Swimming snakes wipe out endemic lizards from Mediterranean islets

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The horseshoe whip snake (Hemorrhois hippocrepis) was by chance launched to Ibiza within the early 2000s, probably by way of imported olive bushes (Silva‐Rocha et al., 2015). Since then, it has quickly unfold throughout the island, severely impacting native biodiversity. Most notably, it has prompted the speedy extirpation of the long-lasting Ibiza wall lizard (Podarcis pityusensis). This is an endemic, keystone species from the 2 foremost Pityusic islands (Ibiza and Formentera) and 39 surrounding islets. This lizard represents over half of the eating regimen of this snake in its invasive vary in Ibiza (Hinckley et al., 2017; Montes et al., 2021, 2022). In response to this menace, the Ibiza wall lizard was not too long ago re‐categorized as Endangered by the IUCN (Bowles, 2024). Despite intensive efforts ensuing within the culling of over 12,000 invasive snakes since 2016 (COFIB, 2022; authors’ information, up to date as of 2025), the species continues to broaden throughout the principle island. In latest years, this unfold has accelerated, thereby driving the speedy extirpation of Ibiza wall lizard populations all through the island (Montes et al., 2022; Vez‐Garzón et al., 2025). Until not too long ago, small populations from surrounding islets, that are important to preserving the evolutionary legacy of the species (Pérez‐Mellado et al., 2017), had been thought-about comparatively secure to the invasion, buffered by sea water. However, populations from these islets may the truth is be at a larger threat than beforehand assumed.

On 15 April 2024, throughout a routine monitoring marketing campaign for marine hen nesting on the islet of Santa Eulària, positioned 430 m off the jap coast of Ibiza (Figure 1), we noticed and filmed a horseshoe whip snake actively swimming from the principle island towards the islet and at last reaching it (Video S1; Appendix S1: Figure S1). To illustrate this conduct extra clearly, we later photographed one other particular person exhibiting related open‐sea swimming conduct (Figure 2A,B). On that very same day of the statement, we additionally captured and euthanized one other snake on this islet. However, these weren’t the one people discovered. On 28 September 2023, 12 snake traps had been put in there, with lively monitoring and trapping since then. Across 13 visits till May 2025, a complete of 58 snakes have been captured, both by way of traps or instantly by hand. The islet of Santa Eulària covers solely 4.67 ha, comparable to an estimated density of roughly 12.4 snakes per 1 ha. While it stays unclear whether or not the preliminary colonization concerned a single gravid feminine or a number of people, our direct statement demonstrates that snakes can colonize islets by actively swimming throughout the ocean and that that is probably a recurring phenomenon.

FIGURE 1.

FIGURE 2.

Driven by the priority that the Santa Eulària islet wall lizard inhabitants was in danger, we began to often conduct transects to quantify the abundance of Ibiza wall lizards because the fall of 2023. During the primary two visits on 28 September and 4 October 2023, we noticed solely two and one lizards, respectively. In distinction, a 2016 census reported within the conservation standing report by Pérez‐Cembranos and Pérez‐Mellado (2022) recorded 72 people alongside an 869 m transect on the identical islet, suggesting that the inhabitants was nonetheless comparatively considerable just some years in the past. Across 11 subsequent visits between 11 October 2023 and 20 May 2025, no lizards had been noticed. The inhabitants on Santa Eulària islet is now thought-about to be fully extirpated. Figure 2D illustrates the coloration of people from this now‐extirpated inhabitants.

Over 30 evolutionarily important models (ESUs) of Ibiza wall lizards have been described (Rodríguez et al., 2013), many generally described as subspecies attributable to their distinctive morphology and coloration (Cirer, 1981; Salvador, 2006; Dappen et al., 2013). With the exception of the 2 ESUs from the principle islands of Ibiza and Formentera, the overwhelming majority of those populations inhabit remoted islets round these two islands (Salvador & Perez Mellado, 1984; Cirer & Berg, 2025). The lack of even a single islet inhabitants can due to this fact characterize the extinction of a singular evolutionary lineage. Their small inhabitants sizes, restricted ranges, and excessive tameness make islet populations notably weak to novel predators (Cooper et al., 2014).

Unfortunately, Santa Eulària isn’t the one islet not too long ago invaded by the invasive horseshoe whip snakes in Ibiza. The populations of Ibiza wall lizard on the close by islet of S’Ora, a small islet off Ibiza’s northeastern coast, vanished round 2018 (Figure 1; Montes et al., 2022). At the time, though snakes had been suspected because the almost certainly reason behind extirpation, the pathway by which they might have reached the islet remained unknown. In reality, there have been anecdotal observations of swimming horseshoe whip snakes, which had been assumed to correspond to hitch‐climbing occasions (Hinckley et al., 2017; Montes et al., 2022). In an effort to analyze how widespread it’s for invasive horseshoe whip snakes to swim within the sea, we gathered stories of sea‐swimming snakes from fishermen, native residents, vacationers, regional information, and social media. These stories had been gathered by way of each direct inquiries to residents and subject collaborators, in addition to unsolicited communications from the general public. Additionally, we searched regional information retailers and social media posts and verified every report’s location and plausibility earlier than inclusion. This effort yielded a complete of 14 impartial sightings of snakes swimming in open water round Ibiza between 2015 and 2025 (Appendix S1: Table S1), comparable to a mean of roughly three documented occasions each 2 years. Most observations (77%) occurred inside 200 m of the shoreline (median 50 m; starting from 10 m to three.2 km). These sightings don’t appear to be related to troublesome climate situations (all media stories printed are in sunny and calm situations), though no formal information had been out there about this. These patterns point out that sea‐swimming conduct probably represents lively, voluntary dispersal from already invaded coastal sectors reasonably than climate‐pushed or unintended drift. While some had proposed boat transport as a believable route (Cirer & Berg, 2025), these accumulating stories, together with the case we doc right here, recommend that lively swimming alone is enough for snakes to colonize close by islets.

The potential for snakes to actively disperse overwater is supported by experimental proof demonstrating that almost all snake species, together with terrestrial taxa, exhibit swimming capabilities (Fosseries et al., 2024). Limited and oblique proof suggests lively overwater dispersal in naturally occurring snake populations exists (e.g., De Queiroz & Lawson, 2008). While most documented island colonization occasions by invasive snakes contain human‐mediated transport (Cabrera‐Pérez et al., 2012; Richmond et al., 2014), our statement means that lively swimming could facilitate secondary unfold. For instance, Burmese pythons (Python bivittatus) in Florida exhibit robust aquatic navigation abilities, reported crossing open water inside wetland ecosystems, however by no means within the marine setting (Pittman et al., 2014). Similarly, the Montpellier snake (Malpolon monspessulanus) has been noticed swimming within the open sea inside its native distribution alongside the coast of France (Deso et al., 2021).

Invasive snakes could be extraordinarily environment friendly predators of native fauna; even just a few particular person snakes (or perhaps a single gravid feminine) might doubtlessly trigger speedy declines or full native extirpations. The existence of lively overwater dispersal by snakes within the sea has profound implications for island biodiversity conservation within the Mediterranean and worldwide. In Guam, the brown tree snake (Boiga irregularis) prompted the extirpation of 10 out of 12 native forest hen species inside 30 years of introduction (Savidge, 1987; Wiles et al., 2003). Similarly, the California kingsnake (Lampropeltis californiae) on Gran Canaria has diminished native reptile populations by 50%–90% inside invaded areas since its introduction in 1998, with the most important declines (over 90%) reported for the Gran Canaria large lizard (Gallotia stehlini). Dispersal is probably going facilitated by each human transport and pure motion (Piquet & López‐Darias, 2021). A comparable case has been reported on Christmas Island, the place the launched wolf snake (Lycodon capucinus) is suspected to have prompted the speedy collapse of practically all the native reptile group inside 20 years (Emery et al., 2021). The statement reported right here demonstrates that actively swimming to islands positioned comparatively removed from the coast is a pathway of colonization that invasive snakes can use with out human voluntary or involuntary help. The potential of horseshoe whip snake to colonize islets positioned nearly 0.5 km away from the coast signifies that even reasonably remoted islets (≤1 km offshore) are probably in danger. Together with proof that seawater isn’t an efficient barrier to dispersal, our observations have robust implications for the administration of endemic species on small islets. In the case of the Ibiza wall lizard, the documented overwater dispersal of snakes means that undetected people could already be current on close by islets near invaded coastlines. Consequently, administration and culling efforts ought to proactively embrace these adjoining islets, which might act as stepping stones for additional colonization.

Our statement of a snake actively swimming towards an offshore islet, together with the confirmed presence of a number of people on that very same islet, means that geographical isolation isn’t all the time an efficient barrier to invasion. In reality, latest international research have additionally challenged this assumption by exhibiting that remoteness can truly make islands extra weak to invasion attributable to their diminished organic resistance and naïve native fauna (Moser et al., 2018). In addition, the ecological penalties of this snake invasion could go far past the lack of a single species. The ecological cascading penalties of the extirpation of Ibiza wall lizards are profound, as they play a keystone function on islet ecosystems, together with seed dispersal, pollination, and the regulation of arthropod communities (Hinckley et al., 2017; Traveset et al., 2019; Donihue et al., 2023). Our discovering prompts new open questions on what drives snakes to disperse overwater. Is this conduct triggered by intra‐particular competitors pushed by prey depletion, or the consequence of elevated exploratory tendencies in invasive lineages? Exploring these questions will assist us take into account all attainable mechanisms of invasive predator unfold and due to this fact enhance our potential to foretell the implications of those invasions.

The islet of Santa Eulària will not be an exception, however a warning sign. As a response to proof of lively overwater colonization by horseshoe whip snakes in Ibiza, now we have launched a monitoring program to evaluate how widespread snake islet colonization is. Early detection of recent colonization occasions whereas monitoring the standing of lizard populations is essential to grasp how invasions unfold over time. Beyond native conservation, this case illustrates how unpredictable the conduct of invasive species could be, and why sustained analysis is vital to anticipate and stop future ecological impacts. Recognizing and responding to those missed dispersal pathways shall be key to defending the biodiversity of small islands within the Mediterranean and past.

AUTHOR CONTRIBUTIONS

Guillem Casbas, Oriol Lapiedra, and Marc Vez‐Garzón wrote the manuscript. All authors contributed with information, fieldwork, and have reviewed and helped write the manuscript. All authors permitted the ultimate model of the manuscript.

FUNDING INFORMATION

The challenge that has generated these outcomes has been supported by EUR2021‐122000 grant funded by the Spanish Ministry of Science, Innovation and Universities (MICIU) (ref. MICIU/AEI/10.13039/501100011033). Marc Vez‐Garzón has acquired assist from the MICIU by way of the predoctoral FPU grant (ref. FPU22/00930) funded by MICIU/AEI/10.13039/501100011033. Guillem Casbas has acquired assist from the MICIU by way of the predoctoral FPI grant (ref. PRE2022‐105446) funded by MICIU/AEI/10.13039/501100011033 and by FSE+.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflicts of curiosity.

Supporting data

Casbas, Guillem
, Vez‐Garzón Marc, Sanglas Ariadna, Colomar Victor, Cardona Esteban, Montes Elba, Pleguezuelos Juan, and Lapiedra Oriol. 2026. “Swimming Snakes Wipe out Endemic Lizards from Mediterranean Islets.” Ecology
107(4): e70373. 10.1002/ecy.70373

Handling Editor: John Pastor

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