and more snakes accumulate at an ever-increasing rate, making the lizards disappear. “There are very few strongholds left, the situation is dramatic,” he says. The snakes have also entered the island of Formentera, and work is being done to control them with traps which caught 806 snakes last year. The ladder snake (Zamenis scalaris) predominates there, although some horseshoe snakes have also been caught.

The focus is on the Ibiza wall lizard because it is the only endemic vertebrate on the island, as well as being “a cultural icon.” But there are other micromammals that have become extinct locally in areas where there are snakes, such as field mice, shrews and geckos. “We have found that their presence also affects bats and small birds such as sparrows, goldfinches, greenfinches,” says Miquel Puig, manager of the Consortium for the Recovery of the Fauna of the Balearic Islands (COFIB).

The disappearance of these animals has a knock-on effect and can lead to other extinctions, because the ecosystem is made up of species, each with an essential role to play, “and when one arrives at the top of the food chain, this hierarchy is totally destabilized,” Lapiedra adds. The mouse, for example, is eaten by owls or kestrels, which disperse seeds. The lizard pollinates many species of plants, especially on islets, “so if they disappear, you are also killing plants that have evolved along with them,” says Puig. The disappearance of species that previously fed on insects can also trigger an explosion of pests that affect agriculture, leading to the use of more chemical substances.

The problem has become worse because snakes are colonizing the islets and, these islets being smaller, “a single specimen can wipe out an entire community and each one is unique because they have evolved in isolation,” warns Lapiedra. To the surprise of the researchers, the snakes can swim to the islets. “There are islets 30 to 40 meters from the coast, but others are hundreds of meters away and we know that they can swim in the open sea and cover great distances,” explains Puig. Some snakes have been found a mile from the coast.

What will happen next? “There will come a point where, sadly, the snakes will have eaten almost everything on the island and there will no longer be room for more snakes, so their numbers could decrease,” Lapiedra says. But “biology is full of surprises; snakes can survive in situations with very little food – they can feed only once every few months,” he adds. This is what has been happening in parts of the island that have been invaded for years, where the snakes are snatching birds or rifling their nests, taking what they can. “Some have been seen trying to eat a rabbit, which is difficult for them because of their size.”

The possibility of eradication

What is essential is to reduce the number of snakes, “to seek a coexistence between both species, because eradication is almost impossible with the numbers that exist now,” says Puig. It is not uncommon to spot them on a wall sunning themselves. “They are not dangerous or poisonous. They usually escape if they hear someone and only become aggressive if they feel attacked,” says Puig who also clarifies that we should not be influenced by the typical negative perception of snakes. “It’s not their fault. They are simply fulfilling their function, which is to eat and reproduce – the more the merrier. The problem is that we have brought them onto islands where previously they did not exist.”

Snakes and lizards do coexist on mainland Spain. They have evolved together and the lizards have developed the mechanisms to escape from the snake. But the Ibiza wall lizard has not had any predators for thousands of years, and it is impossible for it to develop such mechanisms in the space of two decades. The lizard needs help from conservationists, to allow time for that balance to be produced, because “eradicating the snake is a utopia, though I hope I am wrong,” says Puig.

The Ministry of Agriculture, Fisheries and the Natural Environment and the Barcelona Zoo have promoted a pilot project for the breeding of Podarcis pityusensis. After a genetic study carried out by CREAF, 17 healthy and genetically different specimens were captured – 10 in Ibiza and seven in Formentera – and transferred to the zoo. “The real objective is to see if they can be bred in captivity to maintain a gene pool while other measures are developed on a larger scale, such as creating refuges in Ibiza and trying to repopulate the islands with the specimens that are there,” says Lapiedra.

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