La rapid expansion of an invasive snake The arrival of the horseshoe whip snake (Hemorrhois hippocrepis) in Ibiza has triggered scientific and environmental alarms throughout the Balearic Islands. Having arrived accidentally just two decades ago, it now occupies almost the entire island and is pushing the Pitiusa lizard (Podarcis pityusensis) towards local extinction.
Far from being limited to the main island, this A snake has proven that it can swim across the sea and colonize nearby islets, where populations of lizards with unique colorations survive. The latest field data from CREAF and other involved organizations place the snake's occupancy at over 90% of Ibiza's territory, with cascading effects on the entire ecosystem.
An invasive species that colonizes almost all of Ibiza in record time.
The horseshoe whip snake is a species native to the Iberian Peninsula and North Africa which was not part of the original fauna of the Pitiusas Islands. According to research led by the Center for Ecological Research and Forestry Applications (CREAF), its arrival in Ibiza was accidental, hidden in the trunks of ornamental olive trees imported from the mainland at the beginning of the 21st century.
Initially, the snake was only found in very specific areas of the islandHowever, between 2010 and 2015 the process took a turn: a phase of rapid expansion began that scientists describe as "unstoppable." Estimates indicate that, around 2010, it occupied less than 5% of Ibiza; by 2016 it already covered around 40%, and by 2025, it had exceeded 90% of the territory.
Researchers compare this invasion to a advancing fire front as available food dwindles. When the snake enters a new area, it can take less than three years to eliminate virtually all local lizards. The absence of natural predators and the abundance of prey explain much of this invasive success.
The feeding conditions are so favorable that in the Balearic Islands they have been caught specimens up to two meters longsizes that more than double those recorded in many populations on the peninsula. This extraordinary growth reinforces its role as a superpredator in an environment that was not prepared for its presence.
The Pitiusan lizard, a unique treasure on the edge of the precipice
The main victim of this invasion is the Pitiusas lizardThis endemic species is found only in Ibiza, Formentera, and dozens of islets surrounding both islands. Over thousands of years of isolation, these populations have developed their own unique colors and shapes, to the point that some forty subpopulations or local variants have been described.
These reptiles exhibit a exceptional variety of colors on a global scalewith individuals that are green, brown, gray, bluish, or almost black, depending on the islet and the type of habitat. Each small island harbors unique combinations resulting from genetic isolation and adaptation to very specific microenvironments, something that has sparked enormous international scientific interest.
But the value of the Pitiusan lizard is not only aesthetic or academic. From an ecological point of view, Their role in the Pitiusan ecosystems is keyIt helps pollinate flowers, controls pests by feeding on insects, and contributes to seed dispersal. The disappearance of these populations means losing both an irreplaceable natural heritage and a fundamental part of the islands' ecological system.
The sharp decline recorded over the last twenty years has led to the Pitiusa lizard now being listed as an "endangered" species by the IUCNThe work of CREAF, with the participation of the Consortium for the Recovery of Fauna of the Balearic Islands (COFIB), the Environmental Agents Service, the Spanish Herpetological Association and the University of Granada, agrees that the pressure of the horseshoe whip snake is the decisive factor in this collapse.
Snakes swimming between islets: an almost unprecedented behavior
One of the most striking findings of the study is the ability of the horseshoe snake to swim across the sea and colonize islets that, at first glance, might seem safe. At the beginning of the investigation, it was thought that if snakes appeared on these small islands, it would be the result of accidental transfers on boats or materials transported by humans.
However, as the team gathered Videos, photographs and testimonies from fishermen and residents When people saw snakes in the open sea, the scenario changed. The accumulation of evidence led to the conclusion that these snakes are capable of swimming considerable distances on their own, something very poorly documented for this species and practically unusual on a global scale in terms of actively colonizing new territories.
The study, published in the journal Ecology, combines field campaigns with traps specifically designed for snakesComparisons of past and present lizard censuses, film footage, photographs, and observations verified by the scientific team have allowed researchers to track, almost step by step, the invasive species' advance from the main island to the islets.
The researchers describe how, in these very confined spaces, The arrival of just a few snakes is enough to cause the total disappearance of the lizard population in a matter of months. The small size of the islets, combined with the lack of sufficient refuges and the high initial density of lizards, facilitates the immediate and devastating effects of predation.
Santa Eulària and other islets: from dozens of lizards to zero in a few years
One of the most studied cases is the Santa Eulària isletwhere the research team has been able to document the entire invasion process. In 2016, a census conducted along transects—fixed strips that are repeatedly traversed to count animals—recorded 72 Pitiusan lizards. This was before the snake had established itself in the area.
In a second survey, in 2023, the same routes were repeated to compare the data and the result was drastic: Only three lizards were detected at the same locations analyzed years before. Simultaneously, between 2023 and 2025, twelve traps were set up to capture snakes, and 58 horseshoe whip snakes were caught in that short period.
The final blow was confirmed in 2025. In that field campaign, Not a single lizard was observed. in the transects of Santa Eulària, which led scientists to confirm the local extinction of the population. Something similar has been documented on other islets such as s'Ora and on at least a dozen small islands scattered around Ibiza and Formentera.
The results indicate that, once the snake manages to reach the islet, The collapse of the lizard population can occur in a matter of months.Intensive predation, coupled with the inability to escape to other territories, leaves these subpopulations with no room for natural recovery.
This pattern doesn't only affect Ibiza. In Formentera, where the Pitiusan lizard is also native and has its own genetic variations, Horseshoe snakes have already been detectedGiven the island's smaller size, experts fear that rapid expansion could trigger a situation similar to that of its northern neighbor within a few years.
Cascading ecological impact on the Pitiusas Islands
The disappearance of the Ibiza wall lizard is not the only problem. The horseshoe whip snake also feeds on mice, bats, small birds and shrewsThis amplifies its effects on the food chain. Its massive presence disrupts the balance of predators and prey, displacing other species that are left without enough food.
The reduction in lizards implies that Key ecological functions cease to be performedIt reduces the pollination of many wildflowers and some crops, weakens the natural control of insects that can become pests, and decreases seed dispersal, affecting the regeneration of vegetation in different habitats.
Environmentalists and scientists talk about “cascade effects”Because the presence of the snake affects not only the lizard, but the entire Pitiusan ecosystem. With fewer small mammals and reptiles available, other predators, such as certain birds, may also be forced to change their diet, relocate, or reduce their populations.
Furthermore, the fact that the snake has so few natural adversaries in Ibiza and Formentera This helps maintain high populations. In an environment where almost nothing preys on it and where food was abundant until a few years ago, the species has found an ideal setting for its proliferation.
Institutional and scientific response: traps, laws and captive breeding
In response to this situation, the Balearic Government and various institutions have launched control and prevention measures In recent years, one of the first lines of action was to limit the introduction of ornamental olive trees, carob trees, and holm oaks during certain periods of the year, with the aim of preventing new introductions of snakes hidden in the root balls or trunks.
At the same time, there have been installed specific traps to capture snakes In various locations around Ibiza and on the most vulnerable islets, especially in areas like the Ses Salines Natural Park. In this protected area, authorities have decided to keep dozens of traps active year-round, without removing them in winter, given the scale of the problem.
The field teams from the Species Protection Service and the environmental agents have prospected islets one by one to check if the invasion has arrived. In some cases, such as Escull d'en Terra, horseshoe snake specimens have been captured, which has forced increased surveillance and the placement of more trapping devices in nearby areas such as the islet of d'en Caragoler.
At the same time, they have been designed safeguarding programs for the most threatened lizardsIn collaboration with the Barcelona Zoo, specimens from some critical subpopulations are being transferred to ex situ breeding centers, where attempts are made to maintain their genetic lineage and reproduce them in captivity in order to, in the future, reinforce or recover wild populations if conditions allow.
Scientists point out that these conservation initiatives They compete against the clockThe advance of the snakes continues and each passing season may mean the irreversible disappearance of new local lizard populations, with their genetic load and their associated evolutionary uniqueness.
The crisis unleashed by the horseshoe snake in Ibiza and Formentera shows the extent to which A seemingly accidental introduction can completely transform an ecosystem insular in just a few decades. The occupation of more than 90% of the island by this snake, its unsuspected ability to swim to the islets and the collapse of the populations of the Pitiusa lizard have set off all scientific alarms, forcing a combination of invasive species control, regulatory changes and captive breeding programs to try to prevent one of the natural symbols of the Pitiusas Islands from disappearing from the wild.